Resistance

CIE IGCSE Physics· 4.2.4· 25 min read

1. Core: Definition of Resistance and Ohm's Law★★☆☆☆⏱ 8 min

📘 Definition

Resistance

R

Resistance is a measure of how much a component opposes the flow of electric current through it, defined as the ratio of potential difference across the component to the current flowing through it.

Example:

A 10 Ω resistor allows 0.2 A of current to flow when 2 V is applied across it.

The formula relating resistance, potential difference and current is known as Ohm's Law, which applies to ohmic conductors (components where resistance remains constant at constant temperature, regardless of the current flowing through them).

R = \frac{V}{I}

Where: $R$ = resistance in ohms (Ω), $V$ = potential difference in volts (V), $I$ = current in amperes (A).

📐 Worked Example

Calculate the resistance of a fixed resistor if a potential difference of 12 V causes a current of 0.3 A to flow through it.

1
Write down known values and formula: $V = 12$ V, $I = 0.3$ A, $R = V / I$
2
$R = \frac{12}{0.3}$
3
Calculate the result: $R = 40$ Ω

Exam tip:

Always convert units to base units before substituting into the formula: divide current in mA by 1000 to get A, and convert mV to V by dividing by 1000.

2. Extended Only: Resistance Characteristics of Common Components★★★☆☆Extended only⏱ 7 min

Different components have distinct resistance behaviours that are frequently tested in exams. You will be expected to link current-voltage (I-V) graph shapes to these behaviours.

Fixed resistor (ohmic conductor): Resistance is constant at constant temperature, so its I-V graph is a straight line through the origin.
Filament lamp: As current increases, the metal filament heats up, increasing resistance, so the I-V graph curves as current rises.
Diode: Resistance is very high in the reverse direction, so current only flows in the forward direction above a small threshold voltage (~0.7 V for silicon diodes).

📐 Worked Example

A student measures a current of 0.1 A through a filament lamp when the potential difference is 2 V. When the potential difference is increased to 6 V, the current is 0.2 A. Explain why the resistance changes.

1
Calculate resistance at both readings using $R = V / I$
2
$R_1 = \frac{2}{0.1} = 20 \text{ } Ω$
3
$R_2 = \frac{6}{0.2} = 30 \text{ } Ω$
4
The higher current increases the temperature of the filament, which increases the resistance of the metal wire as temperature rises.

3. Extended Only: Series and Parallel Resistance Rules★★★★☆Extended only⏱ 6 min

📘 Definition

Total Circuit Resistance

The equivalent single resistance that would draw the same current from the power supply as the full combination of components in the circuit.

For resistors connected in series, the total resistance is the sum of the individual resistances:

R_{total} = R_1 + R_2 + R_3 + ...

For resistors connected in parallel, the reciprocal of the total resistance is the sum of the reciprocals of the individual resistances:

\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ...

📐 Worked Example

Calculate the total resistance of a circuit with two resistors of 4 Ω and 6 Ω connected (a) in series, (b) in parallel.

1
(a) Series: sum the individual resistances
2
$R_{total} = 4 + 6 = 10 \text{ } Ω$
3
(b) Parallel: use the reciprocal formula
4
$\frac{1}{R_{total}} = \frac{1}{4} + \frac{1}{6} = \frac{3 + 2}{12} = \frac{5}{12}$
5
$R_{total} = \frac{12}{5} = 2.4 \text{ } Ω$

Exam tip:

For two resistors in parallel, you can use the shortcut formula $R_{t o t a l} = (R_{1} \times R_{2}) / (R_{1} + R_{2})$ to save time, but always show your working to earn full marks.

4. Core + Extended: Practical Measurement of Resistance★★★☆☆⏱ 4 min

You will be asked to describe experiments to measure the resistance of a component in both written and practical assessments. Follow this standard method:

Set up a series circuit with the test component, an ammeter (in series), a voltmeter (in parallel across the component), a variable resistor, and a power supply.
Adjust the variable resistor to change the current through the component, recording at least 3 pairs of current and potential difference readings.
Calculate resistance for each pair using $R = V / I$ : average results for ohmic components, or plot an I-V graph for non-ohmic components to analyse resistance changes.

✓ Quick check

Why is the voltmeter connected in parallel across the test component?
Reveal answer
To measure the potential difference across only the test component, not the rest of the circuit. —
Ammeters are connected in series to measure current through the circuit, while voltmeters are connected in parallel to measure the voltage drop across a specific component.

5. Common Pitfalls

Wrong move:

Forgetting to convert mA to A before calculating resistance

Why:

This produces a resistance value 1000x higher than the correct answer, leading to lost calculation marks.

Correct move:

Always divide current in mA by 1000 to get base units (A) before substituting into $R = V / I$ .

Wrong move:

Assuming all components follow Ohm's Law (constant resistance)

Why:

Filament lamps, diodes, thermistors and LDRs are non-ohmic, so their resistance changes with current, temperature or light level.

Correct move:

Only assume constant resistance for fixed resistors at constant temperature; explicitly link resistance changes to the relevant variable (e.g. temperature for lamps) for non-ohmic components.

Wrong move:

Adding parallel resistors directly instead of using the reciprocal formula (Extended only)

Why:

Parallel resistance is always lower than the smallest individual resistor, so direct addition produces an incorrectly high total resistance.

Correct move:

Use the reciprocal formula for parallel resistance, and check your result is smaller than the lowest individual resistor in the combination.

Wrong move:

Stating that diodes have zero resistance in the forward direction

Why:

Diodes have a small threshold voltage (~0.7 V) so their resistance is low but not zero once this voltage is reached.

Correct move:

State that diodes have very high resistance in the reverse direction, and low resistance in the forward direction above the threshold voltage.

6. Quick Reference Cheatsheet

Concept	Core Rule/Formula	Extended Rule/Formula	Unit
Resistance definition	$R = V / I$	Same as Core	Ω (ohm)
Fixed resistor I-V graph	Out of scope	Straight line through the origin (constant resistance at constant temperature)
Filament lamp I-V graph	Out of scope	Curve of decreasing gradient (resistance rises as temperature rises)
Series resistance	Out of scope	$R_{t o t a l} = R_{1} + R_{2} + ...$	Ω
Parallel resistance	Out of scope	$1/ R_{t o t a l} = 1/ R_{1} + 1/ R_{2} + ...$	Ω

7. Frequently Asked

Do I need to learn the R=ρL/A resistance formula for 0625?

No, this formula is part of A-Level Physics and is explicitly out of scope for CIE IGCSE Physics 0625. You only need to use R = V/I for all resistance calculations.

What is the difference between Core and Extended resistance content?

Core candidates only need to know R = V/I, component resistance characteristics, and practical measurement methods. Extended candidates must also calculate total resistance for series and parallel resistor combinations.

Going deeper

SyllabusCIE IGCSE Physics 0625 2026-2028 SyllabusRefer to section 4.2.4 for official resistance content

What's Next

Now that you have mastered resistance concepts, you can apply these rules to more complex circuit problems, including those involving variable components like thermistors and LDRs that are frequently tested in both Core and Extended papers. You will also use resistance calculations when learning about electrical power and energy transfer, a high-weight topic for Paper 2 and Paper 4 exams. Extended candidates should practice combining series and parallel resistance calculations with current and potential difference rules to solve multi-step circuit problems, which are often worth 3-4 marks in extended theory papers.