Topic 5: Electric potential difference, current and resistance

Question 1

Define electric potential difference.

Answer 1

The energy that a charge has as a result of its position in an electric field.

Question 2

What happens when a charge is placed in an electric field?

Answer 2

The charge feels a force.

Question 3

What happens when a charge moves around in an electric field?

Answer 3

Work is done and the charge will either gain or lose electric potential energy.

Question 4

Determine the change in potential energy when a charge moves between two points at different potentials.

Answer 4

1. When a charge moves between two points at different potentials, there is an increase in electric potential energy
2. If the electric potential energy at B is greater than the electric potential at A, work has to be done to push the charge from A to B
3. However, the charge at B would be pushed by the electric field to A
4. This push would accelerate its movement so that the loss in electrical potential energy would be the same as the gain in kinetic energy

Question 5

What is the equation used to determine the change in potential energy when a charge moves between two points at different potentials that involves:

1. electric potential energy change
2. force
3. distance?

Answer 5

electric potential energy change = F d

Where:

electric potential energy change in J

F is force in N

d is distance in m

Question 6

What is the equation used to determine the change in potential energy when a charge moves between two points at different potentials that involves:

1. electric potential energy change
2. energy of the charge
3. charge
4. distance?

Answer 6

electric potential energy change = E q d

Where:

electric potential energy change in J

E is difference in potential energy in J

q is charge in C

d is distance in m

Question 7

Derive an equation for the velocity of a charge

Answer 7

1. kinetic energy gained = potential energy lost
2. 1/2 m v^2 = E q d
3. m v^2 = 2 E q d
4. v^2 = (2 E q d) / m
5. v = sqrt ( (2 E q d) / (m) )

Where:

m is the mass of the charge in kg

v is the velocity of the charge in m s^-1

E is difference in potential energy in J

q is the charge in C

d is the distance in m

Question 8

Define the electronvolt.

Answer 8

The energy that would be gained by an electron moving through a potential difference of 1 volt.

Question 9

What is the charge of an electron?

Answer 9

1.6 X 10^-19 C

Question 10

Derive an equation for the charge of an electronvolt.

Answer 10

energy gained = potential difference X charge

1 electronvolt = 1 volt X 1.6 X 10^-19 C

1 electronvolt = 1.6 X 10^-19 C

Question 11

What does the actual energy difference between two points, A and B, depend on?

Answer 11

1. The charge that is moved
2. If the charge is doubled, then the energy distance would also double
3. The quantity that remains fixed between A and B is the energy difference per unit charge
4. This is called the potential energy difference between the points

Question 12

What is the equation used to determine the potential difference between two points that involves:

1. energy difference
2. charge?

Answer 12

V = E / q

Where:

V is potential difference between two points in V

E is energy difference in J

q is charge in C

Question 13

Define electric current.

Answer 13

The force per unit length between parallel current-carrying conductors. It is the rate of flow of electrical charge.

Question 14

What is the equation used to determine current that involves:

1. charge
2. time?

Answer 14

I = q / t

Where:

I is current in A

q is charge flowed in C

t is time taken in s

Question 15

When is there a direct current?

Answer 15

If a current flows in one direction.

Question 16

When is there an alternating current?

Answer 16

If a current constantly changes direction.

Question 17

When does a current occur?

Answer 17

When charges move.

Question 18

What is a circuit?

Answer 18

The path that currents follow.

Question 19

Why does a current move across a body?

Answer 19

As a result of a potential difference across a body.

Question 20

What creates a potential difference?

Answer 20

A power supply.

Question 21

Why does work occur when charges move?

Answer 21

As a result of interactions between the conduction electrons and the lattice ions.

Question 22

Define drift velocity.

Answer 22

The speed of the electrons due to the current.

Question 23

Define resistance.

Answer 23

The mathematical ratio between potential difference and current.

Question 24

What is the equation that defines resistance?

Answer 24

R = V / I

Where:

R is resistance in Ω

V is potential difference in V

I is current in A

Question 25

What must happen for current to flow if something has a high resistance?

Answer 25

If something has a high resistance, it means that a large potential difference across it is needed in order for current to flow.

Question 26

Define resistor.

Answer 26

A device with constant resistance; an ohmic device.

Question 27

What is the equation used to determine resistance that involves:

1. resistivity
2. length
3. cross-sectional area?

Answer 27

Where:

R is resistance in Ω

ρ is resistivity in Ω m

L is length in m

A is cross-sectional area in m s^-2

Question 28

State Ohm’s Law.

Answer 28

The current flowing through a piece of metal is proportional to the potential difference across it, providing that the temperature remains constant.

V ∝ R (providing temperature is constant)

Question 29

When is a device said to be ohmic?

Answer 29

If current and potential difference are proportional.

Question 30

What is an example of an ohmic device?

Answer 30

Metal at constant temperature.

Question 31

When is a device said to be non-ohmic?

Answer 31

If current and potential difference are not proportional.

Question 32

What is an example of a non-ohmic device?

Answer 32

1. Filament lamp
2. Diode

Question 33

Draw the I-V characteristics of an ohmic resistor.

Question 34

Draw the I-V characteristics of a non-ohmic resistor.

Question 35

Derive an expression for electrical power dissipation in resistors.

Answer 35

1. potential difference = (energy difference) / (charge flowed)
2. current = (charge flowed) / (time taken)
3. P = VI
4. power dissipation = ( (energy difference) / (charge flowed) ) X ( (charge flowed) / (time taken) )
5. P = (energy difference) / (time taken)

Question 36

Define electromotive force (emf).

Answer 36

The total energy difference per unit charge around a circuit. It is not a force measured in Newtons, but an energy difference per charge, measured in volts. It is the same as potential difference if no current flows.

Question 37

Describe the concept of internal resistance.

Answer 37

Energy is used up inside the battery itself as a result of the battery’s internal resistance.

Question 38

Apply the equations for resistors in series.

Question 39

Apply the equations for resistors in parallel.

Question 40

What happens to current and potential difference in a series circuit?

Answer 40

Current is constant

Potential difference is split up

Question 41

What happens to current and potential difference in a parallel circuit?

Answer 41

Current is split up

Potential difference is constant

Question 42

Describe the use of ideal ammeters.

Answer 42

An ideal ammeter has zero resistance.

Question 43

Describe the use of ideal voltmeters.

Answer 43

An ideal voltmeter has infinite resistance.

Question 44

Describe a potential divider.

Answer 44

Potential dividers divide up the potential difference within a circuit, so that parts of a circuit only receive the potential difference that they require. Potential dividers usually consist of two or more resistors arranged in series across a power supply.

Question 45

Explain the use of light dependent resistors in potential divider circuits.

Answer 45

A device whose resistance depends on the amount of light shining on its surface. An increase in light causes a decrease in resistance.

Question 46

Explain the use of thermistors in potential divider circuits.

Answer 46

A resistor whose value of resistance depends on its temperature. Most are semi-conducting devices that have a negative temperature coefficient (NTC). This means that an increase in temperature causes a decrease in resistance.

Question 47

Explain the use of strain gauges in potential divider circuits.

Answer 47

A sensor whose output potential difference depends on any a small extension or compression that occurs, which results in a change of length.

Question 48

What are sensors used for?

Answer 48

These devices can be used in potential divider circuits to create sensor circuits. The output potential difference of a sensor circuit depends on an external factor.