7.1 - 7.4. Fields, Gravitational Fields, Electric Fields and Capacitance: Physics A Level AQA

What are some differences between electrostatic and gravitational forces?

The gravitational forces from masses always attract, whilst charges may repel or attract.

Is force a vector or scalar quantity?

A vector.

The force felt in a force field is a non-contact force. True or False?

True.

What are some similarities between electrostatic and gravitational forces?

● Inverse square force laws

● Potential concept

● Equipotential surfaces

● Use of field lines

What is the work done by moving a mass in a field?

Mass x change in potential

What is the period of a geosynchronous orbit?

Geosynchronous orbits have a period of one day.

What is an equipotential surface?

A surface in which every point on the surface has the same potential.

Why is gravitational potential a negative value?

Work needs to be done to move an object from the inside the field to outside the field. Since outside the field’s potential is defined as 0 then the potential inside the field must be negative.

What is gravity?

Gravity is the universal attractive force which acts between all matter.

What is G?

The universal gravitational constant.

Approximately 6.67 x 10¯¹¹ m³ • kg¯¹ • s¯²

Compare the PE and KE of a lower orbit to a higher one.

A lower orbit (smaller m) has less potential energy and more kinetic energy than a higher orbit (bigger r).

What can field lines tell you about a field?

The direction of the field and the strength of the field depending on the density of the field lines.

What is 𝘨?

𝘨 is the force per unit area in a uniform field. In a radial field the magnitude of 𝘨 is the the proportionality constant at that point between force and mass.

Iet 𝘨 = G•M/r²

What is gravitational potential?

The potential energy per kilogram, at any point in the field. 0 potential is defined at infinity, hence at a point close to a mass the potential of an object would be negative.

How much work is done when you move 1 km in any direction on an equipotential?

No work is done when moving across equipotentials, as the potential at each point is the same.

How is the orbital period related to the radius of a circular orbit?

T² ∝ R³

What is the gravitational potential difference?

Gravitational potential difference is the difference in the gravitational potentials of two points in a gravitational field.

What equations could one use to find the speed of an orbiting satellite?

The orbiting object (mass m) is in circular motion, so we would use F = ma with F = GMm/r^2 rearranged to a = v^2 / r = ω^2/r. This can be sold to find the speed (v), angular speed (ω), the radius of the orbit or using T = 2π/ω its period.

What symbol represents the permittivity

of free space?

𝜀₀

How is electric potential related to electric field strength?

E = ΔV/Δr

The change in electric potential with respects to the change in radius length.

For a charged sphere the charge can be assumed to be at what part of the sphere?

The centre.

Electric field lines always go from _____ to _____.

Electric field lines always go from positive charge to negative charge.

What is the magnitude of E (electric field strength) in an uniform electric field?

Potential difference between plates (V)/distance between plates.

When calculating the force between two particles, what can air can be treated as?

A vacuum.

What is the trajectory of a particle entering a uniform field at right angles?

It is parabolic.

Which is stronger? The gravitational force of subatomic particles or the electrostatic force.

The electrostatic force.

What is electric field strength?

The force per unit charge acting at a point in an electric field.

What does the area under the graph of charge against pd represent ?

The energy stored by the capacitor.

How is capacitance calculated?

C = Q \ V

where:

C is Capacitance (F)

Q is Charge in the plates (C )

V is potential difference across the plates (V)

What is the half time of a capacitor?

T½ = 0.69RC

What is the relative permittivity (a.k.a. dielectric constant)?

● The ratio of the charge stored with the dielectric between the plates to the charge stored when the dielectric is not present.

● ε_r = Q / Q_0

● The greater the relative permittivity, the greater the capacitance of the capacitor.

How does a capacitor charge up?

1. Electrons move from negative to positive around the circuit.

2. The electrons are deposited on plate A, making it negatively charged.

3. Electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge.

4. Electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a potential difference across the plates.

5. When the p.d across plates = source p.d., the capacitor is fully charged and current stops flowing.

State the 3 expressions for the energy stored by a capacitor.

E = ½ (Q^2 /C) = ½ (QV) = ½ (CV^2 )

What 2 factors affect the time taken for a capacitor to charge or discharge?

● The capacitance of the capacitor, C. This affects the amount of charge that can be stored by the capacitors at any given potential difference across it.

● The resistance of the circuit, R. This affects the current in the circuit and how quickly it flows, hence how quickly the capacitor charges/discharges.

Describe and explain in terms of the movement of electrons how the p.d across a capacitor changes, when it discharges across a resistor.

1. Electrons move in opposite direction than when the capacitor was charging up.

2. Charge on one plate A decreases as it loses electrons, and plate B gains electrons, neutralising them.

3. P.d. decreases exponentially across the plates.