Physics 5 - Electricity

Question 1

What is electric current?

Answer 1

The rate of flow of charge. It is measured in amperes (amps).

Question 2

What is potential difference?

Answer 2

The work done moving a unit charge between 2 points in a circuit.
V=W/Q
Where:
V=Potential difference (voltage) (V)
W=Work done (J)
Q=Charge (C)

Question 3

What is resistance?

Answer 3

How difficult is it for current to flow through an appliance.
A component has a resistance of 1Ω if 1A flows through it when a potential of 1V is applied across it.
R=V/I
Where:
R=Resistance (Ω)
V=Potential difference (V)
I=Current (A)

Question 4

What is meant by an ohmic conductor?

Answer 4

A conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remains constant.

Question 5

How can you measure the current in a circuit?

Answer 5

You can measure the current in a circuit with an ammeter connected in series with a component.

Question 6

How do you measure potential difference across a component?

Answer 6

Using a voltmeter, connected in parallel across the component being measured.

Question 7

What does the gradient of a current-potential difference graph represent?

Answer 7

1/R
Because the gradient=change in current/change in potential difference, gradient=I/V.
Since R=V/I, the gradient=1/R

Question 8

Describe the current-voltage graph of an ohmic conductor.

Answer 8

A linear graph, straight line through the origin, which shows that voltage is directly proportional to the current.

Question 9

Describe the current-voltage graph of a filament bulb.

Answer 9

As the current increases, the resistance also increases, so the gradient will decrease as the current increases.
Graph curves towards the parallel to the x-axis the further away from the origin it is.

Question 10

Why does the current increasing on a filament lamp cause an increase in the resistance?

Answer 10

As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions so resistance has increased.

Question 11

What is a diode?

Answer 11

A diode is an appliance that only allows current to flow in one direction.

Question 12

Unless otherwise stated, in a question what assumptions must be made about voltmeters?

Answer 12

They should be assumed to have infinite resistance. Current takes the bath of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through the appliance.

Question 13

Why should you assume that an ammeter has zero resistance unless stated otherwise?

Answer 13

This assumption means that there would be 0 potential difference across the ammeter and no energy lost across it; it doesn’t affect the circuit.

Question 14

What is a light dependent resistor?

Answer 14

A semiconductor that is sensitive to light.

As the light intensity increases, its resistance decreases.

Question 15

What is a thermistor?

Answer 15

A semiconductor the is sensitive to heat.
As the temperature increases, the resistance decreases (so this means that thermistors have a negative temperature coefficient).

Question 16

What is resistivity?

Answer 16

The resistance of a 1m cylinder with a cross sectional area of 1m². Resistivity is an inherent property of a material.
ρ=RA/L
Where:
ρ=Resistivity (Ωm)
R=Resistance (Ω)
A=Cross-sectional area (m²)
L=Length (m)

Question 17

Describe an experiment to determine the resistivity of a metal.

Answer 17

Measure the diameter of a wire with a micrometre, then calculate the cross sectional area A=π(d/2)².
Set up a circuit with the wire on a ruler, with a voltmeter in parallel and an ammeter at the end in series.
Vary the wire length and record the voltage and current for each length.
Use R=V/I to work out the resistance.
Plot a graph of resistance against wire length.
The gradient=resistivity/cross-sectional area
So resistivity=gradient×cross-sectional area.

Question 18

What is a superconductor?

Answer 18

A material that has a resistivity of zero at or below a critical temperature. This critical temperature is an inherent property of the material.

Question 19

How do you find the total resistance in a series circuit?

Answer 19

Rₜₒₜₐₗ=R₁+R₂+R₃+…

Add the individual resistances of each component.

Question 20

What would the voltage of a circuit be where cells of equal voltage are arranged in series?

Answer 20

Vₜₒₜₐₗ=V₁+V₂+V₃+…

Question 21

What would the voltage of a circuit be where cells of equal voltage are arranged in parallel?

Answer 21

Vₜₒₜₐₗ=V꜀ₑₗₗ

Question 22

How does the current vary between each component of a series circuit?

Answer 22

It doesn’t, the current through all the components is the same.

Question 23

Is the current the same in components in parallel?

Answer 23

It can but it often isn’t, each branch of a parallel circuit can have different currents through them according to Kirchhoff’s first law.

Question 24

What is Kirchhoff’s first law?

Answer 24

All of the current going into a junction is equal to the current leaving the junction.

Question 25

What is Kirchhoff’s second law?

Answer 25

For any path (loop) of a circuit, the sum of all of the potential differences must be equal to the total electromotive force of the circuit.

Question 26

In a series circuit, if two cells are connected negative to negative, would their emf add up or cancel out?

Answer 26

Thy will cancel out so the total emf would be equal to

εₜₒₜₐₗ=ε₁-ε₂

Question 27

What is power?

Answer 27

The rate of energy transfer.

Question 28

Give an equation for power in terms of current and voltage.

Answer 28

P=IV
Where:
P=Power (W)
I=Current (A)
V=Voltage (V)

Question 29

What is the purpose of a potential divider?

Answer 29

To provide variable potential difference or to provide a constant specific potential difference.

Question 30

What is electromotive force?

Answer 30

Electromotive force (emf) is the electrical energy transferred by a power supply per unit charge.

Question 31

How can the equation ε=I(R+r) be shown on a graph?

Answer 31

Convert it to the form y=mx+c;
ε=IR+Ir
ε=V+Ir
So for a graph of V against I, V=-rI+ε
So it forms a graph with gradient of -r and a y-intercept of ε.

Question 32

What are two applications of superconductors?

Answer 32

Power cables, which would reduce energy loss through heating to zero during transmission.
Strong magnetic fields, which wouldn’t require a constant power source. These could be used in maglev trains, where there would be no friction between the train and rail, and in certain medical applications like MRI machines.

Question 33

How do you find the total resistance in a parallel circuit?

Answer 33

1/Rₜₒₜₐₗ=(1/R₁)+(1/R₂)+(1/R₃)+…

Add the reciprocal of the total resistances of each branch of the parallel