Series and parallel circuits

Figure 8 shows a battery-powered hand warmer that contains a heating pad.

The battery transfers energy to increase the temperature of the heating pad.

The heating pad has a mass of 0.20 kg.

When the hand warmer was switched on, the energy transferred to the heating pad was 8000 J.

specific heat capacity of heating pad = 1600 J/kg °C

Calculate the temperature increase of the heating pad.

Use the Physics Equations Sheet.

The hand warmer was powered by a 5.0 V battery.

The battery transferred 180 kJ of energy to the heating pad.

Calculate the charge flow through the battery.

Use the Physics Equations Sheet.

The circuit inside the hand warmer can be connected to a mobile phone to recharge the mobile phone.

Figure 9 shows the circuit diagram when the hand warmer is connected to a mobile phone.

The heating pad of the hand warmer is represented by the resistor symbol.

Explain how closing both switches S₁ and S₂ affects the power output of the battery compared with only closing switch S₁.

A hair dryer contains three heating elements.

Figure 7 shows the circuit diagram for the heating elements in the hair dryer.

In Figure 7 the heating elements are represented by resistor symbols.

Complete the sentence.

Which switch must always be closed for the hair dryer to work?

Tick (✓) one box.

Which switches must be closed for the hair dryer to work at maximum power output?

Tick (✓) one box.

Use the Physics Equations Sheet to answer questions 06.4 and 06.5.

Write down the equation which links energy transferred (E), power (P) and time (t).

The heating elements have a maximum power output of 1200 W.

The energy transferred to the heating elements to reach normal operating temperature is 3600 J.

Calculate the time taken for the heating elements to reach normal operating temperature at maximum power output.

The hair dryer has LEDs to indicate the power setting.

What is the circuit symbol for an LED?

Tick (✓) one box.

Figure 12 shows some hair straighteners. Hair straighteners contain heating elements.

When the hair straighteners reach normal operating temperature, an LED turns on. Draw the circuit symbol for an LED in the box.

Figure 13 shows the circuit diagram for the hair straighteners. Each resistor represents a heating element. The power output of the hair straighteners can be changed by closing different switches.

The hair straighteners have a maximum power output of 120 W. The energy transferred to the hair straighteners to reach normal operating temperature is 3.6 kJ. Calculate the time taken for the hair straighteners to reach normal operating temperature when operating at maximum power. Use the Physics Equations Sheet.

Switch S₂ and switch S₃ are then closed at the same time. Explain what happens to the power output of the power supply.

A student investigated how the current in a circuit varied with the number of lamps connected in parallel in the circuit.

Figure 4 shows the circuit with three identical lamps connected in parallel.

Figure 5 shows the results.

Complete the sentences.

Choose answers from the box.

Each answer can be used once, more than once or not at all.

decreased stayed the same increased

As the number of lamps increased, the current

As the number of lamps increased, the total resistance of the

circuit.

As the number of lamps increased, the potential difference across the

battery.

When there were three lamps in the circuit the ammeter reading kept changing between 0.35 A and 0.36 A.

What type of error would this lead to?

Tick (✓) one box.

Figure 6 shows a circuit with five ammeters and three identical lamps.

Complete Table 2 to show the readings on ammeters A₂ and A₅.

The resistance of one lamp is 15 Ω.

The current in the lamp is 0.12 A.

Calculate the power output of the lamp.

Use the equation: power = current² × resistance

Figure 13 shows some overhead power cables in the National Grid.

Explain the advantage of transmitting electricity at a very high potential difference.

It is dangerous for a person to fly a kite near an overhead power cable. Figure 14 shows a person flying a kite.

The person could receive a fatal electric shock if the kite was very close to, but not touching the power cable. Explain why.

A scientist investigated how the potential difference needed for air to conduct charge varies with the distance between a cable and earth. Figure 15 shows the results.

The data in Figure 15 gives the relationship between potential difference and distance when the air is dry. When the humidity of air increases the air becomes a better conductor of electricity. Draw a line on Figure 15 to show how the potential difference changes with distance if the humidity of the air increases.

Figure 16 shows a cross-section through a power cable.

A 1 metre length of a single aluminium wire is a better conductor than a 1 metre length of the steel wire. The individual wires behave as if they are resistors connected in parallel. Explain why the current in the steel wire is different to the current in a single aluminium wire.

A student investigated how the total resistance of identical resistors connected in series varied with the number of resistors. The student used an ohmmeter to measure the total resistance of the resistors. Figure 6 shows the student’s circuit with 3 resistors.

The student repeated each reading of resistance three times. Table 1 shows the student’s results for 3 resistors in series.

Calculate the mean resistance of 1 resistor.

What was the resolution of the ohmmeter the student used? Tick (✓) one box.

How do the results show that the student’s measurements were precise? Tick (✓) one box.

Figure 7 shows the results.

How do the results show that the total resistance is directly proportional to the number of resistors? Tick (✓) one box.

Predict the mean total resistance of 7 resistors. Use Figure 7.

Some resistors are connected in series with a battery. When more resistors are added in series, the total resistance increases. Complete the sentences. Choose answers from the box. Each answer may be used once, more than once or not at all.

When the number of resistors increases, the potential difference across each

When the number of resistors increases, the current in the

A student investigated how the total resistance of identical resistors connected in parallel varied with the number of resistors. The student used an ohmmeter to measure the total resistance of the resistors. Figure 11 shows the student’s circuit with 3 resistors.

The student repeated each reading of resistance three times. Table 1 shows some of the results for 3 resistors in parallel.

Calculate value X in Table 1.

The student thought that taking a fourth reading would improve the precision of the results. The fourth reading was 16.2 Ω. Explain why the student was wrong.

Figure 12 shows the results from the investigation.

The student concluded that the number of resistors in parallel was inversely proportional to the mean total resistance. Explain why the student was correct. Use data from Figure 12 in your answer.

Explain why adding resistors in parallel decreases the total resistance.

Figure 9 shows a circuit diagram.

In which position could a switch be placed so that both lamps can be switched on or off at the same time?

Tick (✓) one box.

Draw the circuit symbol for a switch in the box below.

In 30 seconds, 24 coulombs of charge flow through the battery. Calculate the current in the battery. Use the equation:

There is a potential difference of 3.6 V across the battery. Calculate the energy transferred by the battery when 60 coulombs of charge flows through the battery. Use the equation: energy transferred = charge flow × potential difference

A student built Circuit X and Circuit Y shown in Figure 10. The components used in each circuit were identical.

Figure 10

Circuit X Circuit Y

How would the reading on the ammeter in Circuit Y compare to the reading on the ammeter in Circuit X ?

Tick (✓) one box.

How does the total resistance of Circuit Y compare with the total resistance of Circuit X ?

Tick (✓) one box.

The student built another circuit which is shown in Figure 11.

Write down the equation which links current, potential difference and resistance.

There is a potential difference of 3.6 V across the lamp in Figure 11. The current through the lamp is 0.80 A

Calculate the resistance of the lamp.

Figure 7 shows a person using an electric lawn mower.

The lawn mower is connected to the mains electricity supply.

What is the frequency of the mains electricity supply in the UK?

The lawn mower has a switch on each side of the handle.

Figure 8 shows the circuit diagram for the lawn mower.

The motor in the lawn mower can only be turned on when the person using it holds the handle of the lawn mower with both hands.

Explain why.

The power input to the motor is 1.8 kW

The resistance of the motor is 32 Ω

Calculate the current in the motor.

The useful power output from the motor is 1.5 kW

Calculate the time it takes for the motor to transfer 450 000 J of useful energy.

Draw a diagram to show how 1.5 V cells should be connected together to give a potential difference of 4.5 V.

Use the correct circuit symbol for a cell.

A student built the circuit shown in Figure 3.

Calculate the total resistance of the circuit in Figure 3.

Use the equation:

The resistance of P is 3.5 Ω.

Calculate the resistance of Q.

The student connects the two resistors in Figure 3 in parallel.

What happens to the total resistance of the circuit?

Tick one box.

Give a reason for your answer.

Figure 13 shows a lift inside a building.

Figure 13

The motor in the lift does 120 000 J of work in 8.0 seconds.

Calculate the power output of the motor in the lift.

Use the equation:

The power input to the motor is greater than the power output.

Tick two reasons why.

Figure 14 shows part of the circuit that operates the lift motor.

Figure 14

The lift can be operated using either of the two switches.

Explain why.

Write down the equation that links gravitational field strength, gravitational potential energy, height and mass.

The lift goes up 14 m. The total mass of the people in the lift is 280 kg.

gravitational field strength = 9.8 N/kg

Calculate the increase in gravitational potential energy of the people in the lift.

Give your answer to 2 significant figures.

Complete the sentence. Choose answers from the box.

The current through an ohmic conductor is directly proportional to the

that the
remains constant.

Figure 7 shows a current – potential difference graph for a filament lamp.

Explain how the resistance of a filament lamp changes as the potential difference across it increases.

Many householders are replacing their filament lamps with LED lamps which are more energy efficient.

What does more energy efficient mean?

A Light Dependent Resistor (LDR) is used to turn on an outside lamp when it gets dark.

Part of the circuit is shown in Figure 8.

The light intensity decreases.

What happens to the potential difference across the LDR and the current in the LDR?

Potential difference

What is the resistance of the LDR when the potential difference across it is 4 V?

Give a reason for your answer.

Calculate the current through the LDR when the resistance of the LDR is 5000 Ω.

Give your answer to 2 significant figures.