Electricity

Question 1

Explain the graph of filament light when it is plotted I on the y axis and V on the x axis

Answer 1

as voltage increases, current increases (1)
current heats filament (1)
therefore resistance increases (1)
correct argument to explain curvature(1)
mirror image in negative quadrant (1)

Question 2

What property of the voltmeter allows us to assume that no current flows through it?

Answer 2

very large or infinite resistance

Question 3

Explain why a current flowing through resistors in parallel is higher than a current flowing with resistors in series

Answer 3

parallel resistance gives lower equivalent resistance (1)
total resistance in circuit reduced (1)
current through battery increases since V constant (1)

Question 4

Explain the graph of I/V in a filament bulb

Answer 4

as voltage increases, current increases (1)
current heats filament (1)
therefore resistance increases (1)
correct argument to explain curvature(1)
mirror image in negative quadrant (1)

Question 5

State what is meant by the emf of a battery.

Answer 5

electrical energy produced per unit charge (1)

Question 6

Describe the method of getting the resistivity of a wire and the equipment needed

Answer 6

equip:
circuit with ammeter and voltmeter correct or ohmmeter 
some means of heating eg water bath  
thermometer in water bath
method:
- resistance measured calculated
• water bath used
• ice added to water
• water stirred
• temperature measured with thermometer
• resistance calculated
• graph drawn

Question 7

Define resistivity

Answer 7

resistivity is defined as
ρ =RA/L
where R is the resistance of the material of length l
and cross-sectional area A

Question 8

Explain the shape of the graph and why the filament is more likely to fail when being switched on rather than at other times

Answer 8

1. Initially the temperature is low so current is high
2. Resistance of filament increases as temperature increases
3. Current falls to steady value when temperature is constant
4. Maximum heating is when lamp is switched on
   5 filament breaks due to melting caused by temperature rise

Question 9

State and explain why it would be an advantage for rechargeable batteries to have a low internal resistance

Answer 9

1. Internal resistance limits current

2. Less energy wasted with lower internal resistance

Question 10

What is the definition for internal resistance

Answer 10

The loss of potential difference per unit current in the source when current passes through the source

Question 11

When can maximum power be delivered to a load?

Answer 11

When the resistance of the load is equal to the internal resistance of the source

Question 12

What is the definition for internal resistance

Answer 12

The loss of potential difference per unit current in the source when current passes through the source

Question 13

When can maximum power be delivered to a load?

Answer 13

When the resistance of the load is equal to the internal resistance of the source

Question 14

What three things can a potential divider be used for?

Answer 14

1. To supply a pd which is fixed at any value between zero and the source pd
2. To supply a variable pd
3. To supply a pd that var`ies with a physical condition such as temp or pressure

Question 15

The light intensity in the room is increased. State and explain what happens to the resistance of
the LDR and the reading on the voltmeter.

Answer 15

1. resistance drops
2. voltmeter reading decreases
3. because the number of charge carriers increases

Question 16

What is the definition for ohms law

Answer 16

the pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change

Question 17

What is the definition of electron volt

Answer 17

The work done when an electron moves through a pd of 1V
Work done = qV
1eV = 1.6x10-19 C x 1V = 1.6x10-19

Question 18

A switch is in a circuit. What is the voltmeter reading across the cell when the switch is open equal to?

Answer 18

The emf of the cell