Voltage
Voltage is the electrical potential difference between two points, measured in volts (V). It describes energy transferred per unit of charge, rather than how much charge is flowing. A battery can have voltage across its terminals even when no complete circuit exists and no flows. Voltage and current are different quantities.
Free interactive unit · electrical basics
Make the current change
Change one value, watch the circuit respond, then try three short challenges. No sign-up or equipment needed.
Community draft — not yet reviewedWhat this means
This question has not yet been reviewed by a qualified electrician. It is a community draft: check anything you plan to act on against BS 7671 and your own supervisor before you rely on it. Completing it does not make you competent, certified or registered.
Arrows show conventional current direction
- Current
- 1 A
- Resistor power
- 12 W
I = V ÷ R = 12 ÷ 12 ≈ 1 A. Power P = V × I.
1–24 V · use arrow keys for one-volt steps
2–24 Ω · use arrow keys for one-ohm steps
What to notice
Start at 12 V and 12 Ω: current is 1 A. Hold resistance steady and raise voltage. Then reset and raise resistance instead. Watch how the same current reading responds differently.
Your three challenges
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Ideal DC source, fixed ohmic resistor and zero wire resistance. This model does not simulate heat, component ratings or real installations. On-screen exploration only; no XP or account progress is saved.
Read the Ohm’s law guide · Explore current · Continue on the learning path
Where Voltage fits in the learning path
The Career Taster section covers these units:
- The Absolute Basics
- Meet the Trade
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This section is on the free path. Lesson availability depends on content review and your learning path.
Read, practise, return
1. Read it in context
- Ohm's LawElectrical Science
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
- Electrical Science — quiz previewFree · Electrical Science
3. Return to the meaning
After reading or practising, explain the idea in your own words. Revisit the Voltage definition to check what you remembered.
Recall a related term
Which term fits this definition? Say it to yourself, then reveal it. This is a memory prompt, with no score or saved result.
The voltage lost along a cable as current flows through its resistance. It is Ohm's law working against you, growing with both load current and run length. BS 7671 limits how much of the supply voltage may be dropped in the installation's own wiring, so a long run can force a larger conductor than the current alone would need.
Voltage drop
Read the Voltage drop entryCompare related terms
Open a meaning to compare its role with Voltage, or follow its full entry.
Voltage drop
The voltage lost along a cable as current flows through its resistance. It is Ohm's law working against you, growing with both load current and run length. BS 7671 limits how much of the supply voltage may be dropped in the installation's own wiring, so a long run can force a larger conductor than the current alone would need.
Read the Voltage drop entryCurrent
Current is the rate at which electric charge flows, measured in amperes (A), often called amps. In a simple closed circuit, the supply drives current through the load and back to the source. Current is not used up by a lamp: energy is transferred there. A cable current rating describes how much current it can carry in stated conditions, not how much it always carries.
Read the Current entryResistance
Resistance describes opposition to current that transfers electrical energy into heat, and is measured in ohms (Ω). For an ohmic component at a fixed temperature, greater resistance means less current at the same voltage. A heating element uses resistance deliberately; cable resistance also causes heating and voltage drop. Resistance is only one part of AC impedance.
Read the Resistance entryAC — Alternating Current
Alternating current reverses direction repeatedly, unlike direct current. An AC voltage also changes polarity over its cycle. Frequency, measured in hertz (Hz), tells you how many complete cycles occur each second. In a three-phase system, the three AC waveforms are offset in time; three-phase does not mean three times the frequency.
Read the AC entryA definition is a start. Using it in a real question is where it sticks.