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Science lesson plan · 12th grade (Year 13 in England) · 45 minutes

Transformers and the National Grid

Electricity and magnetism

CCSS NGSS.HS-PS2-5CCSS NGSS.HS-PS3-3England: GCSE Physics: Magnetism and electromagnetism (transformers)England: GCSE Combined Science: Energy (the National Grid)

Objective and success criteria

Students will: Transformers and the National Grid.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: The motor effect (F = B I l) and Fleming's left-hand rule; Power and energy in circuits (P = I²R, E = QV)

Leads to: the next skill in this area.

Materials: Circuit kits (cells, wires, bulbs), Circuit symbol cards, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 5 min

  • Quick retrieval from earlier learning (The motor effect (F = B I l) and Fleming's left-hand rule; Power and energy in circuits (P = I²R, E = QV)). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. A wire 0.4 m long carrying 5 A sits at right angles to a magnetic field of 0.1 T. What is the force on the wire, in N? → 0.2 NPrompt: F = B × I × l
  2. A current of 2 A flows through a 50 Ω resistor. What power is transferred, in W? → 200 WPrompt: P = I² × R
  3. A 0.2 m wire carrying 4 A at right angles to a magnetic field feels a force of 0.5 N. What is the magnetic flux density, in T? (Give 2 decimal places.) → 0.63 TPrompt: Rearrange F = B I l to find B.

Hook · 5 min

  • Show a circuit with a gap and ask why the bulb doesn't light.
  • Share the objective: Transformers and the National Grid. Read the success criteria together.

I do: model it · 10 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: A transformer has 500 turns on the primary coil and 2500 turns on the secondary. The input voltage is 24 V. What is the output voltage, in V? (Vp ÷ Vs = Np ÷ Ns)
  • Step 1: 24 × 2500 ÷ 500 = 120
  • Step 2: So the answer is: 120 V
  • Answer: 120 V
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. A transformer has 1000 turns on the primary coil and 500 turns on the secondary. The input voltage is 230 V. What is the output voltage, in V? (Vp ÷ Vs = Np ÷ Ns) → 115 VPrompt: More turns on the secondary means a higher output voltage.
  2. A 100 % efficient transformer has an input of 25000 V and 10 A. The output voltage is 400000 V. What is the output current, in A? (Vp × Ip = Vs × Is; give 2 decimal places) → 0.63 APrompt: Power in = power out.

You do: independent practice · 10 min

  • Students work on the student sheet. Circulate and check the first two answers of every student early.
  • Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. A 100 % efficient transformer has an input of 230 V and 5 A. The output voltage is 115 V. What is the output current, in A? (Vp × Ip = Vs × Is; give 2 decimal places) → 10 APrompt: Power in = power out.
  2. Why don't transformers work with a steady dc supply? (They need a changing magnetic field to induce a voltage in the secondary coil / dc has no voltage / The iron core melts / dc is too dangerous) → They need a changing magnetic field to induce a voltage in the secondary coilPrompt: Induction needs change.

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: The motor effect (F = B I l) and Fleming's left-hand rule.
  • Use concrete materials: circuit kits (cells, wires, bulbs).
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.

Free lesson plan from eurekami.com/lesson-plan-builder

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Transformers and the National Grid

Name

Date

Score/ 8

  1. Question 1

    What does a step-down transformer near your home do?

    • A. stores electricity for the night
    • B. turns ac into dc
    • C. reduces the voltage to a safer level, about 230 V in the UK
    • D. increases the voltage for homes

  2. Question 2

    A transformer has 500 turns on the primary coil and 5000 turns on the secondary. The input voltage is 230 V. What is the output voltage, in V? (Vp ÷ Vs = Np ÷ Ns)

    AnswerV

  3. Question 3

    A 100 % efficient transformer has an input of 25000 V and 10 A. The output voltage is 400000 V. What is the output current, in A? (Vp × Ip = Vs × Is; give 2 decimal places)

    AnswerA

  4. Question 4

    A transformer has more turns on its secondary coil than its primary coil. What type is it?

    • A. step-up
    • B. step-down
    • C. a generator
    • D. a motor

  5. Question 5

    A transformer has 100 turns on the primary coil and 5 turns on the secondary. The input voltage is 24 V. What is the output voltage, in V? (Vp ÷ Vs = Np ÷ Ns)

    AnswerV

  6. Question 6

    A 100 % efficient transformer has an input of 25000 V and 5 A. The output voltage is 275000 V. What is the output current, in A? (Vp × Ip = Vs × Is; give 2 decimal places)

    AnswerA

  7. Question 7

    A transformer has 500 turns on the primary coil and 250 turns on the secondary. The input voltage is 120 V. What is the output voltage, in V? (Vp ÷ Vs = Np ÷ Ns)

    AnswerV

  8. Question 8

    Why don't transformers work with a steady dc supply?

    • A. The iron core melts
    • B. They need a changing magnetic field to induce a voltage in the secondary coil
    • C. dc is too dangerous
    • D. dc has no voltage

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Answer key: Transformers and the National Grid

  1. 1. C. reduces the voltage to a safer level, about 230 V in the UK

  2. 2. 2300 V

  3. 3. 0.63 A

  4. 4. A. step-up

  5. 5. 1.2 V

  6. 6. 0.45 A

  7. 7. 60 V

  8. 8. B. They need a changing magnetic field to induce a voltage in the secondary coil