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Basic Electricity

Understand electric charge, voltage, current, and resistance — the four fundamental concepts of any circuit.

Last updated: 7 February 2026

★★★☆☆ Level 3electricityCircuitCurrentVoltage4 min read

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Electric Charge

All matter contains electric charges. Electrons carry a negative charge; protons carry a positive charge.

  • Like charges repel each other.
  • Unlike charges attract each other.

Electric Current

Current is the flow of electric charge through a conductor.

Electric Current

I=QtI = \frac{Q}{t}

Q = charge (C = coulombs), t = time (s), I = current (A = amperes).

Current flows from the positive terminal of a battery, through the circuit, to the negative terminal (conventional current direction).

Voltage (Potential Difference)

Voltage is the "push" that drives current around the circuit — the energy provided per unit charge.

Voltage

U=EQU = \frac{E}{Q}

E = energy (J), Q = charge (C), U = voltage (V = volts).

A 9 V battery gives 9 joules of energy to each coulomb of charge.

Resistance

Resistance opposes the flow of current.

Ohm's Law

U=I×RU = I \times R

U = voltage (V), I = current (A), R = resistance (Ω = ohms).

Note Thicker, shorter wires have lower resistance. Materials like copper are good conductors (low R); rubber and plastic are insulators (very high R).

Power

Electrical Power

P=U×IP = U \times I

P = power (W = watts), U = voltage (V), I = current (A).

  1. 1

    Identify the known quantities

    Voltage (V), current (A), or resistance (Ω)?
  2. 2

    Choose the right formula

    Ohm's law: U = IR. Power: P = UI.
  3. 3

    Solve and check units

    V = A × Ω. W = V × A.

A resistor has R = 20 Ω and a current of 0.5 A flows through it. Find the voltage across it.

U=I×R=0.5×20=10 VU = I \times R = 0.5 \times 20 = 10 \text{ V}

Real world: phone charger

A phone charger converts 230 V mains supply to 5 V at 2 A for the phone. The internal resistance of the charger circuit: R=UI=52=2.5 ΩR = \frac{U}{I} = \frac{5}{2} = 2.5\,\Omega. Power consumed: P=5×2=10 WP = 5 \times 2 = 10\text{ W}. Ohm's law governs every consumer electronics device.

Real world: household fuse selection

A kettle operates at 230 V and is rated at 2000 W. Current drawn: I=PU=2000230≈8.7 AI = \frac{P}{U} = \frac{2000}{230} \approx 8.7\text{ A}. The electrician fits a 13 A fuse — the next standard size above 8.7 A — to protect the cable from overheating.

Real world: electric car charging

An electric car battery is charged at 400 V with a current of 50 A. Charging power: P=400×50=20 000 W=20 kWP = 400 \times 50 = 20\,000\text{ W} = 20\text{ kW}. Engineers design the charging circuit using Ohm's law to ensure cables and connectors can carry the load safely.

Tip Always check units after an electrical calculation: V = A × Ω; W = V × A. If your units don't match, you have used the wrong formula or confused voltage and current.

Practice Problems

A 12 V battery drives a current of 3 A. What is the resistance of the circuit?

Show answer
R = U / I = 12 / 3 = 4 Ω.

What current flows through a 60 Ω resistor connected to a 120 V supply?

Show answer
I = U / R = 120 / 60 = 2 A.

A device runs at 230 V and draws 2 A. What is its power?

Show answer
P = 230 × 2 = 460 W.