Edexcel IGCSE Physics Energy and voltage in circuits questions

Energy and voltage in circuits syllabus

Questions on energy and voltage in circuits can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 2.7 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting
• 2.8 understand how the current in a series circuit depends on the applied voltage and the number and nature of other components
• 2.9 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how to investigate this experimentally
• 2.10 describe the qualitative effect of changing resistance on the current in a circuit
• 2.11 describe the qualitative variation of resistance of light-dependent resistors (LDRs) with illumination and thermistors with temperature
• 2.12 know that lamps and LEDs can be used to indicate the presence of a current in a circuit
• 2.13 know and use the relationship between voltage, current and resistance: voltage = current × resistance V = I × R
• 2.14 know that current is the rate of flow of charge
• 2.15 know and use the relationship between charge, current and time: charge = current × time Q = I × t
• 2.16 know that electric current in solid metallic conductors is a flow of negatively charged electrons
• 2.17 understand why current is conserved at a junction in a circuit
• 2.18 know that the voltage across two components connected in parallel is the same
• 2.19 calculate the currents, voltages and resistances of two resistive components connected in a series circuit
• 2.20 know that: voltage is the energy transferred per unit charge passed the volt is a joule per coulomb
• 2.21 know and use the relationship between energy transferred, charge and voltage: energy transferred = charge × voltage E = Q × V

How to answer energy and voltage in circuits questions

1. Identify whether the circuit is series or parallel before combining currents, voltages or resistances.
2. Draw standard circuit symbols cleanly and place ammeters in series and voltmeters in parallel.
3. Show unit conversions, especially milliamps, kilohms, kilowatt-hours and seconds.
4. In explanations, follow charge or energy through the circuit rather than only naming a rule.

Energy and voltage in circuits example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Using a Diagram

Question 1

The student replaces the filament lamp with a light emitting diode (LED).
He notices that there is no current in the diode when the battery is reversed.
He replaces the battery with an a.c. supply.
Which graph shows how the current in the diode varies with time?

A

B

C

D

Final answer

D — a half-wave rectified current.

Mark scheme points

1. M1 Selects D: current consists of repeated positive pulses, with zero current during the opposite half-cycles.

Explanation

An LED conducts current in only one direction. During one half-cycle of the a.c. supply it is forward-biased and current flows. During the opposite half-cycle it is reverse-biased, so the current is zero. Therefore the graph has positive pulses separated by intervals along the zero-current line, as shown by D.

Common mistakes

• Choosing a sinusoidal graph with negative current: an LED does not allow current in the reverse direction.
• Drawing a continuous positive current: the LED conducts only during one half of each a.c. cycle.
• Forgetting the zero-current intervals between the positive pulses.

Example 2: Reading an Experimental Diagram

Question 2

A student investigates the resistance of a thermistor.

Which of these is the correct symbol for a thermistor

A

B

C

D

Final answer

D

See symbol:

Mark scheme points

1. M1 Selects D.

Explanation

A thermistor is represented by the resistor symbol with a diagonal line through it. The diagonal line has no arrowhead.

• Resistor: rectangular resistor symbol only.
• Thermistor: rectangular resistor symbol with a diagonal line.
• Variable resistor: diagonal line with an arrowhead.

Common mistakes

• Choosing the plain resistor symbol, which does not show that resistance changes with temperature.
• Choosing a symbol with an arrowhead; this represents a variable resistor.
• Confusing the thermistor symbol with the light-dependent resistor symbol, which uses arrows directed towards the resistor.

Practise Energy and voltage in circuits questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.