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Science lesson plan · 11th grade (Year 12 in England) · 30 minutes

The motor effect (F = B I l) and Fleming's left-hand rule

Electricity and magnetism

CCSS NGSS.HS-PS2-5England: GCSE Combined Science: Magnetism and electromagnetism (Higher: motor effect)

Objective and success criteria

Students will: The motor effect (F = B I l) and Fleming's left-hand rule.

  • 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: Electromagnets; Magnetic poles, fields and compasses

Leads to: Transformers and the National Grid

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 · 4 min

  • Quick retrieval from earlier learning (Electromagnets; Magnetic poles, fields and compasses). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. A wire carrying a current has… (a static charge / a gravitational field only / a magnetic field around it / no effect on a compass) → a magnetic field around itPrompt: Place a compass near a wire and switch on.
  2. The north pole of one magnet is brought near the south pole of another. What happens? (they attract / they repel) → they attractPrompt: Like poles behave differently from unlike poles.
  3. An electromagnet picked up these numbers of paper clips: 5 turns: 3 clips 10 turns: 6 clips 15 turns: 9 clips 20 turns: 12 clips If the pattern continues, how many clips would 25 turns pick up? → 15Prompt: How many more clips for every 5 extra turns?

Hook · 3 min

  • Show a circuit with a gap and ask why the bulb doesn't light.
  • Share the objective: The motor effect (F = B I l) and Fleming's left-hand rule. Read the success criteria together.

I do: model it · 7 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: A wire 0.05 m long carrying 5 A sits at right angles to a magnetic field of 0.5 T. What is the force on the wire, in N?
  • Step 1: 0.5 × 5 × 0.05 = 0.125
  • Step 2: So the answer is: 0.125 N
  • Answer: 0.125 N
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 6 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 wire 0.1 m long carrying 10 A sits at right angles to a magnetic field of 0.5 T. What is the force on the wire, in N? → 0.5 NPrompt: F = B × I × l
  2. A 0.2 m wire carrying 4 A at right angles to a magnetic field feels a force of 0.1 N. What is the magnetic flux density, in T? (Give 2 decimal places.) → 0.13 TPrompt: Rearrange F = B I l to find B.

You do: independent practice · 7 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 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. Electric car motors and loudspeakers both rely on which effect? (the photoelectric effect / static electricity / the motor effect: a force on a current in a magnetic field / the greenhouse effect) → the motor effect: a force on a current in a magnetic fieldPrompt: Both contain coils and magnets.
  2. A 0.2 m wire carrying 4 A at right angles to a magnetic field feels a force of 0.3 N. What is the magnetic flux density, in T? (Give 2 decimal places.) → 0.38 TPrompt: Rearrange F = B I l to find B.

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: Electromagnets.
  • 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

  • Move on to: Transformers and the National Grid.
  • 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-11.

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

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The motor effect (F = B I l) and Fleming's left-hand rule

Name

Date

Score/ 6

  1. Question 1

    Which change would NOT increase the force on a current-carrying wire in a magnetic field?

    • A. using a stronger magnet
    • B. turning the wire so it is parallel to the field
    • C. having a longer length of wire in the field
    • D. increasing the current

  2. Question 2

    A 0.5 m wire carrying 2 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.)

    AnswerT

  3. Question 3

    A wire 0.2 m long carrying 4 A sits at right angles to a magnetic field of 0.25 T. What is the force on the wire, in N?

    AnswerN

  4. Question 4

    In Fleming's left-hand rule, what do the first finger, second finger and thumb represent?

    • A. voltage, resistance, power
    • B. current, field, motion
    • C. field, current, motion (force)
    • D. motion, current, field

  5. Question 5

    A 0.2 m wire carrying 4 A at right angles to a magnetic field feels a force of 0.6 N. What is the magnetic flux density, in T? (Give 2 decimal places.)

    AnswerT

  6. Question 6

    A wire 0.1 m long carrying 10 A sits at right angles to a magnetic field of 0.2 T. What is the force on the wire, in N?

    AnswerN

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Answer key: The motor effect (F = B I l) and Fleming's left-hand rule

  1. 1. B. turning the wire so it is parallel to the field

  2. 2. 0.5 T

  3. 3. 0.2 N

  4. 4. C. field, current, motion (force)

  5. 5. 0.75 T

  6. 6. 0.2 N