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.

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.

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.

Experiment: How does the length of a wire affect its resistance?

1. 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).
2. Set the clips 20 cm apart. Switch on, read V and I quickly, and switch off.
3. Repeat for 40, 60, 80 and 100 cm. Take each reading twice.
4. Calculate R = V / I for each length.
5. Plot R (y-axis) against length L (x-axis) and draw the line of best fit.
6. Describe the relationship and explain it.

Length L (cm)	V (V)	I (A)	R = V/I (Ω)	Repeat R (Ω)
20
40
60
80
100

Questions

1. State Ohm's law and calculate the current through a 47 Ω resistor connected to 9 V.

2. Calculate the total resistance of 20 Ω and 30 Ω in series and in parallel.

3. A 60 W lamp is on for 5 hours a day for 30 days. How many kWh is that? What would it cost at €0.30 per kWh?

4. Describe your R–L graph and explain the shape. Give one source of error.

Sources for every fact on this sheet: echoesof.science/school/electricity/sources/

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