POTENTIAL DIFFERENCE (pd) — also called voltage — is the energy transferred per unit charge.
Unit: VOLT (V). 1 V = 1 joule per coulomb.
RESISTANCE is the opposition to the flow of current.
Unit: OHM (Ω).
OHM'S LAW:
V = I × R
V = potential difference (V)
I = current (A)
R = resistance (Ω)
Rearranging:
I = V ÷ R
R = V ÷ I
Higher resistance → lower current for a given pd.
Higher pd → higher current for a given resistance.
Ohmic Conductors and I–V Graphs
An OHMIC CONDUCTOR (e.g. a resistor at constant temperature):
Current is DIRECTLY PROPORTIONAL to pd.
Resistance stays CONSTANT as current changes.
On an I–V graph:
Straight line through the origin.
Steeper gradient = lower resistance (more current per volt).
MEASURING RESISTANCE:
Ammeter in series + voltmeter in parallel.
R = V ÷ I
EXAMPLE:
6 V across a resistor, 2 A through it:
R = 6 ÷ 2 = 3 Ω
Potential Difference in Circuits
SERIES — pd SPLITS between components:
V_total = V₁ + V₂ + V₃
Higher resistance → larger share of pd.
PARALLEL — pd is the SAME across all branches:
V₁ = V₂ = V₃ = V_supply
EXAMPLE — series circuit:
Battery = 12 V. R₁ = 4 Ω, R₂ = 8 Ω.
Total R = 12 Ω. I = 12 ÷ 12 = 1 A.
V₁ = 1 × 4 = 4 V. V₂ = 1 × 8 = 8 V. Check: 4 + 8 = 12 V ✓
⚠️ Common Mistake
R = V ÷ I, NOT V × I. Use the triangle: cover the unknown — the remaining two show the operation. 'Voltage' is acceptable in everyday speech but the precise term is POTENTIAL DIFFERENCE.
📐 Variables
VPotential difference (V) is measured in volts (V)
ICurrent (I) is measured in amperes (A)
RResistance (R) is measured in ohms (Ω)
📐 Key Equations
V = I × R
📌 Key Note
V = IR. I = V/R. R = V/I. Ohmic: I ∝ V, straight I–V line. Series: pd splits (sum = total). Parallel: same pd across each branch. Ammeter in series; voltmeter in parallel.
🎯 Matching Activity — V = IR Calculations
Match each circuit scenario to the correct calculated value. — drag the symbols on the right to match the component names on the left.
R = 6 Ω
Drop here
I = 0.5 A
Drop here
V = 9 V
Drop here
I doubles
Drop here
Pd doubles for an ohmic conductor — current proportional to pd
V = 3 V, R = 6 Ω — I = 3 ÷ 6
I = 3 A, R = 3 Ω — V = 3 × 3
V = 12 V, I = 2 A — R = 12 ÷ 2
⚽ FIFA Worked Examples
Resistance Calculation
A component has 12 V across it and 0.4 A through it. Calculate its resistance.
F
R = V ÷ I
I
V = 12 V, I = 0.4 A
F
R = 12 ÷ 0.4
A
R = 30 Ω
🧪 Required Practical
🔬 RP15 (Physics) — Measure V and I for different components; calculate resistance using R = V/I.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 2
1. A 6 Ω resistor has 3 A through it. What is the potential difference?
2. On an I–V graph for an ohmic conductor, what does a steeper gradient indicate?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
Don't get itGetting thereNailed it!
🤖 Ask Mr Badmus AI
Stuck? Just ask! 💬
I'll use FIFA for calculations and flag Higher/Triple content clearly.