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IB DemystifiedMYP Sciences

Electromagnetism and motors

Every fan, washing machine, electric car and hard drive depends on one simple discovery: a wire carrying a current in a magnetic field feels a push. Put that push in a spinning coil and you have an electric motor.

Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D

NSforcewire: current into the page (⊗)magnetic field lines: N → S
Figure 1. A current-carrying wire in a magnetic field experiences a force.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Motors and electromagnets
  6. Electromagnetism in the real world
  7. Worked examples
  8. In the eAssessment
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • describe magnetic fields and magnetic materials
  • explain how electromagnets work and what affects their strength
  • explain the motor effect and use F = BIL
  • explain how d.c. motors, loudspeakers and other devices work
  • analyse data from electromagnet and motor experiments
  • discuss the implications of electric motors and magnet supply

Before you start

You will use these skills. If any feel shaky, review them first.

  • current, voltage and circuits (see Electrical power)
  • forces and turning effects
  • energy transfers and efficiency

Key vocabulary

Magnetic field
The region around a magnet where a magnetic material or current experiences a force.
Electromagnet
A coil of wire, usually around an iron core, that is magnetic only when current flows.
Motor effect
The force on a current-carrying wire in a magnetic field.
Magnetic flux density (B)
The strength of a magnetic field, measured in tesla (T).
Commutator
A split ring that reverses the current in a motor coil every half turn.
Electromagnetic induction
Producing a voltage by changing the magnetic field through a conductor.

Understanding the ideas

  1. What is it?

    Magnets have north and south poles; like poles repel and unlike poles attract. The field around a magnet can be shown with field lines, which point from north to south. Only some materials, such as iron, steel, nickel and cobalt, are magnetic.

  2. Why does it happen?

    A current in a wire creates a magnetic field around it. Winding the wire into a coil and adding a soft-iron core makes a strong electromagnet that can be switched on and off. When a current-carrying wire sits in another magnetic field, the two fields interact and the wire feels a force, F = BIL, whose direction is given by Fleming's left-hand rule.

  3. How do we know?

    The force can be measured with a top-pan balance and a magnet, and field strength with a magnetic field sensor or phone magnetometer. Hans Christian Ørsted discovered in 1820 that a current deflects a compass, and Michael Faraday built the first electric motor in 1821.

  4. Why does it matter?

    Electric motors drive most of the world's machines and a growing share of transport. Electromagnets are used in scrapyard cranes, door locks, relays, loudspeakers and MRI scanners.

  5. What does it connect to?

    Electromagnetism links to circuits and energy in physics, to generating electricity (the reverse process) and to the minerals needed for magnets in Earth science.

Motors and electromagnets

  • Electromagnet strength increases with more turns, more current and an iron core.
  • Motor effect: F = B × I × L (B in tesla, I in amps, L in metres), greatest when current and field are at right angles, zero when parallel.
  • Direction: reversing the current or the field reverses the force (Fleming's left-hand rule: First finger = Field, seCond finger = Current, thuMb = Motion).
  • A d.c. motor spins because opposite forces act on the two sides of the coil; the commutator keeps it turning one way.
  • Faster motor: more current, stronger magnet, more turns.

Electromagnetism in the real world

Ceiling fans, water pumps and electric rickshaws all use electric motors. Recycling plants use electromagnets to lift out steel. Electric cars use regenerative braking, running the motor as a generator to recharge the battery.

Worked examples

Example 1: the motor effect

A 0.05 m wire carries 4.0 A at right angles to a 0.30 T field. Find the force.

  1. F = BIL = 0.30 × 4.0 × 0.05.
  2. F = 0.060 N.

Example 2: finding the current

A 0.20 m wire in a 0.50 T field feels a force of 0.15 N. Find the current.

  1. I = F ÷ (BL) = 0.15 ÷ (0.50 × 0.20).
  2. I = 0.15 ÷ 0.10 = 1.5 A.

In the eAssessment

Electromagnetism questions combine explanations of devices with calculations and data. Expect:

  • Explain how electromagnets, motors and loudspeakers work, step by step.
  • Calculate force, current or field strength with F = BIL, including units.
  • Analyse data from electromagnet and wire-force experiments.
  • Discuss the use of motors and magnets in transport and energy.

Common ways to lose marks: saying all metals are magnetic; forgetting to convert mN to N or cm to m; mixing up the motor effect and induction; and forgetting to explain what the commutator does.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: mapping magnetic fields

Partially guided investigation · about 40 minutes · pairs

Research question
How does the magnetic field pattern between two bar magnets change when unlike poles face each other compared with like poles, and where is each field strongest?
Scientific background
Field lines show the direction a compass needle points. Where lines are close together, the field is stronger. Between unlike poles the lines join up; between like poles they push apart and there is a neutral point.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Two bar magnets, several plotting compasses, A3 paper, pencil, ruler; optionally a phone magnetometer app.
Method
  1. Place one magnet on the paper and draw round it. Move a compass around it, marking the direction of its needle with dots, and join the dots into field lines.
  2. Repeat with two magnets 6 cm apart, first with N facing S, then with N facing N.
  3. Use the phone magnetometer at the same points to compare field strengths.
  4. Mark any neutral point where the compass does not settle in one direction.

Safety. Keep magnets away from phones' bank cards, computers and anyone with a pacemaker; do not let strong magnets snap together on fingers.

Then evaluate: how could you make your field-line drawing more accurate?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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