Energy and voltage in circuits

Edexcel International GCSE Physics· 2.7-2.21 (Section 2: Electricity)· 25 min read

1. Series and Parallel Circuit Properties and Applications★★☆☆☆⏱ 5 min

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Circuits are classified as series (components connected in a single loop) or parallel (components connected across separate branches). Each type has unique properties that make it suitable for different uses.

📘 Definition

Series vs Parallel Circuits

Series: Single loop, current identical through all components, supply voltage shared between components. Parallel: Multiple branches, voltage identical across all branches, supply current split between branches.

Series circuits are used for low-power fairy lights, where multiple bulbs share the supply voltage to reduce power use
Parallel circuits are used for domestic lighting, so each bulb can be switched independently and receives full mains voltage

📐 Worked Example

A 2Ω resistor and a 3Ω resistor are connected in series to a 10V battery. Calculate the total resistance, total current, and voltage across each resistor.

1
Total resistance for series circuits is the sum of individual resistances:
$R_{total} = R_1 + R_2 = 2 + 3 = 5\Omega$
2
Use Ohm's Law to calculate total current:
$I = \frac{V}{R} = \frac{10}{5} = 2A$
3
Current is identical across all series components, so calculate voltage across each resistor:
$V_1 = I \times R_1 = 2 \times 2 = 4V$
4
$V_2 = I \times R_2 = 2 \times 3 = 6V$
5
Verify: 4V + 6V = 10V, matching the supply voltage.

Exam tip:

When asked to explain domestic lighting circuit design, always mention independent switching and full voltage per bulb as key benefits of parallel layouts.

2. Core Circuit Formulae and Quantities★★★☆☆⏱ 6 min

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Three core formulae must be memorized for this topic, as they are not provided on the exam formula sheet. Always convert all units to SI base units before calculation.

📘 Definition

Voltage

Energy transferred per unit charge passed between two points in a circuit. 1 volt = 1 joule per coulomb (1V = 1J/C).

V = I \times R \text{ (Ohm's Law: Voltage = Current } \times \text{ Resistance)}

Q = I \times t \text{ (Charge = Current } \times \text{ Time)}

E = Q \times V \text{ (Energy Transferred = Charge } \times \text{ Voltage)}

📐 Worked Example

A 12V battery supplies a current of 2A for 30 seconds. Calculate the total charge flow and energy transferred by the battery.

1
Calculate charge flow using Q = I × t:
$Q = 2 \times 30 = 60C$
2
Calculate energy transferred using E = Q × V:
$E = 60 \times 12 = 720J$

3. I-V Characteristics of Common Components★★★☆☆⏱ 6 min

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I-V graphs plot current against voltage for a component, showing how resistance changes with voltage. You need to know the shape of these graphs for four common components, and the experimental method to measure them.

📘 Definition

Ohmic Conductor

A component (e.g. fixed resistor, wire at constant temperature) where current is directly proportional to voltage, producing a straight-line I-V graph through the origin.

Component	I-V Graph Shape	Reason for Shape
Fixed Resistor (Ohmic)	Straight line through origin	Resistance constant at constant temperature
Filament Lamp	Curved line, gradient decreases at high voltage	Resistance increases as filament temperature rises with current
Diode	Zero current for negative voltage, sharp rise in current for positive voltage	Only conducts in forward bias (positive direction)

📐 Worked Example

Describe an experiment to measure the I-V characteristic of a filament lamp.

1
Set up a series circuit with a cell, variable resistor, ammeter, filament lamp, and a voltmeter connected in parallel across the lamp.
2
Adjust the variable resistor to change the voltage across the lamp, recording pairs of current (ammeter) and voltage (voltmeter) readings for at least 5 settings.
3
Reverse the cell connections to take negative voltage and current readings.
4
Plot a graph of current (y-axis) against voltage (x-axis) to produce the I-V characteristic.

Exam tip:

If asked to explain the filament lamp curve, always link the shape to increasing temperature causing increasing resistance, which reduces the rate of current rise at high voltages.

4. Variable Resistance Components: LDRs, Thermistors and Diodes★★☆☆☆⏱ 4 min

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Specialist resistors change their resistance in response to external conditions, making them useful for sensor circuits. Lamps and LEDs can also be used to indicate the presence of current, as they light up when current flows through them.

📘 Definition

LDR and NTC Thermistor

LDR (Light Dependent Resistor): Resistance decreases as light intensity increases. NTC Thermistor: Resistance decreases as temperature increases.

📐 Worked Example

An LDR is used in an automatic porch light circuit. Explain why the light turns on when it gets dark.

1
When light intensity is low (dark), the resistance of the LDR increases.
2
Higher resistance reduces the current in the control circuit.
3
This triggers a relay switch to close and turn on the porch light.

5. Current Conservation and Junction Rules★★☆☆☆⏱ 4 min

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Current is conserved in all circuits, meaning no charge is lost at junctions in parallel circuits. Voltage across components in parallel is identical to the supply voltage.

📘 Definition

Junction Rule

The total current flowing into a junction in a parallel circuit is equal to the total current flowing out of the junction, as charge cannot be created or destroyed.

📐 Worked Example

At a junction in a parallel circuit, 3A of current flows into the junction, and 1A flows out through one branch. Calculate the current flowing out through the second branch.

1
Apply the junction rule: total current in = total current out
$I_{in} = I_{1} + I_{2}$
2
Rearrange to solve for the second branch current:
$I_{2} = 3A - 1A = 2A$

Exam tip:

For parallel circuits, you only need to describe trends qualitatively: do not attempt to calculate combined parallel resistance, as this is out of scope for the Edexcel IGCSE specification.

6. Common Pitfalls

Wrong move:

Assuming current is identical across all parallel branches

Why:

Current splits at junctions, only voltage is identical across parallel branches

Correct move:

Use the junction rule: total current into a junction equals total current out

Wrong move:

Using the parallel combined resistance formula for calculations

Why:

Edexcel IGCSE only requires series resistance calculations for two components; parallel resistance calculations are out of scope

Correct move:

Only add resistances directly for series circuits, describe parallel behavior qualitatively

Wrong move:

Drawing the filament lamp I-V graph as a straight line

Why:

Filament temperature rises with current, increasing resistance, so the graph curves

Correct move:

Draw an S-shaped curve for the filament lamp, with decreasing gradient at high voltages

Wrong move:

Defining voltage as a 'push of current' in exam questions

Why:

Mark schemes require the precise definition: energy transferred per unit charge, 1V = 1J/C

Correct move:

Use the exact syllabus definition for all 2+ mark voltage explanation questions

Wrong move:

Mixing up LDR and thermistor resistance trends

Why:

Both components have resistance that falls with an increase in their trigger stimulus, but students often mix up the stimulus for each

Correct move:

Use the mnemonic: Lighter = Lower LDR resistance, Hotter = Lower thermistor resistance

7. Quick Reference Cheatsheet

Quantity/Rule	Formula/Statement	Key Detail	Exam Tip
Ohm's Law	V = I × R	Units: V (V), I (A), R (Ω)	Memorize, not provided on formula sheet
Charge Flow	Q = I × t	Units: Q (C), t (s)	Convert time to seconds before calculation
Energy Transferred	E = Q × V	1V = 1 J/C	Use exact definition for voltage explain questions
Series Circuits	I same, V_total = V1+V2, R_total = R1+R2	Only for 2 resistive components	Add resistances directly only for series
Parallel Circuits	V same across branches, I_in = I_out at junctions	No parallel resistance calculations needed	Domestic lighting uses parallel for independent control
I-V Graphs	Ohmic: straight line, Filament: curved, Diode: forward only	Diode only conducts with positive voltage	Draw correct shapes for graph questions
Variable Resistors	LDR: higher light = lower R, Thermistor: higher temp = lower R	Only NTC thermistors are assessed	Use the mnemonic to avoid mixing trends

8. Frequently Asked

Do I get the V=IR, Q=It, and E=QV formulae on the exam formula sheet?

No, these three formulae must be memorized for your Edexcel IGCSE Physics exam, as they are not provided on the formula sheet. Always show your full working when using them to gain maximum marks.

What is the difference between conventional current and electron flow?

Conventional current is defined as flowing from the positive terminal to the negative terminal of a cell, and is used for all circuit diagrams and calculations. In solid metallic conductors, current is actually a flow of negatively charged electrons, which move from negative to positive. You must be able to state both correctly if asked.

Do I need to calculate combined resistance for parallel circuits?

No, for Edexcel IGCSE Physics, you only need to calculate combined resistance for two resistive components in series. For parallel circuits, you only need to know that voltage is the same across all branches, and current is conserved at junctions.

Going deeper

official_specEdexcel 4PH1 2017 SpecificationRefer to Section 2 Electricity for full content guidance
practical_guideI-V Characteristic Required Practical GuideStep-by-step method for the assessed core practical

What's Next

Now you have mastered core energy and voltage in circuits content for Edexcel IGCSE Physics, you are ready to progress to mains electricity and circuit safety, which builds on your current and voltage knowledge to explain fuse operation and the P=IV power formula. You can also move to the next electricity sub-topic, electrostatic charge. This content is assessed in both Paper 1 and Paper 2, as well as the Double Award (4SD0) specification. Make sure you practice past paper questions focusing on series circuit calculations and I-V characteristic descriptions, and memorize the three core formulae as they are not provided on the exam formula sheet.