Current and Voltage

Electric current is the flow of charge around a circuit, measured in amperes (A). Voltage (potential difference) is the energy given to each unit of charge by the battery, measured in volts (V). Ohm's Law connects current, voltage, and resistance.

Electric Current

Current is the flow of electric charge through a conductor.

Symbol: I
Unit: Ampere (A)
Measured with an ammeter (connected in series)

Current flows from the positive (+) terminal of the battery to the negative (–) terminal in conventional current.

Voltage (Potential Difference)

Voltage (also called potential difference) is the energy transferred per unit of charge.

Symbol: V
Unit: Volt (V)
Measured with a voltmeter (connected in parallel)

The battery gives each unit of charge a certain amount of energy. The voltage tells us how much.

Resistance

Resistance opposes (slows down) the flow of current.

Symbol: R
Unit: Ohm (Ω)

Ohm's Law

$V = I \times R$

Where:

$V$ = voltage (V)
$I$ = current (A)
$R$ = resistance (Ω)

Ohm's Law

V = I \times R

Voltage equals current multiplied by resistance. Used to find any one of the three quantities if the other two are known.

Finding Current

A bulb has a resistance of 10 Ω and is connected to a 5 V battery. What is the current?

Given: V = 5 V, R = 10 Ω Find: I

$I = \frac{V}{R} = \frac{5}{10} = 0.5 \text{ A}$

Current in Series and Parallel

Circuit type	Current	Voltage
Series	Same everywhere	Splits between components
Parallel	Splits between branches	Same across each branch

Real world: AA battery powering a torch bulb

A single AA battery has a voltage of 1.5 V. A small torch bulb has a resistance of about 3 Ω. Using Ohm's Law: I = V/R = 1.5/3 = 0.5 A. That 0.5 A of current flows through the filament, heating it to white heat and producing light — all from a battery you can hold in one hand.

Real world: Electric doorbell

A doorbell button closes a circuit — connecting a low-voltage supply (typically 8–12 V AC) through a transformer to a buzzer or bell. When you press the button, current flows and the electromagnet inside the buzzer vibrates a metal strip, creating the ringing sound. The transformer reduces the mains voltage to a safe level before it reaches the button.

Real world: Car starter motor

A car's 12 V battery pushes current through the starter motor to crank the engine. The motor's resistance is very low (about 0.1 Ω), so by Ohm's Law: I = V/R = 12/0.1 = 120 A — a huge surge of current for a few seconds, which is why starter cables must be very thick to handle it without overheating.

Tip Where do you see current and voltage in real life? AA batteries, car starters, phone chargers, doorbells, and every mains appliance — they all rely on V = IR!

Practice Problems

A resistor of 20 Ω has 4 A flowing through it. What is the voltage across it?

Show answer

V = I × R = 4 × 20 = 80 V

The voltage across a lamp is 12 V and its resistance is 6 Ω. Find the current.

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I = V ÷ R = 12 ÷ 6 = 2 A

In a parallel circuit, what is the same across each branch: current or voltage?

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Voltage — the same voltage acts across each parallel branch.