What is Newtons law of universal gravitation?
This states that every particle of matter in the universe attracts every other particle of matter with a force whose magnitude is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres of masses.
What is the superposition of forces?
When applying two or more forces to a rigid body, the resultant force is the vector sum of these forces.
What is a Gravitational field?
The gravitational field is a vector quantity, defined at all point in space. Its value at any particular point is given by the gravitational force per unit ‘test’ mass at that point.
What are fields in physics?
A field is the spacial distribution of a physical quantity.
What are the two types of fields that I will be looking at?
1 - Scalar Fields which are connected points that only need a value and a unit.
2 - Vector fields which are points with magnitude and direction.
What is the conservation of charge?
In an isolated system, the total electric charge is always conserved.
What are the 5 main properties of electric charge?
- The presence of a net electrical charge on a body can only be detected via electrostatic forces.
- There are only two types of charge: positive and negative. Like charges repel and unlike charges attract.
- Charge is a conserved quantity.
- All normal matter contains charge, and in certain circumstances this can be transferred from one body to another.
- Charge can flow freely through certain materials: conductors. Substances in which charge cannot freely flow are insulators.
Two propertied of equipotential surfaces and gravitational field lines
- The magnitude of the gravitational field is weakest where the gravitational equipotentials are closest together.
- The Gravitational field vectors are always perpendicular to the gravitational equipotential surfaces.
Two properties of electric field lines and equipotential surfaces.
- The magnitude of the electric field is strongest where the electric equipotentials are closest together, and weakest where they are furthest apart.
- The electric field vectors are always perpendicular to the electric equipotential surfaces.
What is Capacitance?
If there are 2 parallel plates each storing unlike charges of equal magnitude, then the ratio of charge on either plate to the potential difference between the two plates is called capacitance, C.
C = Q / V
(Capacitance is always a positive value).
What are the differences in how materials conduct a charge in different states?
- Metals contain a sea of free electrons which can move through the solid structure with very little resistance.
- Ions dissolved in a solvent can move freely in the solution, but are at a much lower concentration to free electrons and will collide with solvent molecules whilst moving. Positive ions go with the electric field whilst negative ions go against it.
- Plasma is a gas containing free electrons and positive ions. As there is no solvent molecules to collide with the charged particles are more free to move.
What is the difference between potential energy and potentials?
The potential energy, U, is a vector quantity which dictates the energy associated with the position of a mass/charge in a field.
A potential, V, is a scalar quantity that dictates the energy associated with just a position within a field.
Ve = U / q
Vg = U / m
What is electric current, I?
The rate at which electric charge passes through a plane perpendicular to the axis of the wire.
I = dQ / dt
Where dQ is the amount of charge moving through a certain point, in a time dt.
How to draw electrical current on a diagram vs which direction the electrons flow
When drawing an electrical diagram, all that you need to do is show the direction that a (hypothetical) positive charge would flow (from the positive to the negative). This is know as the direction of conventional flow.
The current in a wire is actually created by the flow of negative electrons moving in the opposite direction to the conventional flow.
What is Drift speed?
This is the average speed at which electron are flowing through a wire.
This is not accounting for the random movement of electrons which can be at speeds of much higher magnitude.
What is Kirchhoff’s voltage law?
The sum of the potential changes along any closed path in a given sense around a circuit is zero.
What is Kirchhoff’s current law?
At a junction of two or more wires, the total current flowing into the junction is equal to the current leaving the junction.
What is potential difference (voltage)?
This is described as the difference in potential between two points.
Or as the difference in electrical energy per unit charge between two points in a circuit.
What is electric current?
This can be described as the flow rate of electric charge.
What is an ampere?
It is the unit of current, I.
What are the conditions for a charge to experience a magnetic force?
For a particle to experience a magnetic force in a uniform magnetic field , it must be (a) charged, and (b) moving at an angle to the magnetic field.
Formula for The magnetic force?
Fm = q Vxb
Where q is the charge of the particle and Vxb is the cross product of the velocity and magnetic field vectors.
What are two rules for drawing magnetic field lines?
- the tangent to the field line at any point indicates the direction of the magnetic field vector at that point.
- The lines are close together where the field is strongest and further part where the field is weaker.
- Magnetic field lines always form closed loops; they never start or end.
(Bonus - magnetic field lines never intersect)
