School › Electricity › Ages 15–16
Current, voltage, resistance and power
Current as flow of charge, Ohm's law, series and parallel resistance, electrical power and energy in kWh. With a resistance-of-a-wire investigation.
Read
Current is the rate of flow of charge: I = ΔQ / Δt, measured in amperes (1 A = 1 C/s). By convention current flows from + to −, the direction positive charge would move; in metal wires the charge carriers are electrons, which drift the other way. A steady current needs a continuous energy input, such as a battery.[1]
Ohm's law: the current through a conductor is proportional to the potential difference across it, V = I·R, where R is the resistance in ohms (1 Ω = 1 V/A). Materials such as copper and aluminium are ohmic: their resistance does not depend on the current, at least at constant temperature.[2]
Conductors such as metals have free electrons that let charge move easily; insulators such as glass, rubber and plastic do not. That is why cables are insulated copper.[5]
For resistors in series, Rs = R₁ + R₂ + …, and the same current flows through each. In parallel, 1/Rp = 1/R₁ + 1/R₂ + …: the total is smaller than the smallest single resistance, and each branch has the full source voltage. Household devices are connected in parallel so each works independently.[3]
Power is the rate of energy transfer: P = I·V, which with Ohm's law also gives P = I²R and P = V²/R. Energy used is E = P·t. Electricity bills use the kilowatt-hour: 1 kWh = 3.6 × 10⁶ J.[4]
Try it: How does the length of a wire affect its resistance?
You need
- About 1.2 m of thin constantan or nichrome resistance wire, taped along a metre rule
- A low-voltage DC supply or battery pack (max 3 V) and a switch
- An ammeter and a voltmeter (or two multimeters)
- Crocodile-clip leads
- Optional: a fixed resistor to protect the circuit
Safety
Low voltage only (max 3 V). The resistance wire can get hot at short lengths: switch off between readings and don't touch the wire while current flows. Never connect the wire straight across the supply without the ammeter and switch.
What to do
- Connect the supply, switch, ammeter and the wire in series; connect the voltmeter across the length of wire being tested (between the two crocodile clips on the wire).
- Set the clips 20 cm apart. Switch on, read V and I quickly, and switch off.
- Repeat for 40, 60, 80 and 100 cm. Take each reading twice.
- Calculate R = V / I for each length.
- Plot R (y-axis) against length L (x-axis) and draw the line of best fit.
- Describe the relationship and explain it.
What happens, and why?
R increases in proportion to L: the graph is a straight line through (or close to) the origin, because a longer wire gives the drifting electrons more material to pass through. Readings are taken quickly because a warm wire has a slightly higher resistance. A small offset at L = 0 can come from contact resistance at the clips.[2][1]
Play: Circuit calculator
Work out each answer, then pick the right box. Use V = I·R, P = I·V, series and parallel rules.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11