Current, Resistance and Potential Difference | Physics

📖 In-Depth Theory

Potential Difference and Resistance

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?

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🤖 Ask MrBadmus

Stuck? Just ask! 💬

I'll use CFIFA for calculations and flag Higher/Triple content clearly.