Resistance
Resistance describes opposition to that transfers into heat, and is measured in ohms (Ω). For an ohmic component at a fixed temperature, greater resistance means less current at the same . A heating element uses resistance deliberately; cable resistance also causes heating and . Resistance is only one part of AC impedance.
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
0 of 3 explored this visit
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 Resistance fits in the learning path
The Career Taster section covers these units:
- The Absolute Basics
- Meet the Trade
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Read, practise, return
1. Read it in context
- Ohm's LawElectrical Science
- R1+R2 Continuity TestingTesting & Inspection
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
- Electrical Science — quiz previewFree · Electrical Science
- Inspection & Testing — quiz previewPlus · Testing & Inspection
3. Return to the meaning
After reading or practising, explain the idea in your own words. Revisit the Resistance 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 combined resistance of a circuit's line conductor (R1) and its circuit protective conductor (R2), measured end-to-end during dead testing. It proves the protective conductor is continuous, and added to Ze it predicts the circuit's earth fault loop impedance before the supply is ever switched on.
R1+R2
Read the R1+R2 entryCompare related terms
Open a meaning to compare its role with Resistance, or follow its full entry.
R1+R2
The combined resistance of a circuit's line conductor (R1) and its circuit protective conductor (R2), measured end-to-end during dead testing. It proves the protective conductor is continuous, and added to Ze it predicts the circuit's earth fault loop impedance before the supply is ever switched on.
Read the R1+R2 entryVoltage 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 entryInsulation resistance
The resistance between conductors that should be completely separated from each other. It is measured in megohms, line to neutral and each to earth, with a test instrument applying an elevated DC voltage. Healthy insulation reads very high; damaged cables, moisture or pinched conductors show up as a low reading long before they show up as a shock or a fire.
Read the Insulation resistance 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 entryImpedance
Impedance is the combined opposition that resistance and reactance present to alternating current, measured in ohms (Ω). Reactance comes from inductance and capacitance, so impedance is not always the same as a resistance reading. Earth fault loop impedance, Zs, describes the whole fault-current path; R1+R2 describes the resistance of two circuit conductors.
Read the Impedance entryA definition is a start. Using it in a real question is where it sticks.