Current, potential difference and resistance GCSE physics lesson: Q = It and V = IR

Current and resistance: the virtual practical

A virtual experiment: the circuit follows a model, and it is not the required practical. Record your readings first, then answer the questions about them.

1. Virtual practical. R1's resistance is hidden. Change the cell's potential difference (B1) and record at least 5 readings, each at a different p.d. Watch both ammeters: A1 is before R1 and A2 is after it.They set up a virtual circuit and record its meters' readings, each checked.
2. Look at A1 and A2 in your readings. What do they show?
   1. They read the same only at a low p.d.; at a high p.d. A2 is less
   2. A2 reads less than A1: R1 uses up some of the current
   3. A2 reads more than A1: R1 speeds the charge up
   4. A1 and A2 read the same every time: the current is not used up in R1
3. Use one of your readings and R = V ÷ I. What is R1's resistance, in ohms?
4. Your readings are drawn as a graph of current against potential difference. What shape is it, and what does that tell you?
   1. A straight line through the origin: the current is directly proportional to the p.d.
   2. A straight line that doesn't go through the origin: there is a current at 0 V
   3. A curve that gets less steep: the resistance goes up as the p.d. goes up
   4. A straight line through the origin: the current goes up by the same amount as the p.d.
5. Without measuring it: what current would flow through R1 at 9.0 V, in amps? Give your answer to 2 decimal places.

Current, p.d. and resistance: practice

Show your working on paper: write the equation, put the numbers in with their units changed first, then the answer with its unit. The stretch is optional.

Diagnostic question: choose and explain

1. A 12 V cell drives a current of 0.5 A through a 24 Ω resistor. The resistor is swapped for a 48 Ω one, and the cell stays the same. What is the current now?
   1. 1.0 A
   2. 0.25 A
   3. 0.5 A
   4. 4 A

Practice questions

1. A current of 0.4 A flows for 30 s. How much charge flows, in coulombs?
2. A current of 0.5 A flows through a 12 Ω resistor. What is the potential difference across it, in volts?
3. A charge of 90 C flows past a point in 2 minutes. What is the current, in amps? Give your answer to 2 decimal places.
4. The p.d. across a resistor is 9.0 V and the current through it is 250 mA. What is its resistance, in ohms?
5. Virtual experiment. The cell B1 stays at 6.0 V. Change R1's resistance and record at least 4 readings, each at a different resistance.They set up a virtual circuit and record its meters' readings, each checked.
6. In your readings for question 5, R1 went from 10 Ω to 20 Ω. What happened to the current?
   1. It stayed at 0.6 A: the cell sets the current
   2. It doubled, from 0.6 A to 1.2 A
   3. It halved, from 0.6 A to 0.3 A
   4. It went down by 10 A
7. A student wants to measure the current through a resistor and the potential difference across it. Where should the meters go?
   1. Ammeter in series with the resistor; voltmeter in parallel with it
   2. Ammeter in parallel with the resistor; voltmeter in series with it
   3. Both meters in series with the resistor
   4. Both meters in parallel with the resistor
8. Which sentence is right?
   1. The cell stores charge, and the current is that charge running out
   2. The potential difference flows through the resistor, just as the current does
   3. The current is used up in the resistor, so less flows out of it
   4. The current flows through the resistor; the potential difference is across it, between its two ends
9. A p.d. of 6.0 V is across a 15 Ω resistor for 5.0 minutes. How much charge flows through it, in coulombs?
10. Stretch: a resistor at constant temperature gives 0.08 A at 2.0 V and 0.20 A at 5.0 V. A student records 0.34 A at 8.0 V. What should the current be at 8.0 V, in amps? Give your answer to 2 decimal places.

Support: For question 3: change 2 minutes to seconds first. Then Q = I × t, so I = Q ÷ t.

Current, p.d. and resistance: exit

One calculation, then two or three sentences.

1. A 3.0 V cell drives a current of 0.15 A through a resistor. What is its resistance, in ohms?
2. A friend says a resistor uses up some of the current. Use the readings from the virtual practical to explain why that is wrong, and say what the resistor does instead.

Exit check

One calculation and one explanation show who has it.

A calculation with V = I R, then an explanation, using the readings, of why the current is not used up in a resistor.

Current, p.d. and resistance: use Q = It and V = IR

For each calculation: write the equation, change the units first (minutes to seconds, mA to A), put the numbers in, and give the answer with its unit.