Teacher sheet · Ages 15–16 · about 45–50 minutes
Current, voltage, resistance and power
Lesson page: echoesof.science/school/electricity/ages-15-16/ · Pupil worksheet: echoesof.science/school/electricity/ages-15-16/print/
Learning aims
- Use I = ΔQ/Δt and know conventional current vs electron flow.
- Apply V = I·R and the series and parallel resistance rules.
- Calculate power (P = IV, I²R, V²/R) and energy in J and kWh.
- Investigate the resistance of a wire and evaluate the method.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why does a phone charger say 5 V, 2 A? What is its power? |
| 10 min | Read the text; one Ohm's law and one parallel example. |
| 25 min | Resistance-of-a-wire practical in pairs. |
| 5 min | Plot R against L; discuss heating and contact resistance. |
| 5 min | Quiz, or Circuit calculator as homework. |
Materials for the experiment
- 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.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Current is used up as it goes round a circuit. | Current is the same all round a series loop; energy is transferred to components. |
| Adding resistors in parallel increases the total resistance. | It decreases it: there are more paths for the current. |
| A kWh is a unit of power. | It is a unit of energy: 3.6 × 10⁶ J. |
Worksheet answers
- V = I·R. I = 9 / 47 ≈ 0.19 A.
- Series: 50 Ω. Parallel: 1/Rp = 1/20 + 1/30 = 5/60, so Rp = 12 Ω.
- E = 0.06 kW × 150 h = 9 kWh; 9 × €0.30 = €2.70.
- A straight line through (about) the origin: R ∝ L. Errors: wire heating (switch off between readings), contact resistance at the clips, reading the length to the clip edge.
Quiz answers
- A charge of 30 C passes a point in 10 s. The current is… 3 A. I = ΔQ/Δt = 30/10 = 3 A.[1]
- A 6 V battery drives 0.5 A through a resistor. Its resistance is… 12 Ω. R = V/I = 6/0.5 = 12 Ω.[2]
- Two 10 Ω resistors in parallel have a total resistance of… 5 Ω. 1/Rp = 1/10 + 1/10 = 1/5.[3]
- A kettle draws 10 A from a 230 V supply. Its power is… 2300 W. P = I·V = 10 × 230 = 2300 W.[4]
- A 2 kW heater runs for 3 hours. The energy used is… 6 kWh. E = P·t = 2 kW × 3 h = 6 kWh.[4]
- In a metal wire, the electrons move… opposite to the conventional current. Conventional current is the direction positive charge would flow.[1]
Going further
Repeat with a thicker wire of the same material. Predict and then test how the cross-sectional area affects R.
Sources used in this lesson
- [1] College Physics 2e AP, 20.1 Current (OpenStax, Rice University): reliability High
- [2] College Physics 2e, 20.2 Ohm's Law: Resistance and Simple Circuits (OpenStax, Rice University): reliability High
- [3] College Physics 2e, 21.1 Resistors in Series and Parallel (OpenStax, Rice University): reliability High
- [4] College Physics, 20.4 Electric Power and Energy (OpenStax, Rice University): reliability High
- [5] College Physics 2e AP, 18.2 Conductors and Insulators (OpenStax, Rice University): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/electricity/sources/
Privacy: the lesson pages have no ads, no tracking and no cookies. Quiz answers and game scores stay in the pupil's browser. A short video for each age is coming.