Chapter 9 - Current and Voltage in Circuits

Question 1

State the equation linking voltage, current and resistance (1)

Answer 1

voltage = current x resistance

Question 2

Calculate the voltage across the 4.0 resister

Answer 2

1.2 x 4.0 = 4.8V

Question 3

Show that the voltage across the heater coil is about 7V

Answer 3

12 - 4.8 = 7.2V

Question 4

what is the formula linking energy transferred, voltage, time, current

Answer 4

energy transferred = voltage x time x current

Question 5

At first, the temperature of the water increases.

After a while, the temperature reaches a steady value below the boiling point of water

Explain why the temperature reaches a steady value (2)

Answer 5

Rate of energy losses = rate of energy supply (at steady temp)

Question 6

Answer 6

X - Series

Y - Parallel

Question 7

Describe the advantages and disadvantages of design X when used as a heater in a car window (3)

Answer 7

• It is a series circuit
• Series circuits have fewer wires and lower resistance
• In series, if one fails, the circuit fails, no independent control

Question 8

Time - 5 minutes

Voltage - 7.2 V

Current - 1.2 A

Work out the energy transferred (3)

Answer 8

(5 x 60) = 300

300 x 1.2 x 7.2 = 2592

= 2600J

Question 9

At first, the temperature of water increases

After a while, the temperature reaches a steady value below the boiling point of water.

Explain why the temperature reaches a steady value (2 marks)

Answer 9

• Energy losses occur
• Rate of energy loss = Rate of energy supply (at steady temp)

Question 10

Advantages and disadvantages of using a series circuit when used as a heater in a car window (3 marks)

Answer 10

• Advantages : Fewer wires, lower resistance
• Disadvantages : One fault causes entire circuit to fail, no independent control

Question 11

Describe the method a student should use to investigate how the resistance of a wire depends on its length [5 marks]

Answer 11

• Connect the circuit / connect (crocodile) clip to wire
• Read ammeter; Read voltmeter

For known length; measure length with a ruler

• Repeat readings / average
• Take readings for different lengths
• Check meters for zero errors
• Switch off between readings to avoid heating the wire;

Question 12

Give two reasons why the lamps are wired in parallel (2)

Answer 12

• so that lamps work independently
• so that they all get mains/same voltage/230V;
• so that different areas/rooms can have different brightness/power/light intensities of lamps;

Question 13

What is the current at P? (1)

Answer 13

1.38 A