1
Q
What is a force field?
A
- Any region of space where a body will experience a non-contact force
- This will cause the body to move, interact or be deformed in some way
2
Q
Why do force fields occur?
A
- Arise from the interactions between bodies of particles
3
Q
What type of particles experience what forces?
A
- Static or moving charged particles experience a force in an electric field
- Moving charged particles experience a force in a magnetic field
- Particles with mass experience a force in a gravitational field
4
Q
What is Newtons law of gravitation?
A
- The gravitational force between two point masses is proportional to the product of the masses and inversely proportional to the square of their seperation
5
Q
What is the gravitational field strength at a point?
A
- Describes how weak or strong the gravitational field strength is at that point
6
Q
What is the gravitational force?
A
- There is a universal force of attraction between all matter with mass
- This is known as the “force due to gravity”, or the weight
7
Q
What is a gravitational field?
A
- A region of space where a mass experiences a force due to the gravitational attraction of another mass
8
Q
What is the direction of a gravitational field?
A
- The direction of a gravitational field is always towards the centre of the mass
- Gravitational forces cannot be repulsive
9
Q
What is the range of a gravitational force?
A
- Gravity has an infinite range, meaning it affects all objects in the universe
10
Q
How does mass affect gravitational force?
A
- Objects with a larger mass have a greater gravitational force
- Objects with a small mass have a smaller gravitational force (almost negligible for atoms)
11
Q
How do you calculate gravitational field strength?
A
- g = F / m
- Gravitational field strength = force due to gravity (weight) / mass
12
Q
How does the mass and weight of an object differ at different points in space?
A
- An object’s mass is the same at all points in space
- The weight of an object will differ depending on the gravitational field strength at that point in space
13
Q
What factors affect the gravitational field strength on the surface of a planet?
A
- Radius of the planet
- Mass of the planet
14
Q
How can you represent the strength and direction of a gravitational field?
A
- Can be represented using gravitational field lines
- The gravitational field lines around a point mass are radially inwards
- The gravitational field lines of a uniform field, where the field strength is the same at all points, are represented by equally spaced parallel lines
15
Q
What is a radial field?
A
- Radial fields are considered to be non-uniform fields
- The gravitational field strength is dependent on how far you are from the centre
16
Q
What is a point mass?
A
- For a point outside of a uniform field, the mass of the sphere may be considered to be a point mass at its centre
- A uniform sphere is one where its mass is evenly distributed
17
Q
When can an object be considered to be a point mass?
A
- It has an even mass distribution and the distance being considered is larger than its size
18
Q
What is Newton’s law of gravitation?
A
- The gravitational force between two bodies outside a uniform field is defined by Newton’s law of gravitation
- Newton’s law of gravitation states that the gravitational force between two point masses is proportional to the product of the masses and inversely proportional to the square of their separation
19
Q
How do you calculate the gravitational force between two masses?
A
- F = Gm1m2 / R^2
20
Q
What bodies does Newton’s law of gravitation apply to?
A
- Applies to orbiting bodies
- Although stars and planets are very large, they can be considered to be point masses as they are approximately uniform spheres and their separation is much larger than their radii
21
Q
What is the gravitational field strength due to a point mass?
A
- g = GM / r^2
- g = gravitational field strength
- M = mass of the body producing the gravitational field
- r = distance between point source (mass) and the position in field
22
Q
What is gravitational potential energy (on Earth)?
A
- Gravitational potential energy on Earth is the energy an object has when lifted off the ground
- GPE = mgh
- The GPE on the surface of the Earth is taken to be 0. This means work is done to lift the object
23
Q
What is gravitational potential energy (not on Earth)?
A
- Outside the Earth’s surface, GPE is defined as the energy an object possesses due to its position in a gravitational field
24
Q
What is gravitational potential?
A
- Gravitational potential at a point is the gravitational potential energy per unit mass at that point (J/kg)
- The work done per unit mass in bringing a test mass from infinity to a defined point
25
Why is gravitational potential always a negative value?
- Gravitational potential is 0 at infinity, and work must be done on the mass to bring it to infinity
- Therefore, it is negative on the surface of a mass (planet) or at a defined point and will increase as it gets further away from the mass and towards infinity (tending towards 0)
26
What is gravitational potential difference?
- Two points at different distances from a mass will have different gravitational potentials
- Therefore, there will be a gravitational potential difference between the two points
- A difference in gravitational potentials will also give a difference in gravitational potential energies
27
How do you calculate gravitational potential?
- V = -GM / r
- V = gravitational potential (J/kg)
- M = mass of body producing the gravitational field (kg)
- r - distance from the centre of mass to the point mass (m)
28
What type of quantity is gravitational potential?
- Scalar quantity
- Gravitational field strength is a vector quantity
29
What is the graphical representation for V = -GM / r
- The values of V are always negative
- As r increases, V also increases towards 0
- V against r follows a -1/r relation
- The gradient of the graph at any particular point is the value of 6 at that point
30
What is the graphical representation for g = GM / r^2
- The values of g are all positive
- As r increases, g decreases towards 0
- g against r follows a 1/r^2 relation (inverse square law)
- The area under this graph is the change in gravitational potential
31
What is work done in gravity and how do you calculate it?
- When a mass is moved against the force of gravity, work is done on the mass
- W = mΔV
- W = work done (J)
- m = mass (kg)
- ΔV = change in gravitational potential (J / kg)
32
What is the relationship between work done and GPE?
- The change in work done against a gravitational field is equal to the change in gravitational potential energy
33
What are equipotential lines?
- Equipotential lines join together points that have the same gravitational potential
34
What are the characteristics of equipotential lines?
- They are always perpendicular to the gravitational field lines
- They are represented by dotted lines
35
What are equipotential lines in a radial field?
- A radial field is the same as the field of a planet
- The equipotential lines are concentric circles around the planet
- They become further apart as they move further away from the planet
36
Why do equipotential lines move further apart?
- Potential decreases with distance away, and the gravitational field gets weaker
- Hence, a greater distance needs to be moved to obtain the same change in potential
37
What is true about moving along equipotential lines?
- No work is done as an object moves along equipotential lines
38
How do objects stay in circular orbit in a gravitational field?
- Since most planets and satellites have a near circular orbit, the gravitational force Fg between the sun and another satellite / planet provides the centripetal force needed to stay in orbit
- The gravitational force is centripetal; therefore, it is perpendicular to the direction of travel of the planet
39
How do we calculate the linear speed of an object in orbit (derivation)?
- As we know the gravitational force is also the centripetal force:
- Fg = Fcentripetal
- GMm / r^2 = mv^2 / r
- Hence: v^2 = GM / r
- v = linear speed of mass in orbit
- G = Newton's gravitational constant
- M = mass of object being orbited
- r = orbital radius
40
What do we know about the speed of different objects in orbit?
- As we know v^2 = GM / r and we know G and M are constants:
- All satellites at a distance r will travel at the same speed, no matter their mass
41
How do we calculate the time period of the orbit of an object (derivation)?
- As we know v = 2πr / T (speed = distance (circumference) / time)
- We can square both sides to get (2πr / T)^2 = GM / r
- And make T the subject to get T^2 = 4π^2r^3 / GM
42
What is Kepler's third law?
- As we know that T^2 = 4π^2r^3 / GM
- And π, G and M are constants
- We can see that T^2 ∝ r^3
- Kepler's third law: for planets and satellites in a circular orbit about the same central body, the square of the time period is proportional to the cube of the radius of the orbit
43
What is the energy of an orbiting satellite?
- An orbiting satellite follows a circular path around a planet
- Therefore, it has KE and GPE and its total energy is always constant
44
What happens to the energy of an orbiting satellite if its orbital radius changes?
- If the orbital radius increases, its GPE increases (further away from the centre) and so the KE decreases
- As the KE decreases, it will have a longer time period
45
What is the escape velocity (explanation)?
- To escape a gravitational field, a mass must travel at the escape velocity
- This is dependent on the mass and radius of the object creating the gravitational field (the planet)
46
What is the escape velocity (definition)?
- The minimum speed that will allow an object to escape a gravitational field with no further energy input
47
What is true about the escape velocity of different objects?
- The escape velocity of different objects in the same gravitational field is the same, no matter the mass of the object
48
When does an object reach its escape velocity?
- An object reaches its escape velocity when all of its kinetic energy has been transferred to gravitational potential energy
49
How do you calculate escape velocity (derivation)?
- Escape velocity is reached when all of the KE is transferred to GPE:
- 1mv^2 = GMm / r
- Hence, v = sqrt(2GM / r)
50
Why do rockets not need to reach the escape velocity?
- They are constantly given energy through fuel and thrust
- Less energy is needed to achieve orbit than to escape from Earth's gravitational field
51
What is a common misconception about escape velocity?
- The escape velocity is the velocity required to escape a planet's gravitational field NOT just to escape the planet
52
What is a synchronous orbit?
- When an orbiting body has a time period equal to that of the body being orbited and in the same direction of rotation as the body
- These usually refer to satellites orbiting planets
53
What is the difference between a synchronous and a geosynchronous orbit?
- The orbit of a synchronous satellite can be above any point on any planet's surface and in any plane
- A geosynchronous orbit is a synchronous orbit around the Earth with a orbital period of 24 hours
54
What is a geostationary orbit?
- A geostationary orbit is a special case of a geosynchronous orbit
- A geostationary orbit remains directly above the equator, is in the plane of the equator, always orbits the same point above the Earth's surface and moves from West to East (same direction of rotation as the Earth)
55
What are geostationary orbits used for and how?
- Geostationary orbits are used for telecommunication transmissions and television broadcasts
- A base station on Earth sends the TV signal up to the satellite, where it is amplified and broadcast back to the ground to the desired location
- The satellite receiver dishes on the surface must point to the same point in the sky
- Since the geostationary orbits are fixed, the receiver dishes can be fixed too
56
What is a low orbit and why are they used?
- A low orbit means the altitude of the satellite is closer to the Earth's surface
- Low orbits are useful for taking high-quality photographs, E.G: for the weather or for military purposes
57
What is an electric field?
- All charged particles generate an electric field
- This field exerts a force on charged particles which are nearby
58
What is Coulomb's law?
- The electrostatic force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation
59
How do you calculate the electrostatic force between two charges?
- F = Q1Q2 / 4πε₀r^2
- F = electrostatic force between two charges (N)
- Q1, Q2 = magnitude of the charges (C)
- r = distance between the centres of the charges (m)
- ε₀ = permittivity of free space
60
What is a similarity between electric and gravitational forces?
- Both electric and gravitational forces follow an inverse square law between charge or mass
61
What is the permittivity of free space?
- A measure of the resistance offered by a material in creating an electric field within it
62
What is the difference between electric and gravitational forces?
- A gravitational force can only be attractive, while an electric field can be attractive or repulsive
63
How do you know if an electric field is attractive or repulsive?
- A positive electrostatic force means the charge experiences repulsion
- A negative electrostatic force means the charge experiences attraction
64
How do we use the permittivity of air in calculations?
- When calculating the force between two charges, air is treated as a vacuum
- Hence, we use the permittivity of free space
65
What are field lines used to show in electric fields?
- Field lines are used to represent the direction and magnitude of an electric field
- In an electric field, field lines are always directed away from the positive charge and towards the negative charge
66
Characteristics of a uniform electric field
- The field lines are equally spaced at all points
- The field strength is the same at all points
- A force on a test charge has the same magnitude and direction at all points in the field
67
Characteristics of a radial electric field
- The field lines are equally spaced when they exit the surface of the charge
- The separation increases with distance
- The magnitude of the electric field strength and force on a test charge decreases with distance
68
Electric field around a point charge
- The electric field is radial and the lines are directed radially inwards or outwards
- If the charge is positive, the field lines point outwards
- If the charge is negative, the field lines point inwards
69
Electric field around a conducting sphere
- When a conducting sphere becomes charged, the resulting electric field around it is the same as it would be if all of the charge was concentrated at the centre
- This means that a charged sphere can be treated in the same way as a point charge
70
Electric field between two point charges (opposite charges)
- The field lines are directed from the positive to the negative charge
- The closer the charges are brought together, the stronger the attractive electric force between them becomes
71
Electric field between two point charges (like charges)
- Field lines are directed away from two positive charges and towards two negative charges
- The closer the charges are brought together, the stronger the repulsive electric force becomes
- There is a neutral point at the midpoint between the charges where the resultant electric force is zero
72
Electric field between two parallel plates
- When a potential difference is applied between two parallel plates, they become charged
- The electric field between the plates is uniform
- The electric field beyond the edges of the plates is non-uniform
73
Electric field between a point charge and a parallel plate
- The field around a point charge travelling between two parallel plates combines the field around a point charge and the field between two parallel plates
74
What is an electric field?
- A region of space in which an electric charge experiences a force
75
What is the electric field strength at a point?
- The force per unit charge experienced by a small positive test charge placed at that point
76
How do you calculate electric field strength?
- E = F / Q
- E = electric field strength (N/C)
- F = electric force on the charge (N)
- Q = magnitude of the charge (C)
77
Why is a positive test charge used?
- Electric field strength is a vector quantity
- A positive charge experiences a force in the direction of the field
- A negative charge experiences a force in the opposite direction
78
How do you calculate the electric field strength in a uniform field?
- E = V / d
- E = electric field strength (V/m)
- V = potential difference between the plates (V)
- d = separation between the plates (m)
79
What do we know about the electric field strength between two parallel plates?
- The greater the voltage between the plates, the stronger the field
- The greater the separation between the plates, the weaker the field
80
What direction is the electric field between two parallel plates?
- The electric field between two parallel plates is directed from the positive plate towards the negative plate
81
How is work done in an electric field?
- When two points in an electric field have a different potential, there is a potential difference between them
- Work must be done to move a charge across the potential difference
82
Derivation of the electric field strength between plates
- E = F / Q = V / d
- Fd = VQ
- W = Fd
- W = work done on charge (J)
- F = electrostatic force (N)
- d = distance between plates (m)
83
Describe the strength of an electric field in a radial field
- The strength of an electric field due to a point charge decreases with the square of the distance
- This is an inverse square law, similar to Coulomb's law
84
How do you calculate the electric field in a radial field?
- E = F / q = Q / (4πε₀r^2)
- Q = point charge producing the radial field
- q = charge of the small positive test charge
85
How do you calculate the force or electric field strength due to multiple charges?
- Both electric force and field strength are vector quantities
- To find the electric force or field strength at any point due to multiple charges, each field can be combined by vector addition
86
What are the differences between gravitational and electrostatic fields?
- The gravitational force acts on particles with mass, whilst the electrostatic force acts on particles with charge
- The gravitational force can be attractive, while the electrostatic force can be attractive or repulsive
- The gravitational potential is always negative, while the electric potential can be positive or negative
87
What is a magnetic field?
- A magnetic field is a region of space in which a magnetic pole will experience a force
88
What can a magnetic field be created by?
- A permenant magnet
- A moving electric charge
89
What are permenant magnets and what will not produce a magnetic field?
- A permenant magnet is a material that always produces a magnetic field
- A stationary charge will not produce a magnetic field
90
What is the magnetic flux density?
- The number of magnetic flux lines passing through a region of space per unit area
- Used to describe the strength of a magnetic field
91
- What is 1 tesla?
- 1 tesla (1T) is defined as the flux density that causes a force of 1N on a 1m wire carry a current of 1A at right angles to the flux
92
How are flux lines used to represent a magnetic field?
- Flux lines go from North to South
- They never cross
- They are closest together at the poles as this is where the field is the strongest
93
What is electric potential?
- The electric potential of a point charge is defined as the work done per unit charge in taking a small positive test charge from infinity to a defined point
94
What is electric potential measured in and what type of quantity is it?
- Electric potential is measured in J/C or V
- It is a scalar quantity but has a sign depending on the sign of the charge
95
When is electric potential 0?
- Electric potential is 0 at infinity
- This is because the work done in bringing a test charge from infinity to infinity is 0 (it is already at infinity)
96
What does the electric potential at a point depend on?
- The magnitude of the point charge
- The distance between the charge and the point
97
What are the characteristics of the electric potential around an isolated positive charge?
- Electric potential has a positive value
- Increases when a test charge moves closer: energy must be supplied to overcome the repulsive force (the test charge is always positive)
- Decreases when a test charge moves away
98
What are the characteristics of the electric potential around an isolated negative charge?
- Electric potential has a negative value
- Decreases when a test charge moves closer
- Increases when a test charge moves away
99
How do you calculate the electric potential around a point charge?
- V = Q / (4επr)
100
What do you do if there are multiple test charges (electric potential)?
- To find the potential at a point caused by multiple charges, the potential of each charge is combined using addition
101
How do you use electric potential for the capacitance of a sphere?
- Using C = Q / V, we can substitute V = Q / (4επr) to find that the capacitance of a sphere C = 4επr
- The capacitance of a sphere is entirely dependent on its radius
102
What is the potential gradient?
- The rate of change of electric potential with respect to displacement in the direction of the field
- The gradient at a point on the graph of V = Q / (4επr)
103
What is the gradient of a V-r graph?
- The gradient of a V-r graph at any particular point is equal to the electric field strength E at that point
104
Why is the potential gradient negative?
- E = -ΔV / ΔR
- The direction of the field opposes the direction of the increasing potential, hence negative
105
What is an E-r graph and what can it be used for?
- The E-r graph is the graphical representation of E = Q / (4επr^2)
- The potential difference due to a charge can be determined from the area under the E-r graph
106
What is the area under an E-r graph between two points?
- The potential difference between two points
107
When is work done on a charge and how do you calculate it?
- When a charge moves through an electric field, work is done
- The work done in moving a charge, q, is given by ΔW = qΔV
108
What can be said about two different points from a charge?
- Two points at different distances from a charge will have different electric potentials
- This is because the electric potential increases with distance from a negative charge and decreases with distance from a positive charge
- Therefore, there will be an electric potential difference between the two point
109
What is the electric potential energy of two point charges?
- The electric potential energy of two point charges is given by Ep = Q1Q2 / 4επr
110
How is the work done and change in electric potential energy related?
- The work done on a point charge and its change in electric potential energy are equal
111
When is work done on a charge?
- Work is done when a positive charge in an electric field moves against the electric field lines or when a negative charge moves with the electric field lines
112
What is the graph of potential energy against distance?
- Can be drawn for two like charges or two opposite charges
- The gradient of the graph at any particular point is equal to the electric force F at that point
113
What are equipotential lines and surfaces?
- Equipotential lines (2D) and surfaces (3D) join together points that have the same potential
114
What are the characteristics of equipotential lines?
- Perpendicular to the electric field lines in both radial and uniform fields
- Represented by dotted lines
- An equal distance from the source charge
115
What are the equipotential lines in a radial field?
- In a radial field, such as around a point charge, the equipotential lines are concentric circles around the charge
- They become progressively further apart with distance
116
What happens if a charge moves along equipotential lines?
- No work is done as a charge moves along equipotential lines
117
Equipotential surface between two opposite charges
- An equipotential surface between twoo opposite charges can be identified by a central line at a potential of 0V
- This is the point where the opposing potentials cancel
118
Equipotential surface between two like charges
- Identified by a region of empty space between them
- This is the point where the resultant force is 0
119
Equipotential surface in a uniform field
- Horizontal straight lines
- Parallel
- Equally spaced
120
What is special about the equipotential surface in a uniform field?
- The equipotential lines are equally spaced
- This means that the potential gradient is constant, and there is a constant electric field strength
121
Why does a current-carrying conductor experience a force?
- A current-carrying conductor produces its own magnetic field
- When this interacts with an external magnetic field, it will experience a force
122
How do you calculate the force experienced by a current-carrying conductor?
- F = BILsinθ
- F = force on conductor (N)
- B = magnetic flux density of applied field (T)
- I = current in the conductor (I)
- L = length of the conductor IN the field (M)
- θ = angle between the conductor and applied magnetic field (degrees)
123
How can the force on a current-carrying conductor be increased?
- Increase the strength of the magnetic field
- Increase the current flowing through the conductor
- Increase the length of the conductor that is in the field
124
When does a current-carrying conductor experience the maximum and minimum force?
- The conductor will experience the maximum magnetic force if the current through it is perpendicular to the direction of the magnetic field (θ = 90, sin90 = 1)
- The conductor will experience NO magnetic force if the current through it is parallel to the direction of the magnetic field (θ = 0, sin0 = 0)
125
How can you observe the force on a current-carrying conductor due to a magnetic field?
- Placing a copper rod in a uniform magnetic field
- Connecting the copper rod to a circuit
- When current is passed through the copper rod, it experiences a force. This causes it to accelerate in the direction of the force
126
What is Fleming's left hand rule used for?
- To find the direction of the force, B-field and current on a current-carrying conductor if it is placed perpendicularly in a magnetic field
127
How do you use Fleming's left hand rule?
- Thumb = direction of the force
- Index finger = direction of the applied magnetic field (B-field)
- Middle finger = direction of the current
128
How is 3d force shown on a page?
- Dots: magnetic field directed out of the page
- Crosses: magnetic field directed into the page
129
Why does a moving charged particle experience a force?
- A moving charged particle produces its own magnetic field
- When interacting with an applied magnetic field, it will experience a force
130
What force does a moving charged particle experience?
- The force F on an isolated particle with charge Q moving with speed v at an angle θ to a magnetic field with flux density B is given by:
- F = BQv sinθ
131
What must you note about the current for a moving charge?
- Current is taken as the flow of POSITIVE charge (conventional current)
- This means that the direction of the current for a flow of negative charge (a beam of electrons) is in the opposite direction to its motion
132
What is the maximum force experienced by a moving charged particle?
- The maximum force on a moving charged particle occurs when it travels perpendicular to the field (θ = 90, sin90 = 1)
- Hence, F = BQv
133
What is the minimum force experienced by a moving charged particle?
- If the particle travels parallel to the magnetic field, it will experience no magnetic force
134
What is the direction of a magnetic force on a moving charged particle dependent on?
- The direction of flow of positive current
- The direction of the magnetic field
135
What is the path of a moving charged particle passing through a uniform magnetic field?
- The magnetic force acts perpendicular to the field and the particle's velocity
- Hence, the particle will follow a circular path
136
What force keeps the moving charged particle in circular orbit?
- The magnetic force F provides the centripetal force on the particle
137
What equation can we find using the centripetal force on a moving charged particle?
- mv^2 / r = BQv
- r = mv / BQ
- r = radius of the path (m)
- m = mass of the particle (kg)
- v = linear velocity of the particle (m/s)
- B = magnetic field strength (T)
- Q = charge of the particle
138
What does the equation for the radius of the orbit of a moving charged particle show us?
- Faster moving particles will move in larger circles
- Particles with greater mass will move in larger circles
- Particles with greater charge will move in smaller circles
- Particles moving in a stronger magnetic field move in smaller circles
139
What is the centripetal acceleration of a moving charged particle in a magnetic field?
- The centripetal acceleration is in the same direction as the centripetal force (the magnetic force)
- This can be found using Newton's second law
140
What is electromagnetic induction?
- The process of inducing an emf in a conductor when there is relative movement between a charge and a magnetic field
141
What is magnetic flux?
- The product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density
- The total magnetic field that passes through a given area
142
When is the magnetic flux minimum and maximum?
- It is a maximum when the magnetic field lines are perpendicular to the plane of the area
- It is 0 when the magnetic field lines are parallel to the plane of the area
143
How do you calculate magnetic flux?
- ϕ = BA
- ϕ = magnetic flux (Wb)
- B = magnetic flux density (T)
- A = cross-sectional area (m2)
144
What is electromagnetic induction?
- The process by which an emf is induced in a circuit due to changes in magnetic flux (when a conductor moves through a magnetic field)
145
What happens when a conductor moves through a magnetic field?
- The conductor will cut through the magnetic field lines
- This causes a change in magnetic flux
- This causes work to be done
- This work is transferred into electrical energy
146
When can an electromagnetic current be induced?
- A conductor cuts through a magnetic field
- The direction of a magnetic field through a coil changes
147
Where is electromagnetic induction used?
- Transformers and generators
148
What two ways can we study electromagnetic induction?
- Moving a magnet through a coil
- Moving a wire between a magnetic field (two magnets)
149
What happens when a magnet is moved through a coil?
- When the magnet is stationary, there is no change in magnetic flux. Hence, there is no induced emf
- As the bar magnet moves, it cuts through the magnetic field lines of the coil hence, inducing an emf
- When the bar magnet is moved in the opposite direction, an emf is induced in the opposite direction
150
How can we change the induced emf when moving a magnet through a coil?
- We must increase the change in magnetic flux by:
- Moving the magnet faster through the coil (rate of change increases)
- Adding more turns to the coil
- Increasing the strength of the bar magnet
151
What happens when a wire is moved through a magnetic field?
- When the wire is not moving, there is no change in magnetic flux. Hence, there is no emf induced
- As the wire is moved between the magnets, there is a change in magnetic flux as the wire cuts through the magnetic field lines of the magnets, resulting in an induced emf
- When the wire is moved back out of the magnetic field, an emf is induced in the opposite direction
152
How can we change the induced emf when moving a wire through a magnetic field?
- We must increase the change in magnetic flux by:
- Increasing the length of the wire in the field
- Moving the wire between the magnets faster
- Increasing the strength of the magnets
153
What is the magnetic flux linkage?
- The product of the magnetic flux and the number of turns in a coil
- Magnetic flux linkage = ϕN = BAN
- N = number of turns of the coil
154
How does magnetic flux linkage differ with the angle between the magnetic field lines and the line perpendicular to the plane?
- When the magnetic field lines are not perpendicular to the area A, the magnetic flux is given by ϕ = BAcosθ
- θ = angle between magnetic field lines and line perpendicular to the plane of area
- Magnetic flux linkage = ϕN = BANcosθ
155
What is Faraday's law?
- The magnitude of the induced emf is directly proportional to the rate of change in magnetic flux linkage
156
What is the equation for Faraday's law?
- ε = N ( Δϕ / Δt)
- ε = induced emf (v)
- N = number of turns in the coil
- Δϕ = change in magnetic flux (Wb)
- Δt = change in time (t)
- Magnetic flux linkage = Nϕ hence, ε = rate of change of magnetic flux linkage
157
What is Lenz's law?
- The induced emf acts in such a direction to produce effects that oppose the change causing it
158
Explain the effects of Lenz's law
- When a bar magnet goes through a coil, an emf is induced within the coil due to a change in magnetic flux
- A current is also induced, which means the coil now has its own magnetic field
- The coil's magnetic field acts in the opposite direction to the magnetic field of the bar magnet
- This means that the induced field in the coil will repel the bar magnet
159
What happens when a straight conducting rod moves through a magnetic field?
- It will have an emf induced in it
- The maximum emf is induced when the conductor moves perpendicular to the magnetic field, where it cuts through the greatest amount of magnetic flux
160
How do you calculate the emf induced in a straight conducting rod moving through a magnetic field?
- s = vΔt
- A = LvΔt
- Δϕ = BA = BLvΔt
- ε = N(Δϕ / Δt) (N = 1 in conducting rod)
- ε = BLvΔt / Δt
- ε = BLv
161
How can we increase the emf induced in a straight conducting rod moving through a magnetic field?
- Increase the length of the conductor in the magnetic field
- Increase the magnetic field strength
- Increase the speed at which the conductor cuts through the field lines
162
What happens when a coil rotates through a magnetic field?
- The flux through the coil will vary as it rotates
- The emf induced in the coil will also change as it rotates
- The maximum emf is when the coil cuts through the most magnetic field lines
163
When is the emf induced in a rotating coil max and min?
- Max emf is induced when the coil is parallel to the magnetic field lines. This is because the coil will cut through the most amount of field lines
- Min emf (emf=0) is induced when the coil is perpendicular to the field lines as it will rotate parallel to the field lines
164
How is the emf induced related to the magnetic flux in a rotating coil?
- The maximum emf is induced when the magnetic flux is at its minimum
165
What type of graph is the emf induced in a rotating coil?
- The emf induced follows a sin wave and is 90° out of phase with the flux linkage
- The emf induced is an alternating voltage
166
What is an alternating current?
- A current which periodically varies between a positive to a negative value with time
- This means that the direction of an alternating current changes every half cycle
167
What type of motion is an alternating current?
- The variation of current (or pd) with time can be described using a sine curve (sinusoidal)
- This means that the electrons in a wire carrying a.c move back and forth with simple harmonic motion
168
What is the peak current / voltage in an alternating current?
- Peak current / voltage is the maximum value of the alternating current / voltage
- It is determined by the amplitude of a current-time or voltage-time graph
169
What is the peak-to-peak voltage / current?
- The distance between a positive and a consecutive negative peak
- Amplitude x 2
170
What is rms in alternating current?
- The root mean square values of current or voltage
- They are a useful way of comparing alternating current / voltage to their equivalent direct current / voltage
171
What do the rms values represent in an alternating current?
- The rms value is the steady direct current, or voltage, that delivers the same average power in a resistor as the alternating current, or voltage
172
How do you calculate the corresponding direct current / voltage of an ac?
- Irms = I0 / √2
- Vrms = V0 / √2
- I0 / V0 = peak current / voltage
173
What is the average power of a supply (alternating current)?
- The product of the rms current and voltage
- Average power = Irms x Vrms
174
How is mains electricity supplied in the UK?
- Mains electricity is supplied as an alternating current
- This is supplied at 230V and 50 Hz (this is the corresponding rms values)
175
How is mains electricity used in appliances in the UK?
- Most devices have a step-down transformer which converts the 230V a.c into a d.c
176
What is an oscilloscope?
- A laboratory instrument used to display, measure and analyse waveforms of electrical circuits
- Be used as an a.c and d.c voltmeter
177
How is an a.c voltage shown on an oscilloscope?
- A transverse wave
- You can calculate its frequency, time period and peak voltage
178
How is a d.c voltage shown on an oscilloscope?
- A horizontal line at the relevant voltage
179
What is shown on the different axes of an oscilloscope?
- X-axis: time
- Y-axis: voltage
180
What is time-base and y-gain on an oscilloscope?
- Time-base: how many seconds each division represents
- Y-gain: how much voltage each division represents
181
How is the time-base altered on an oscilloscope?
- When the time-base is switched off, only a vertical line will be displayed with the relative amplitude (the wave is squashed horizontally)
- When the time-base is switched on, a wave will appear across the whole screen (the wave will spread out horizontally)
182
How is the y-gain altered on an oscilloscope?
- This controls the vertical deflection, or amplitude, of the wave
- The peak voltage is the maximum vertical displacement from the time axis
- When the voltage-gain is switched off, only a horizontal line on the time axis can be seen (the wave is squashed vertically)
183
What is a transformer and what is its purpose?
- A device that changes high alternating voltage at a low current to a low alternating voltage at a high current
- This is designed to reduce heat energy lost while electricity is transmitted down electrical power lines from power stations to the national grid
184
What is a transformer made up of?
- A primary coil
- A secondary coil
- A soft iron core
185
What is the purpose of the soft iron core in a transformer?
- The primary and secondary coils are wound around the soft iron core
- The soft iron core is necessary because it focuses and directs the magnetic field from the primary coil to the secondary coil
- Soft iron is used because it can easily be magnetised and demagnetised
186
How does a transformer work?
- An alternating current producing an alternating voltage is applied to the primary coil. This creates an alternating magnetic field inside the soft iron core and hence, a changing magnetic flux linkage
- The changing magnetic field passes through to the secondary coil through the soft iron core. This results in a changing magnetic flux linkage in the secondary coil and hence, an emf is induced (due to Faraday's law)
- The output alternating voltage is the same frequency as the input voltage
187
How is the output voltage in a transformer calculated?
- Ns / Np = Vs / Vp
- Ns = number of turns in the secondary coil
- Np = number of turns in the primary coil
- Vs = output voltage from the secondary coil
- Vp = input voltage in the primary coil
188
What is a step-up transformer?
- Increases the voltage and decreases the current
- Decreases the loss of electricity due to heat
- Used between power stations and transmission wires
- Number of turns in the secondary coil > number of turns in the primary coil
189
What is a step-down transformer?
- Decreases the voltage and increases the current
- Makes electricity safe to use in appliances
- Used between transmission wires and buildings
- Number of turns in the primary coil > number of turns in the secondary coil
190
What is needed for a transformer to be 100% efficient?
- There must be no power loss
- Power in = power out
- IpVp = IsVs
191
How do you calculate transformer efficiency?
- Efficiency = output power / input power
- Efficiency = IsVs / IpVp
192
What are eddy currents and how do they arise?
- They are a key source of energy loss in a transformer
- A changing magnetic field from the alternating current creates a changing magnetic field in the iron core that acts against the field that induced them
- An emf is therefore induced. As the core is made up of a conducting material, a current flows
- This current dissipates energy by generating heat
193
How can eddy currents be reduced?
- Laminating the iron core with layers of insulation
- Having a core made from a high-resistivity material
194
How is the iron core laminated in transformers?
- Building the core with thin layers of metal, instead of solid metal - the eddy currents are therefore decreased a lot
- The laminations are insulated from each other, so current doesn't flow between them
195
How do inefficiencies appear in the coils of a transformer?
- The coils of wire have resistance
- This causes heat energy to be lost from the current flowing through the coil
- The larger the current, the greater the amount of heat energy lost
196
How do inefficiencies appear in the core of a transformer?
- Induced eddy currents
- The reversal of magnetism
- Poor insulation between the primary and secondary coil
197
How can you reduce energy loss in a transformer?
- Making the core from soft iron to allow easy magnetisation and demagnetisation
- Laminating the core to reduce eddy currents
- Using thick wires, especially in the secondary coil of step-down transformers
- Using a coil that allows all the flux due to the primary coil to be linked to the secondary coil
198
Why is energy lost in transmission lines?
- Due to heating
- Electrical energy is transmitted across long distances from power stations to buildings
199
How do transformers reduce energy lost when transferring energy?
- Step-up transformers are used to increase the voltage. As P = IV, increasing the voltage decreases the current
- Smaller currents have a smaller heating effect on the wires
- This reduces energy loss to the surroundings
- This is because P = I^2 x R, hence a lower current means less power lost
Physics A-Level
flashcards
Decks in class (22)
# Cards
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Resistors in series & parallel2
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Series and parallel circuits7
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Electrical energy and power2
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Potential divider circuits4
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Electromotive force5
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Electricity58
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Scalars and vectors4
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Thermal energy transfer12
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Ideal gases17
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Mechanics61
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Rotational dynamics45
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Thermodynamics61
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Circular motion13
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Particles and radiation22
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Measurements and error16
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Fields199
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Simple harmonic motion15
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Particles and radiation157
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Waves8
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Capacitors47
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Nuclear186
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Waves158
