Electric fields and Capacitors

Question 1

What is Coulomb’s Law?

Answer 1

The force between two charges is proportional to the product of the charges and inversely proportional to the square of the distance between them
F = kQ1Q2 / r^2

Question 2

If 2 charges are alike, what sort of value will Coulomb’s Law give?

Answer 2

A positive value, so the force is repulsive

Question 3

If 1 charge is positive and 1 is negative, what sort of value will Coulomb’s Law give?

Answer 3

A negative value, so the force is attractive

Question 4

What adjustment needs to be made when calculating force between 2 charges that are not in a vacuum?

Answer 4

ε = ε0 εr
Where εr is the relative permittivity of the material. It has no units
Air can be treated as a vacuum when calculating force between 2 point charges

Question 5

What is an electric field?

Answer 5

An electric field is a region of space in which a force acts on an object which carries a charge

Question 6

What is electric field strength?

Answer 6

The electric field strength at a point in a field is defined as the force per unit charge (per coulomb) on a positive charge at that point / a positive point charge

Question 7

What can field lines tell you about an electric field and how?

Answer 7

Strength of the field - the closer the field lines are, the stronger the force exerted by the field
Direction - field lines always go from positive to negative, and the direction of the arrow tells you the direction a small positive charge would move

Question 8

How are uniform fields produced?

Answer 8

They are produced by two parallel plates with a potential difference across

Question 9

What are the electric field lines like in a uniform field?

Answer 9

Parallel and equally spaced. This shows that the field strength is the same everywhere

Question 10

How can a uniform electric field be made stronger?

Answer 10

By increasing the potential difference or reducing the distance between the plates

Question 11

How would an electron move in a uniform field?

Answer 11

Parallel to the field lines, it would accelerate towards the positive plate, in the opposite direction to the field lines
Perpendicular to the field lines, it will follow a parabolic path (constant horizontal velocity, acceleration changes vertical velocity

Question 12

What direction do the field lines of a radial field go in if the charge at the centre is positive?

Answer 12

Outwards

Question 13

What direction do the field lines of a radial field go in if the charge at the centre is negative?

Answer 13

Inwards

Question 14

How does field strength change in a radial field?

Answer 14

As you move away from a point charge, the separation of the field lines increase, showing the field strength reducing by an inverse square law

Question 15

Is electric field strength a scalar or a vector quantity?

Answer 15

A vector

Question 16

If a positive charge is moved from a positive plate to a negative plate, is work being done?

Answer 16

No, it will have gained kinetic energy when it reaches the negative plate

Question 17

If a positive charge is moved from a negative plate to a positive plate, is work being done?

Answer 17

Yes

Question 18

What is electric potential?

Answer 18

The electric potential at a point in an electric field is the potential energy per unit charge of a positive charge placed at that point

Question 19

What are the units of electric potential?

Answer 19

Volts (V), or JC^-1

Question 20

What is the electric field at a point defined as?

Answer 20

The work done in bringing a unit positive charge from infinity to that point in the field

Question 21

What is the potential gradient at a point in a field?

Answer 21

It is the change of potential per unit distance at that point

Question 22

What sign does the electrical potential have if a positive charge is brought into a field due to a positive charge?

Answer 22

Work is done against the field so its potential is positive

Question 23

What sign does the electrical potential have if a negative charge is brought into a field due to a positive charge?

Answer 23

The field does work on the charge so its potential is negative

Question 24

What are equipotentials?

Answer 24

Lines joining points in an electric field with the same potential. They are always perpendicular to field lines. No work is done when moving a charge along an equipotential line. The potential difference is the same between each equipotential and the next, and they are closer together where the field is stronger

Question 25

Millikan’s oil drop

Answer 25

A closed chamber is fitted with 2 parallel metal plates, and one acquires a positive charge and one a negative charge when current is applied.
An atomizer sprays a fine mist of oil droplets into the chamber, and these are ionised so electrons will attach themselves to the oil droplets.
Electrostatic attraction between the positively charged upper plate and the electrons balances the weight of the oil drops

Question 26

What is a capacitor?

Answer 26

A device which is used to store electric charge and it can be discharged to create a momentarily large current

Question 27

What is a capacitor made of?

Answer 27

A pair of conducting plates separated by an insulator. The insulator is called a dielectric and it is often air, oil or paper.
It is often rolled up to save space

Question 28

How is a capacitor charged?

Answer 28

The battery is first connected and there is a flow of current. Electrons are drawn from one plate by the positive terminal of the battery and electrons are deposited on the other plate by the action of the negative terminal. Electrons in the first plate are repelled by the build up of electrons on the first plate, effectively completing the circuit. The second plate becomes more negative and so less attractive to electrons, and eventually the potentials on the plate become equal to the terminals of the battery respectively. The pd across the capacitor will be equal to the emf of the battery and oppositely directed to it, so no further current can flow so the capacitor is fully charged

Question 29

What is capacitance?

Answer 29

The measure of the extent to which a capacitor can store charge

Question 30

What do the charging curves for charge, voltage and current look like?

Answer 30

Charge and voltage increase to a maximum (exponential), initially steepest gradient, flattens out
Current decreases from a maximum (exponential), initially steepest gradient, flattens to an asymptote

Question 31

Explain the shapes of the charging curves (capacitors)

Answer 31

Initially the current is high, but as charge builds up, electrostatic repulsion makes it more and more difficult to get more electrons onto the plates, which is why the charge and voltage graphs flatten out with time. When the pd across the capacitor is equal to the pd across the battery, current falls to zero

Question 32

What do the discharge curves for charge, voltage and current look like?

Answer 32

All negative exponentials. Initially high, decrease quickly then flatten out.

Question 33

Explain the shapes of the discharging curves (capacitors)

Answer 33

Initially there is a large current due to the large pd across the plates. The current drops as pd drops. There is the greatest repulsion between electrons initially, so the greatest rate of flow of charge therefore a greater current.
The charge drops quickly at first (the large current means a large rate of flow of charge), and as the charge and therefore the pd across the plates drops, the charge drops more slowly

Question 34

Constant current charging of a capacitor

Answer 34

The charging current can be controlled by varying the resistance in the charging circuit as the capacitor is charging.
If resistance is gradually decreased as the capacitor charges then charging current may be kept constant until the capacitor is fully charged

Question 35

Calculating energy stored in a capacitor (using V against Q graph)

Answer 35

Total energy = 1/2 x base x height = 1/2 x Q x V
Therefore E = 1/2 C V ^2
or E = 1/2 Q^2 / C
This is always true, as this is the shaded area under the graph which will not change if a resistor is added etc
Energy lost = I^2 R T

Question 36

Discharging a capacitor through a fixed resistance - graph shape

Answer 36

It allows electrons from the negative plate to pass through the resistance and move onto the positive plates so there is a neutral charge. Initially rate of discharge is high, however as p.d. across the plates decreases, the rate of discharge falls too

Question 37

What would happen to the process of charging a capacitor if there was no resistance?

Answer 37

It would charge instantly. Resistance introduces the idea τ = RC