Ohm's Law
Electrical Science · C&G 2365 Units 202/302 · Free course section with an account
Volts push, ohms resist, amps flow: V = I × R captures the relationship, and the classic triangle gives you all three rearrangements. Add the power equations and you can move between watts, volts, amps and ohms from any starting pair.
The trick in exams isn't knowing the formula, it's spotting which quantity hides in the question's units and keeping milliamps and kilohms honest. Our practice questions vary the numbers and the disguises until neither trips you.
This is the foundation of the electrical-science units of the 2365 — series and , and fault- calculations are all Ohm's law wearing different hats.
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
Ready to change the connections? Build a circuit and compare series with parallel paths. Explore what happens to the other load when one path opens.
Try the free sample belowFree interactive sample
Choose an option, check it and retry here. No account, payment or energy needed. These public samples are separate from the full course and practice papers; progress is not saved.
Original public revision material · A4:2026. For revision only; do not use samples to guide work on site.
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.
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.
Common questions
- Can I try Ohm's Law free on this page?
- Yes. The public sample below accepts answers and gives feedback and retries without an account, payment or energy. It is a short revision sample, not the complete course or a full mock paper. This course section is also free with an account.
- Where does Ohm's Law sit in the City & Guilds scheme?
- Ohm's Law maps to Units 202/302. On TradeTraining it lives in the Electrical Science section as bite-size lessons with practice questions.
- What is Ohm's law in simple terms?
- Ohm's law says the flowing through a is set by the voltage across it: V = I × R. Know any two of voltage, current and resistance and you can find the third — which is why it underpins nearly every electrical calculation you'll meet.
Explore key terms
Open a term to read its meaning here, then close it or explore another. Free to use without an account.
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 full Voltage drop glossary entryPFCProspective Fault Current
The highest current that could flow at a given point if a fault of negligible impedance occurred there — whether between live conductors or from live to earth. Every protective device must be able to break the prospective fault current at its position without failing, which is why PFC is measured or calculated at the origin of every installation.
Read the full PFC glossary entryZsEarth Fault Loop Impedance
The total opposition to current around the loop a fault current would travel. That loop runs from the supply, along the line conductor to the fault, and back through the protective conductor and the earthing. It is the supply's own part (Ze) plus the circuit's R1 + R2, the resistance of its line and protective conductors. A low Zs lets a big fault current flow, which trips the protective device quickly, so electricians measure it and compare it with the maximum allowed for that device.
Read the full Zs glossary entry