D: Fields

Question 1

What is Newton’s law of gravitation?

Answer 1

Newton’s law of gravitation states that the gravitational force between two point masses is directly proportional to the product of the masses and inversely proportional to the square of their separation.

Question 2

What is the relationship between gravitational force and distance?

Answer 2

The relationship between gravitational force and distance is an inverse square law.

This means that when the distance between two point masses doubles, the gravitational force between them falls by 1/4

Question 3

What is the significance of a test mass?

Answer 3

Test masses are used to define the strength of a field at a point and the direction a mass will move in the field.

This is because gravitational field strength is a vector quantity.

Question 4

Which two quantities does the strength of a gravitational field at the surface of a planet depend on?

Answer 4

The strength of a gravitational field at the surface of a planet depends on:

the radius of the planet

the mass of the planet

Question 5

Inside the planet gravitational field strength…

Answer 5

Inside the planet, the gravitational field strength decreases linearly from a maximum value (at the surface) to zero at the centre.

Question 6

State Kepler’s first law.

Answer 6

Kepler’s first law states that the orbit of a planet is an ellipse, with the Sun at one of the two foci.

Question 7

Define gravitational potential at a point.

Answer 7

The gravitational potential at a point is the work done per unit mass in bringing a small test mass from infinity to a defined point.

Question 8

State Kepler’s second law.

Answer 8

The imaginary line joining the sun to the moving planet sweeps out equal areas in equal times. This means that for eliptical orbits the speed of the planet changes (faster nearer the sun).

Question 9

State Kepler’s third law.

Answer 9

Kepler’s third law states that for planets or satellites in a circular orbit about the same central body, the square of the time period is proportional to the cube of the orbital radius.

Question 10

Gravitational potential is always…

Answer 10

negative

Question 11

At what point in a comet’s orbit does it travel fastest?

Answer 11

Comets travel fastest when they are at the closest point to the Sun in their elliptical orbit.

Question 12

What happens to gravitational potential as r approaches infinity?

Answer 12

As r approaches infinity, gravitational potential approaches zero.

Question 13

Define gravitational potential energy of a system.

Answer 13

The gravitational potential energy of a system is the work done when bringing all the masses in a system to their positions from infinity.

Question 14

Define gravitational potential difference.

Answer 14

Gravitational potential difference is the difference in gravitational potential between two points.

It is equal to the work done when a mass of 1 kg is moved between the points.

Question 15

What does the area under a force-distance graph represent?

Answer 15

The area under a force-distance graph represents the change in gravitational potential energy or the work done in moving the mass from one point to another.

Question 16

Define the potential gradient of a gravitational field.

Answer 16

Gravitational gradient is the rate of change of gravitational potential with respect to displacement in the direction of the field.

Question 17

What does the gradient of a V minus r graph represent?

Answer 17

The gradient of a potential-distance (V-r) graph represents the gravitational field strength at that point.

Question 18

What does the area under a g-r graph represent?

Answer 18

The area under a field-distance (g-r) graph represents the potential difference between the two points.

Question 19

True or False?
The curve of a field-distance ,g-r, graph is steeper than its corresponding potential-distance, V-r, graph

Answer 19

True
The curve of a g-r graph is steeper than its corresponding V-r graph.

This is because g-r follows a 1/r^2 relation whereas V-r graphs follow a -1/r relation.

Question 20

True or False?

Work is done when a mass moves along an equipotential surface.

Answer 20

False.

No work is done when a mass moves along an equipotential surface.

Question 21

What are the key features of the equipotential lines in a radial gravitational field?

Answer 21

The key features of the equipotential lines in a radial gravitational field are:

concentric circles

become further apart with distance

Question 22

What are the key features of the equipotential lines in a uniform gravitational field?

Answer 22

The key features of the equipotential lines in a uniform gravitational field are:

horizontal straight lines

parallel

equally spaced

Question 23

Define escape speed.

Answer 23

Escape speed is the minimum speed that will allow an object to escape a gravitational field with no further energy input. Escape speed is the same for all masses in the gravitational field.

Question 24

How does the mass of the planet affect escape speed?

Answer 24

The greater the mass of the planet, the greater the escape speed.
escape velocity is proportional to the square root of the mass of the planet.

Question 25

Why don't satellites being launched into an orbit around the Earth need to achieve escape speed?

Answer 25

Satellites launched from Earth's surface do not need to achieve escape velocity to reach orbit because the escape speed equation assumes the mass starts with zero kinetic energy, but the thrust from fuel provides a continuous source of energy less energy is needed to achieve orbit than to escape from Earth's gravitational field

Question 26

Define orbital speed.

Answer 26

Orbital speed is the minimum speed required for an object to maintain a circular orbit.

Question 27

True or False? As a satellite's orbital radius decreases, its kinetic energy increases.

Answer 27

True. As a satellite's orbital radius decreases, its kinetic energy increases.

Question 28

What is the effect of drag on a satellite's orbit over time?

Answer 28

The effect of drag on a satellite's orbit over time is a decrease in height and an increase in orbital speed.

Question 29

What happens to the orbital speed of a satellite in a circular orbit as its orbit becomes lower due to drag?

Answer 29

As a satellite's orbit becomes lower due to drag, its orbital speed increases. This is because some of its potential energy has been transferred to kinetic energy.

Question 30

What happens to the total energy of a satellite in a circular orbit as its orbit becomes lower due to drag?

Answer 30

As a satellite's orbit becomes lower due to drag, its total energy decreases. This is because some energy has been dissipated.

Question 31

Define 1 coulomb (C)

Answer 31

1 coulomb is the charge carried by an electric current of one ampere in one second

Question 32

Is charge scalar or vector?

Answer 32

Charge is a scalar quantity. It has magnitude only, not direction.

Question 33

How is charge quantised? (Quantised means distinct values rather than a continuous range)

Answer 33

Charge is quantised because any quantity of charge will always equal a whole number of protons or electrons, which have a charge equal to +- 1.6 x 10^-19 C

Question 34

What is the law of conservation of charge?

Answer 34

The law of conservation of charge states that the total charge in an isolated system remains constant.

Question 35

What happens when two charged spheres come into contact?

Answer 35

The charges are shared between them until they are evenly distributed. As a result, both spheres will have an equal charge.

Question 36

What does Millikan's oil drop experiment provide evidence for?

Answer 36

Millikan's oil drop experiment provides evidence for the quantisation of charge.

Question 37

What is meant by the term fundamental charge?

Answer 37

The fundamental / elementary charge is the charge of a single proton (+1.6 x 10^-19 ) or electron (-1.6 x 10^-19)

Question 38

In Millikan's experiment, why are oil drops used instead of water?

Answer 38

In Millikan's experiment, oil drops are used as they do not evaporate quickly like water. This allows the mass of the drops to remain constant throughout the experiment.

Question 39

In Millikan's experiment, how do the oil drops become charged?

Answer 39

friction (using a spray nozzle) or ionisation (using x-rays)

Question 40

In Millikan's experiment, what happens to the oil drops in the absence of an electric field?

Answer 40

In Millikan's experiment, when no electric field is applied, the oil drops fall under gravity until they reach a terminal velocity.

Question 41

What is the role of the uniform electric field in Millikan's experiment?

Answer 41

In Millikan's experiment, the uniform electric field provides an electric force which is equal and opposite to the gravitational force on an oil drop. The resultant force will then be zero and the drop will become stationary.

Question 42

What was the conclusion of Millikan's experiment?

Answer 42

The conclusion of Millikan's experiment was: the charges of all drops were found to be multiples of the same number (-1.60 × 10^−19 C) therefore, electric charge must be a quantised quantity

Question 43

What is charging by friction?

Answer 43

Charging by friction is the transfer of charge (electrons) when two insulating substances move past one another. One substance gains an excess of positive charge and the other gains an excess of negative charge.

Question 44

What is charging by contact?

Answer 44

Charging by contact is the transfer of charge (electrons) between two substances when there is physical contact between them. The excess, or deficit, of electrons, is shared between the two substances.

Question 45

What is charging by induction?

Answer 45

Charging by induction is the separation of charge caused by the influence of a nearby charged object without any physical contact. The charged object causes electrons near the surface of the uncharged substance to be either repelled or attracted.

Question 46

What does earthing do?

Answer 46

Earthing a charged body causes it to discharge until it has a potential of 0V

Question 47

How can a metal sphere become charged by induction?

Answer 47

bringing a charged rod near the sphere without touching it (this separates the charges) earthing the sphere (this allows electrons to flow to or from the sphere) removing the earth connection and the rod (this leaves an excess charge). When inducing a charge on a metal sphere, its final charge will always be opposite to the rod.

Question 48

How can a metal sphere become charged by contact?

Answer 48

A metal sphere can become charged by contact by bringing a charged rod into contact with the sphere (this allows electrons to flow to or from the sphere) removing the rod (this leaves an excess charge)

Question 49

How can sparks occur when refuelling an aircraft and how can this risk be reduced?

Answer 49

Sparks occur when there is a large build-up of charge on the aircraft and the potential difference becomes large enough for the current to travel through the air (sparks). The risk can be reduced by earthing the aircraft and fuel tank to carry excess charge away.

Question 50

What is Coulomb's law?

Answer 50

Coulomb's law states that the electric force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation.

Question 51

Is the electric force between two opposite charges positive or negative?

Answer 51

The electric force between two opposite charges is negative. A negative force means the force is attractive.

Question 52

Is the electric force between two similar charges positive or negative?

Answer 52

The electric force between two similar charges is positive. A positive force means the force is repulsive.

Question 53

True or False? Coulomb's law can be applied to any charged object regardless of shape or size.

Answer 53

False. It can only be applied to charged spheres.

Question 54

What is the permittivity of a medium?

Answer 54

The permittivity of a medium represents the medium's ability to transfer an electric field and force between charges in it.

Question 55

True or False? All materials have a higher relative permittivity than air.

Answer 55

True. All materials have a higher relative permittivity than air. The relative permittivity of air is 1 and the relative permittivity of other materials is less than 1. (relative permittivity is: permittivity of a medium/ permittivity of free space.)

Question 56

Define electric field strength at a point.

Answer 56

The electric field strength at a point is the force per unit charge experienced by a small positive test charge placed at that point.

Question 57

Define electric field strength at a point.

Answer 57

N C^-1 V m^-1

Question 58

What is the significance of a test charge?

Answer 58

Test charges are used to define the strength of a field at a point and the direction a charge will move in the field. This is because electric field strength is a vector quantity.

Question 59

How is the resultant electric field due to multiple charges determined?

Answer 59

vector addition or pythagoras

Question 60

How is a uniform electric field set up?

Answer 60

A uniform electric field can be set up between two parallel metal plates by connecting them to the terminals of a power supply.

Question 61

What do electric field lines represent?

Answer 61

The strength and direction of the electric field. The arrows go from positive to negative.

Question 62

True or false? Field lines are always perpendicular to the surface of a conducting sphere.

Answer 62

True, field lines are always perpendicular to the surface of a conducting sphere.

Question 63

What two factors does the strength of a radial electric field depend on?

Answer 63

The magnitude of the charge producing the field The distance between the charge and a point.

Question 64

True or False? Magnetic fields are produced around all charges.

Answer 64

False. Magnetic fields are produced around all MOVING charges.

Question 65

Define electric potential at a point.

Answer 65

The electric potential at a point is the work done per unit charge in taking a small positive test charge from infinity to a defined point.

Question 66

Define magnetic flux density.

Answer 66

The magnetic flux density of a field is the number of magnetic field lines passing through a region of space per unit area.

Question 67

Is electric potential scalar or vector?

Answer 67

electric potential is a scalar quantity.

Question 68

Define 1 tesla (T)

Answer 68

One tesla is the flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the field

Question 69

What is the unit of electric potential?

Answer 69

J C^-1 or V

Question 70

What is the relationship between flux density and the strength of a magnetic field?

Answer 70

The flux density is a measure of the strength of a magnetic field the higher the flux density, the stronger the magnetic field the lower the flux density, the weaker the magnetic field

Question 71

How is a uniform magnetic field set up?

Answer 71

A uniform magnetic field can be set up between two flat permanent magnets when the north pole of one magnet is parallel to the south pole of the other.

Question 72

True or False? Electric potential increases when a test charge moves closer to an isolated positive charge.

Answer 72

True. Electric potential increases when a test charge moves closer to an isolated positive charge.

Question 73

What is the right hand grip rule?

Answer 73

Used to determine the direction of a magnetic field around a current. The thumb points in the direction of the current. The fingers curl in the direction of the magnetic field.

Question 74

True or False? Electric potential decreases when a test charge moves away from an isolated negative charge.

Answer 74

False. Electric potential increases when a test charge moves away from an isolated negative charge.

Question 75

True or False? Electric potential increases in the direction of electric field lines.

Answer 75

False. Electric potential decreases in the direction of electric field lines.

Question 76

What is the electric potential inside a charged sphere?

Answer 76

A constant non-zero value.

Question 77

Define electric potential energy.

Answer 77

Electric potential energy is the work done when bringing all the charges in a system to their positions from infinity.

Question 78

What happens to the electric potential energy of two similar charges as their separation increases?

Answer 78

The electric potential energy of two similar charges decreases as the separation between them increases.

Question 79

What happens to the electric potential energy of two opposite charges as their separation increases?

Answer 79

The electric potential energy of two opposite charges increases as the separation between them increases.

Question 80

What does the area under a force-distance graph represent?

Answer 80

The area under a force-distance graph represents the change in electric potential energy or the work done in moving the charge from one point to another.

Question 81

Define electric potential difference.

Answer 81

Electric potential difference is the difference in electric potential between two points. It is equal to the work done when a charge of 1 C is moved between the points.

Question 82

Define the potential gradient of an electric field.

Answer 82

Potential gradient is the rate of change of electric potential with respect to displacement in the direction of the field.

Question 83

What does the gradient of a potential-distance graph represent?

Answer 83

The electrical field strength at that point.

Question 84

What does the area under a field-distance graph represent?

Answer 84

The potential difference between the two points.

Question 85

What is an electric equipotential surface?

Answer 85

Equipotential surfaces (or lines) connect points of equal electric potential.

Question 86

True or False? Equipotential lines are always parallel to electric field lines.

Answer 86

False. Equipotential lines are always perpendicular to electric field lines.

Question 87

True or False? No work is done as a charge moves along an equipotential line.

Answer 87

True. No work is done as a charge moves along an equipotential line.

Question 88

The key features of the equipotential lines in a radial electric field are

Answer 88

concentric circles become further apart with distance

Question 89

What are the key features of the equipotential lines in a uniform electric field?

Answer 89

The key features of the equipotential lines in a uniform electric field are: horizontal straight lines parallel equally spaced

Question 90

How can an equipotential surface be used to determine the sign of a charge?

Answer 90

Equipotential lines represent the potential gradient, so the sign of a charge can be identified by looking for positive potentials which decrease with distance represent a positive charge negative potentials which increase with distance represent a negative charge

Question 91

Name 3 ways the magnitude of the force on a current-carrying conductor can be increased.

Answer 91

Increasing the strength of the magnetic field Increasing the current flowing through the conductor Increasing the length of the conductor in the field. (F= BIL sin theta)

Question 92

True or False? A current-carrying conductor in a magnetic field will experience a maximum force when the current flows perpendicular to the field.

Answer 92

True. A current carrying conductor in a magnetic field will experience a maximum force when the current flows perpendicular to the field.

Question 93

What is the force on a current-carrying conductor in a magnetic field when the current flows parallel to the field?

Answer 93

When the current flows parallel to the magnetic field, the force on the conductor is zero.

Question 94

What is Fleming's left-hand rule?

Answer 94

Fleming's left hand rule: FBI Thumb - direction of the force First finger - direction of magnetic field (pointing north to south) Second finger - direction of current (flow of positive charge)

Question 95

What two symbols are used to represent a magnetic field directed into or out of the plane of a page?

Answer 95

Dots represent a magnetic field directed out of the plane of the page. Crosses represent a magnetic field directed into the plane of the page.

Question 96

When the currents in two parallel conductors flow in the same direction, is the force between them attractive or repulsive?

Answer 96

When the currents in two parallel conductors flow in the same direction, the force between them is attractive.

Question 97

When the currents in two parallel conductors flow in opposite directions, is the force between them attractive or repulsive?

Answer 97

When the currents in two parallel conductors flow in opposite directions, the force between them is repulsive.

Question 98

When does a charged particle experience a maximum force?

Answer 98

A charged particle experiences a maximum force when it travels perpendicular to a magnetic field.

Question 99

How can the direction of the force experienced by an electron in a magnetic field be determined?

Answer 99

The direction of the force experienced by an electron in a magnetic field can be determined using Fleming's left-hand rule. The second finger (representing the flow of positive current) points in the opposite direction to its motion.

Question 100

In what direction to the magnetic field is the force experienced by a moving charge?

Answer 100

The force experienced by a moving charge is always perpendicular to the direction of the magnetic field

Question 101

If a charge is stationary will it experience a force in a magnetic field?

Answer 101

If a charge is stationary, it does not experience a force in a magnetic field.

Question 102

Describe the motion of a charged particle in a magnetic field.

Answer 102

A charged particle in a magnetic field moves in a circular path. This is because the magnetic force is always perpendicular to motion.

Question 103

How would the path of a charged particle moving in a magnetic field change if the magnetic field strength is increased?

Answer 103

The path of the charge particle would become more curved, the radius decreases.

Question 104

How would the path of a charged particle moving in a magnetic field change if it travels at a greater speed?

Answer 104

The path becomes less curved ie radius increases.

Question 105

Describe the motion of a stationary charged particle in a uniform electric field.

Answer 105

A stationary charged particle in a uniform electric field will move parallel to the field lines.

Question 106

Describe the motion of a charged particle in a perpendicularly orientated uniform electric field.

Answer 106

A charged particle moving perpendicular to a uniform electric field will follow a parabolic path.

Question 107

What three factors affect the amount of deflection of a charged particle in a uniform electric field?

Answer 107

The mass of the particle (greater mass, smaller deflection) The magnitude of the charge of the particle (more charged, greater deflection) The speed of the particle (greater speed, smaller deflection)

Question 108

A charged particle will continue to move at a constant speed in a straight line if...

Answer 108

the electric and magnetic forces acting on it are equal and opposite.

Question 109

What is the speed, v, of a charged particle moving in perpendicularly orientated uniform electric and magnetic fields?

Answer 109

electric field strength / magnetic flux density = v

Question 110

What is the charge-to-mass ratio of a particle?

Answer 110

The charge-to-mass ratio of a particle is the total charge of a particle divided by its total mass.

Question 111

What four quantities are required to determine a particle's charge-to-mass ratio experimentally?

Answer 111

To determine a particle's charge-to-mass ratio experimentally, the four quantities required are: the potential difference between the parallel plates, V the separation of the parallel plates, d the magnetic field strength, B the radius of the particle's path in the magnetic field, r (when the electric field is switched off)