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IB DemystifiedMYP Sciences

Generating electricity

Almost all the electricity you use was made by spinning a magnet near a coil of wire, then sent hundreds of kilometres at half a million volts before arriving at your socket at 230 V. Electromagnetic induction and transformers make the modern grid possible.

Recommended for MYP 5 · eAssessment priority · About 3 lessons · Criteria A, B, C and D

Power stationgenerator25 kVStep-uptransformer500 kVTransmissionlines (pylons)Step-downsubstation11 kVLocaltransformer230 VHomes230 V mainsHigh voltage means a small current,so less energy is wasted heating the cables.
Figure 1. Transformers change the voltage at each stage of the grid.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Key equations
  6. Generating electricity 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:

  • explain electromagnetic induction and what affects the induced voltage
  • explain how generators and power stations produce alternating current
  • explain transformers and use the turns-ratio equation
  • explain why electricity is transmitted at high voltage, using P = VI and I²R
  • analyse data from induction and transformer experiments
  • discuss decisions about the electricity grid

Before you start

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

  • the motor effect and electromagnets (see Electromagnetism and motors)
  • current, voltage, power and resistance (see Electrical power)
  • energy transfers and efficiency

Key vocabulary

Electromagnetic induction
Producing a voltage in a conductor by changing the magnetic field through it.
Generator
A device that induces a voltage by rotating a coil and magnet relative to each other.
Alternating current (a.c.)
Current that repeatedly reverses direction; mains in Pakistan is 50 Hz.
Transformer
Two coils on an iron core that change the size of an alternating voltage.
Step-up / step-down
A transformer that increases / decreases the voltage.
National grid
The network of cables and transformers linking power stations to users.

Understanding the ideas

  1. What is it?

    When the magnetic field through a coil changes, a voltage is induced. Moving a magnet faster, using a stronger magnet or adding more turns increases the voltage. A generator rotates a coil and magnet relative to each other, producing alternating current.

  2. Why does it happen?

    In most power stations, a turbine driven by steam, water or wind turns the generator. Transformers then change the voltage: Vs ÷ Vp = Ns ÷ Np. Transmitting at very high voltage makes the current small, and since the heating loss in cables is I²R, the energy wasted falls enormously.

  3. How do we know?

    Michael Faraday discovered electromagnetic induction in 1831. Today engineers measure grid voltages, currents and losses continuously, and in the laboratory you can measure induced voltages with a data logger.

  4. Why does it matter?

    Reliable electricity underpins homes, hospitals, industry and communications. Grid losses, from heating and theft, are a major cost in many countries, and new grids must connect solar and wind farms that may be far from cities.

  5. What does it connect to?

    This topic links to magnetism and energy in physics, to energy resources and sustainability in Earth science, and to costs and planning in economics.

Key equations

  • Transformer: Vs ÷ Vp = Ns ÷ Np.
  • Ideal transformer power: Vp × Ip = Vs × Is.
  • Power: P = V × I.
  • Power lost in cables: P = I² × R.
  • Induced voltage increases with faster movement, stronger magnet and more turns.

Generating electricity in the real world

Pakistan's grid links hydro stations on the Indus, thermal and nuclear stations, and growing solar and wind farms, using 500 kV and 220 kV lines. Bicycle dynamos, wireless chargers and electric-car regenerative braking all use induction.

Worked examples

Example 1: a transformer

A transformer steps 11 kV down to 230 V. The secondary has 46 turns. How many turns are on the primary?

  1. Np ÷ Ns = Vp ÷ Vs.
  2. Np = 46 × 11 000 ÷ 230.
  3. Np = 2200 turns.

Example 2: transmission current

A power station sends 50 MW at 250 kV. Find the current.

  1. I = P ÷ V = 50 × 10⁶ ÷ 250 × 10³.
  2. I = 200 A.

In the eAssessment

Generating electricity questions combine explanations of devices with calculations. Expect:

  • Explain induction, generators and transformers step by step.
  • Calculate with the turns ratio, P = VI and I²R, converting kV and MW.
  • Analyse induction and transformer data.
  • Discuss grid decisions such as upgrading, burying cables and reducing losses.

Common ways to lose marks: saying transformers work with d.c.; saying high voltage makes electricity travel faster; forgetting to square the current in I²R; and mixing up the motor effect and induction.

Check your understanding

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

Practice questions

Show

Investigation: building a simple transformer

Partially guided investigation · about 45 minutes · pairs

Research question
How does the number of turns on the secondary coil of a simple C-core transformer affect the output voltage, with a fixed primary coil and input voltage?
Scientific background
A transformer's alternating current creates a changing magnetic field in the iron core, which induces a voltage in the secondary coil. The turns-ratio equation predicts the output voltage.
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 C-cores (or a laminated iron core) with clip, insulated wire, low-voltage a.c. supply (maximum 12 V), two a.c. voltmeters or multimeters, connecting leads.
Method
  1. Wind a primary coil of 20 turns and connect it to a 2 V a.c. supply.
  2. Wind secondary coils of 10, 20, 30, 40 and 50 turns in turn and measure the output voltage.
  3. Compare each measurement with the value predicted by the turns-ratio equation.
  4. Repeat with the core clip loosened to see how the core affects the output.

Safety. Use only low-voltage a.c. from the laboratory supply, never mains. Coils can get warm; switch off between readings, and do not leave the circuit on unattended.

Then evaluate: why were your measured voltages lower than predicted, and what could make your transformer more efficient?

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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