Chapter 5: Electricity

Question 1

What is electric current? State its units.

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

Question 3

What is resistance?

Answer 3

How difficult it is for current to flow through an appliance.
A component has a resistance of 1Ω if 1A flows through it when p.d. of 1V is applied across it.
R=V/I

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) remain constant.

Question 5

How can you measure the current in a circuit?

Answer 5

You can measure the current in an ammeter connected in series with the 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

Rate of charge of current with respect to voltage.
This is not the same as 1/R

Question 8

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

Answer 8

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

Question 9

What is a diode?

Answer 9

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

Question 10

What is a Light Dependent Resistor (LDR)?

Answer 10

A semiconductor that is sensitive to light.
As the light intensity increases, its resistance decreases.

Question 11

How does a thermistor work?

Answer 11

Similar to an LDR but, as the temperature increases, the resistance decreases (This means thermistors have a negative temperature coefficient).

Question 12

What is resistivity?

Answer 12

The resistance of a 1m cylinder with a cross sectional area of 1m^2. Resistivity is an inherent property of a material.
ρ=RA/L

Question 13

Describe an experiment to determine the resistivity of a metal.

Answer 13

1. Measure the diameter of the wire with a micrometer. Then calculate the cross-sectional area
2. Set up the circuit as shown.
3. Vary the wire length and record the voltage and current for each length.
4. Use the R=V/I to work out the resistance
5. Plot a graph of resistance against wire length
6. The gradient = resistivity / cross-sectional area
7. So resistivity = gradient x cross-sectional area

Question 14

What is a superconductor?

Answer 14

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

Question 15

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

Answer 15

Rtotal = R1 + R2 + R3 + …

Question 16

If 6 cells, each of voltage 5V, are arranged in parallel what is the voltage in the circuit?

Answer 16

5V

Question 17

If 6 cells, each of 5V, are arranged in series what voltage is provided to the circuit?

Answer 17

30V

Question 18

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

Answer 18

The current through all of the component is the same so the current does not vary.

Question 19

Is the current in parallel components the same?

Answer 19

No, each branch of a parallel circuit can have different currents through them according to Kirchhoff’s first law.

Question 20

What is Kirchhoff’s first law?

Answer 20

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

Question 21

What is Kirchhoff’s second law?

Answer 21

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

Question 22

What is power?

Answer 22

The rate of energy transfer.

Question 23

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

Answer 23

P=IV

Question 24

What is the purpose of a potential divider?

Answer 24

• to provide variable p.d., or
• to provide a constant specific potential difference

Question 25

A circuit is set up with a cell providing a voltage of 12V to 2 resistors of 6 Ω and 7Ω respectively, in series. What is the voltage across the 7Ω resistor?

Answer 25

Total resistance = 6+7 = 13Ω
(7/13) x 12 = 6.5V

Question 26

What is emf?

Answer 26

Electromotive force: the electrical energy transferred by a power supply per unit charge.

Question 27

Rearrange the equation ε = I(R+r) into the form y=mx+c

Answer 27

ε = IR + Ir
ε = V + Ir
So for a graph of V against I
V = -rI + ε
Gradients = -r and y-intercept = ε

Question 28

What are two applications of superconductors?

Answer 28

• power cables which would reduce energy loss through heating to zero during transmission.
• strong magnetic fields, which would not 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.

Question 29

What is the total resistance of a parallel arrangement of 3Ω, 7Ω and 9Ω resistors?

Answer 29

1/Rt = 1/3 + 1/7 + 1/9
1/Rt = 37/63
Rt = 63/37 = 1.7Ω