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Current, p.d. and resistance What decides how big the current in a circuit is?

Foundation and Higher · AQA · 45 minutes

Students predict whether a resistor uses current up, then test it in a virtual circuit: they change the cell's p.d., record the current before and after a hidden resistor, and find its resistance from their own readings. Slides to present, a vote that shows who's stuck, practice and homework.

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I = V ÷ R for one resistor. Along the bottom (x) is the potential difference V in volts, up the side (y) the current I in amps, so y = x/4 means I = V ÷ 4. Move r, the resistance in ohms. At 6 V, what happens to the current when the resistance doubles?

36912

y = x/4

The idea in one minute

Current is the rate at which charge flows (Q = I × t), and in a single loop it is the same at every point: nothing is used up. The potential difference across a component pushes charge through it, and its resistance opposes that: V = I × R. For a resistor at a constant temperature the current is directly proportional to the p.d., so its I–V graph is a straight line through the origin.

The lesson starts with a prediction (is current used up in a resistor?) and answers it with the students' own readings: a virtual circuit with an ammeter on each side of a resistor whose value is hidden. They change the cell's p.d., record five readings and work out the resistance, then see that at a fixed p.d. a bigger resistance gives a smaller current. Then Q = I t and V = I R in numbers, with the unit changes examiners say cost marks (minutes to seconds, milliamps to amps). It is a virtual experiment, not a substitute for the required practical.

By the end, students can

  • say what current, charge, potential difference and resistance are, with their units
  • use Q = I t and V = I R, changing minutes to seconds and milliamps to amps first
  • place an ammeter in series and a voltmeter in parallel, and say why
  • find a resistance from readings of I and V, and say what a straight I–V line through the origin means

Key words

  • current (I, amps, A)
  • charge (Q, coulombs, C)
  • potential difference (p.d., V, volts, V)
  • resistance (R, ohms, Ω)
  • ammeter, voltmeter
  • ohmic conductor

Before this lesson

Students should substitute into a formula, solve an equation such as 0.3R = 6 and plot points. Circuits from KS3 (cells, bulbs, a single loop) help but aren't needed.

Next: Series and parallel circuits, I–V characteristics.

The lesson, step by step

60 minutes in total. Each step says what changes for this length.

  1. 1 · Think5 min

    Is current used up?

    One loop: a cell, ammeter A1, a resistor, ammeter A2.

    A1 reads 0.24 A.

    What does A2 read: more, less or the same?

    Write your answer and one reason.

    Predict A2 before anyone explains.

    Write here

    5 min 1 · Hook · students think · the slides say “Think”

    Is current used up?

    Students predict what an ammeter after a resistor reads, before anyone explains.

    Say

    “A cell, then an ammeter, then a resistor, then a second ammeter, all in one loop. The first ammeter reads 0.24 A. Write down what the second one reads, and why. Don't show anyone yet.”

    Ask
    • Does the resistor use some of the current up?
    • What does a resistor do to the charge flowing through it?
    Watch for

    Predictions such as 0.12 A or 0 A: keep them, because the virtual practical answers them.

    For the teacher

    Give a minute of silent thinking. Gather predictions: more, less or the same as 0.24 A, each with a reason.

    Don't say who is right. Ask what the resistor might use up instead, and keep the word 'energy' for later.

    Collect predictions with Ask the class and leave them unanswered: the students' own readings settle it in Your turn.

    Shorter: Take three predictions only, without reasons (−2 min).

  2. 2 · Watch10 min

    Current is a flow of charge

    Current I: the rate of flow of charge.

    Q = I × t

    Q in coulombs (C), I in amps (A), t in seconds (s).

    0.5 A for 2 minutes: Q = 0.5 × 120 = 60 C

    Charge needs a closed loop and a source of p.d.

    The same current all round a loop? You test it next.

    Minutes to seconds first.

    2 · Watch

    P.d. pushes, resistance opposes

    P.d. V (volts) is across a part.

    Resistance R (ohms, Ω) opposes I.

    V = I × R, so I = V ÷ R

    The line: I = V ÷ r, r in ohms.

    Along: V in volts. Up: I in amps.

    Same p.d., more R: less current.

    Vote, then move r from 4 to 8.

    2 · Watch

    Where the meters go

    Ammeter: in series, in the loop: current goes through.

    Almost no resistance: in parallel it shorts the part.

    Voltmeter: in parallel, across the part.

    P.d. is between two points, so it measures across.

    Example: 6.0 V across, 0.30 A through: R = 20 Ω

    Current through, p.d. across.

    10 min 2 · Explain · you explain · the slides say “Watch”

    Charge, current, p.d. and resistance

    What each quantity is and its unit; Q = I t and V = I R; where the meters go.

    Say

    “Current is charge flowing. Potential difference pushes it; resistance opposes it. Two equations to learn by heart (the exam doesn't give them): Q = I t and V = I R.”

    Ask
    • If the p.d. stays the same and the resistance goes up, what happens to the current?
    • Why must time be in seconds in Q = I t?
    • Which meter goes across the resistor, and which goes in the loop?
    Watch for

    Saying the voltage 'goes through' the resistor: insist on current through, p.d. across.

    Charge from current and time

    A current of 0.5 A flows for 2 minutes. How much charge flows?

    t = 2 × 60 = 120 s

    Q = I × t = 0.5 × 120 = 60 C

    Resistance from p.d. and current

    6.0 V across a resistor, 0.30 A through it. What is its resistance?

    V = I × R, so R = V ÷ I

    R = 6.0 ÷ 0.30 = 20 Ω

    For the teacher

    Current I is the rate of flow of charge: Q = I × t, charge in coulombs (C), current in amps (A), time in seconds (s). A closed loop with a source of p.d. is needed for any charge to flow. Whether the current is the same all round the loop is left for the practical's readings to settle.

    Potential difference V (volts) across a component is what pushes charge through it; resistance R (ohms, Ω) opposes the current. V = I × R. For the same p.d., more resistance means less current. 'Voltage' is credited too, but questions say potential difference.

    An ammeter goes in series, in the loop, so the current flows through it; it has almost no resistance, so in parallel it would short the component out. A voltmeter goes in parallel, across the component, because p.d. is between two points.

    A resistor at a constant temperature is an ohmic conductor: its resistance stays the same, so I is directly proportional to V and its I–V graph is a straight line through the origin. Lamps, diodes and thermistors are different: that is the next lesson.

    On the p.d. slide, move r on the graph (Ask the class to vote first): r from 4 Ω to 8 Ω halves the current at every p.d.

    Shorter: Leave out the graph slide; one worked example (−3 min).

  3. 3 · Your turn14 min

    The virtual practical

    Open the quiz: the virtual practical.

    R1's resistance is hidden.

    Change the cell's p.d.; record 5 readings.

    Watch A1 (before R1) and A2 (after R1).

    Then find R1's resistance from your readings.

    A virtual experiment, not the required practical.

    Five readings, each at a different p.d.

    3 · Your turn

    Worked, then yours

    I do: 6.0 V across, 0.30 A through: R = 20 Ω

    You do: 4.5 V across, 0.15 A through. R?

    I do: 0.2 A for 45 s. Q = 0.2 × 45 = 9 C

    You do: 0.6 A for 1 minute. Q?

    I do: 0.25 A through 40 Ω. V = 0.25 × 40 = 10 V

    You do: 0.4 A through 15 Ω. V?

    Equation, numbers, answer with its unit.

    Write here

    Its answers' slide comes with your copy.

    14 min 3 · Your turn · students write

    The virtual practical, then worked examples

    Students take readings on a resistor whose value is hidden, find its resistance, then solve twins of worked examples.

    Say

    “Open the virtual practical. Change the cell's p.d., record five readings, and watch both ammeters. Then work out the hidden resistance from your readings.”

    Ask
    • What did A2 read compared with A1, every time?
    • Which reading gives the resistance most easily?
    • Your I–V graph: straight or curved? Through the origin?
    Watch for

    Dividing I by V for the resistance: R = V ÷ I. Ask what the units of their answer would be.

    Resistance

    I do: 6.0 V across, 0.30 A through

    R = V ÷ I = 6.0 ÷ 0.30 = 20 Ω

    Then theirs

    You do: 4.5 V across, 0.15 A through. R?

    Charge

    I do: 0.2 A for 45 s

    Q = I × t = 0.2 × 45 = 9 C

    Then theirs

    You do: 0.6 A for 1 minute. Q?

    Potential difference

    I do: 0.25 A through 40 Ω

    V = I × R = 0.25 × 40 = 10 V

    Then theirs

    You do: 0.4 A through 15 Ω. V?

    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.
    For the teacher

    The virtual practical: a cell B1 whose p.d. students set from 0 to 12 V, ammeter A1, a resistor R1 whose value is hidden, ammeter A2, and voltmeter V1 across R1. They record at least five readings, each at a different p.d.; each is checked against the circuit. The readings are drawn as a graph of I against V.

    Then the questions on their readings: A1 against A2, R1's resistance, the shape of the graph, and a current they predict without measuring. Go back to the hook's predictions once most have answered.

    Say plainly that this is a virtual experiment, not the required practical: in a lab the readings wobble, the resistor warms up if left on, and the meters have their own small errors.

    Then each worked example aloud, its twin straight after.

    Give the virtual practical first; once most have their readings, go back to the hook's predictions and let a student read out A1 and A2 from their table. Then say what the resistor transfers instead (energy, not charge). Then the worked examples and their twins; open the answers slide when most have finished.

    Shorter: After the practical, the resistance pair of examples only (−5 min).

  4. 4 · Vote and practise21 min

    Vote first

    A 12 V cell, a 24 Ω resistor: the current is 0.5 A.

    Swap in a 48 Ω resistor; the same cell.

    What is the current now?

    A: 1.0 A

    B: 0.25 A

    C: 0.5 A

    D: 4 A

    Choose on your own, then say why.

    4 · Vote and practise

    Practise: equation first

    1. 0.4 A for 30 s. Charge, in C?

    2. 0.5 A through 12 Ω. P.d., in V?

    3. 90 C in 2 minutes. Current, in A?

    4. 9.0 V across, 250 mA through. Resistance, in Ω?

    5–6. The second virtual circuit: 6.0 V, change R1.

    Stuck on 3? 2 minutes is 120 s.

    Write here

    4 · Vote and practise

    Reason it out

    7. Where do the ammeter and the voltmeter go?

    8. Which sentence about current and p.d. is right?

    9. 6.0 V across 15 Ω for 5.0 minutes. Charge?

    Stretch: 0.08 A at 2.0 V, 0.20 A at 5.0 V.

    Is 0.34 A at 8.0 V right? What should it be?

    9 takes two steps: the current first.

    Write here

    21 min 4 · Who's stuck? · students vote · the slides say “Vote and practise”

    Vote, then practise

    One question shows who's stuck; then practice in levels, with a second virtual circuit.

    For this length: The exam-style question and the stretch, gone through together

    Say

    “Choose an answer on your own. Nobody talks until everyone has chosen.”

    Ask
    • Does the cell decide the current on its own?
    • Which quantity stays the same here, and which changes?
    Watch for

    Who works it out from the equation, and who answers from a feeling about resistance.

    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.

    The question that shows who's stuck

    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
    In your Classroom, as they answer (an example)
    • A6
    • B10
    • C8
    • D3

    Each wrong answer is a different mistake. While they answer, your Classroom shows who chose which, so you know in seconds whom to help with what.

    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.

    For the teacher

    Everyone chooses before anyone explains. Then ask for a reason for each option, and settle it with I = V ÷ R for each resistor.

    Questions 1–2 are fluency; 3–4 change units; 5–6 are a second virtual circuit (a fixed p.d., the resistance changed); 7–8 are about meters and words; 9 is exam-style, in two steps; the stretch tests a set of readings.

    Students show their working on paper or the board; the quiz keeps their final answers.

    If many chose the same wrong option, ask a student who chose it to explain, then put the two resistances into I = V ÷ R together before they practise.

    Shorter: After the vote, questions 1–6 only (−5 min).

    Longer: The exam-style question and the stretch, gone through together (+10 min).

  5. 5 · Show what you know10 min

    Show what you know

    1. 3.0 V drives 0.15 A through a resistor. R?

    2. 'A resistor uses up some of the current.'

    Use your readings to say why that is wrong,

    and what the resistor does instead.

    Two or three sentences for question 2.

    Write here

    10 min 5 · Exit · students show · the slides say “Show what you know”

    Show what you know

    One calculation and one explanation show who has it.

    For this length: Draw the circuit for the practical, with the meters in the right places

    Say

    “Answer the two exit questions on your own: one calculation, one explanation in two or three sentences.”

    Watch for

    Explanations that still say the resistor uses current up: what does it transfer instead?

    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.
    For the teacher

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

    The explanations show who still holds the 'used up' idea: start the next lesson (series and parallel circuits) with two of them. Set the homework.

    Longer: Draw the circuit for the practical, with the meters in the right places (+5 min).

One to one: let the student run the virtual practical themselves and say, before each new p.d., what they expect both ammeters to read. Then ask them to explain to you what A1 and A2 show, and what the resistor does to the charge.

Homework: 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.

  1. A current of 0.25 A flows for 4 minutes. How much charge flows?
  2. A 9.0 V battery is connected to a 45 Ω resistor. What current flows?
  3. A charge of 18 C passes a point in 1 minute 30 seconds. What is the current?
  4. The current through a resistor is 150 mA and the p.d. across it is 4.5 V. What is its resistance?
  5. Draw the circuit you would use to find a resistor's resistance: a cell you can change, an ammeter and a voltmeter. Say why each meter goes where it does.
  6. A student's readings on a resistor: V = 0, 2.0, 4.0, 6.0 V and I = 0, 0.10, 0.20, 0.30 A. Plot I against V. What is the resistance, is the resistor ohmic, and what current would flow at 9.0 V?

Print the worksheet with working space and homework. Teacher answers come with your Library copy.

Teaching current, p.d. and resistance: common questions

Can the virtual practical replace the required practical?

No. It follows an exact model (an ammeter with no resistance, a voltmeter that takes no current, a resistor that never warms up), so it is good for seeing the pattern and practising taking readings, but students must still do the required practical with real equipment. Use it before the lab to prepare, or after it to compare their real readings with ideal ones.

Should students say voltage or potential difference?

AQA questions say potential difference, and students get credit for either. Teaching 'p.d. across' alongside 'current through' helps with the idea that p.d. is between two points and doesn't flow.

Is this lesson for Foundation or Higher?

Both: none of AQA's 4.2.1 is Higher only. Foundation students can stop at the standard questions; the exam-style question and the stretch suit Higher.

How do I get past 'the current is used up'?

Let the students' own readings argue: the virtual practical puts an ammeter on each side of a resistor, so they can compare the current going in with the current coming out at every p.d. Then give them something that is used up: energy. The cell transfers energy to the charge, and the resistor transfers it away by heating; the charge itself keeps going round.

Where are the I–V graphs of lamps, diodes and thermistors?

In the next lesson, on I–V characteristics. This one keeps to the resistor at constant temperature, whose straight line through the origin is the one the others are compared with.

Can I use the lesson without creating an account?

Yes. Read the lesson, print the worksheet and open its slides without signing in. A free teacher account keeps the whole lesson in your Library to change and teach, with the virtual practicals, the quizzes, the answers and the answer slides.

Curriculum reference: AQA GCSE Physics 8463: 4.2.1.1 (circuit symbols), 4.2.1.2 (electrical charge and current, Q = I t), 4.2.1.3 (current, resistance and potential difference, V = I R) and 4.2.1.4 (resistors: ohmic conductors). Links to required practicals 3 and 4.

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