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21.0Electromagnetic Induction and Electromagnetism

Overview: Electromagnetism: Induction, Generators, Transformers and Electromagnetic Waves — American System Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 2 of 5 Unit quiz

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Lesson summary

Explain electromagnetic induction.Transformers work only with alternating current.Power lines use high voltage to reduce I²R losses.All EM waves travel at c in vacuum.Eddy currents cause magnetic braking.Accelerating charges produce EM waves (antennas).Explain electromagnetic ind…

  • A transformer has 100 primary and 2000 secondary turns; 120 V in. Output?
  • EMF (motional)
  • Faraday's law
  • Lenz's law
  • Electromagnetic wave
  • Mass spectrometer

Section 1 of 7Overview: Electromagnetism: Induction, Generators, Transformers and Electromagnetic Waves

1
21.1 Electromagnetism: Induction, Generat… concept map: A transformer has 100 primary, Electromagnetic induction, EMF (motional), Faraday's law, Lenz's law
Visual guide to the key ideas explained in this lesson.
2
AC generators produce sinusoidal voltage.
3
Effective (rms) voltage = Vmax/√2.
4
Self-inductance opposes changes in current.
5
Induction cooktops use eddy currents.
6
Radio waves have the longest wavelengths.

Section 2 of 7Process & sequence

1
Identify a changing magnetic flux.
2
Calculate EMF (BLv or Faraday's law).
3
Use Lenz's law for direction.
4
For transformers, apply the turns ratio.
5What the symbols mean:
Power in ≈ power out: VpIp = VsIs.
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil

Generator principlewrite these steps in this order; each step is a marking point.

  1. Coil rotates
  2. Flux changes
  3. EMF induced
  4. Current flows
  5. AC output

Section 3 of 7Compare

Transformers
TypeTurnsVoltageCurrent
Step-upNs > NpIncreasesDecreases
Step-downNs < NpDecreasesIncreases
Electromagnetic spectrum
RegionUse
RadioBroadcasting
MicrowaveCooking, radar
InfraredHeat imaging
VisibleSight
UltravioletSterilisation
X-rayMedical imaging
GammaCancer treatment

Section 4 of 7Worked examples

  1. A transformer has 100 primary and 2000 secondary turns; 120 V in. Output?

Section 5 of 7Key terms

Section 6 of 7Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 7 of 7Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Electromagnetic induction · EMF (motional) · Faraday's law · Lenz's law · Generator · Transformer · Electromagnetic wave · Mass spectrometer
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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