Magnetism & Electromagnetism — Practice · Grade 12 Science

Chapter: Electricity, Magnetism & Circuits

What this practice covers

By the end of Magnetism & Electromagnetism you should be able to:

11 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: magnetic field, electromagnet, electromagnetic induction, electric motor, generator.

Problem 1

The north poles of two magnets are brought close together. They will:
  1. stick together
  2. repel each other
  3. lose their magnetism
  4. attract strongly
Answer and explanation

repel each other

Like magnetic poles repel, just as like charges do, so two north poles push apart.

Problem 2

An electromagnet is created by:
  1. rubbing a balloon to charge it
  2. rubbing a magnet on a piece of iron
  3. passing an electric current through a coil of wire
  4. cooling a wire until it is very cold
Answer and explanation

passing an electric current through a coil of wire

A current flowing through a coil produces a magnetic field, forming an electromagnet that can be switched on and off.

Problem 3

A current is induced in a coil of wire only when:
  1. the wire is warm
  2. the coil is made of copper
  3. a magnet is held motionless inside it
  4. the magnetic field through it is changing
Answer and explanation

the magnetic field through it is changing

Electromagnetic induction requires a changing magnetic field; a stationary field induces no current.

Problem 4

A wire 0.5 m long carries a current of 4 A perpendicular to a uniform 0.2 T magnetic field. Find the force on the wire. (F = BIL)
Answer and explanation

0.4 N

F = BIL = 0.2 × 4 × 0.5 = 0.4 N. The BIL form applies because current and field are perpendicular; otherwise include sinθ.

Problem 5

An electric motor converts:
  1. electrical energy into mechanical (motion) energy
  2. mechanical energy into electrical energy
  3. heat energy into light energy
  4. sound energy into motion
Answer and explanation

electrical energy into mechanical (motion) energy

A motor uses the force on a current-carrying coil in a magnetic field to turn electrical energy into rotational motion.

Problem 6

Explain how a generator produces electricity, and why it can be described as an electric motor run in reverse.
Answer and explanation

A generator spins a coil of wire inside a magnetic field (or spins a magnet near a coil). The constantly changing magnetic field through the coil induces a voltage by electromagnetic induction, driving a current. It is a motor in reverse because a motor takes in electricity to produce motion, while a generator takes in motion to produce electricity — the same parts, run the opposite way.

Problem 7

Find force magnitude for q=2 C, v=3 m/s, B=0.5 T at 90° in an idealized case. Show the reasoning, not just the conclusion.
Hints

Zero force here does not mean no magnetic field.

Answer and explanation

F=2×3×0.5=3 N.

Problem 8

Why can changing magnetic flux induce circuit current?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

A changing flux produces an induced emf; current depends on the circuit being closed and its impedance.

Problem 9

Magnetic flux includes field, area and orientationMagnetic flux includes field, area and orientationarea normalBΦB=BAcosθ for a uniform field; θ from area normal.
Original tilted-area and normal schematic; arrow directions clarify angle definition rather than a numerical geometry.
Interpret the supplied visual. What flux results if B lies parallel to the loop plane?
Answer and explanation

Zero for a uniform field and flat loop, because θ=90° from the area normal.

Magnetic flux measures the field passing through an oriented surface. For a uniform field and flat area, ΦB=BAcosθ, where θ is between B and the area normal, not the surface itself. A loop can have nonzero field but zero net flux when its plane is parallel to B. Flux is measured in webers and differs from field strength in teslas. Changing field, loop area or orientation can change flux.

Zero for a uniform field and flat loop, because θ=90° from the area normal.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

A charge+2 μC moves at 3.0×10⁴ m/s perpendicular to a 0.20 T field. Find magnetic force magnitude. What happens if velocity becomes parallel to the field?
Answer and explanation

F=|q|vB=0.012 N. Parallel motion gives sin 0=0 and zero magnetic-force magnitude in this model; electric forces would need a separate account.

F=|q|vB=0.012 N.

Parallel motion gives sin 0=0 and zero magnetic-force magnitude in this model; electric forces would need a separate account.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 11

OpenSciEd sketch of a wire coil facing a separate magnet across a gap; the diagram is a template for adding the two interaction-force arrows.
The source sketch omits current direction, pole labels and a completed circuit, so attraction/repulsion or induction cannot be inferred from it alone. Coil and magnet before force arrows are added · OpenSciEd · 8.3 Forces at a Distance, Lesson 2, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
Assume explicitly that the energized coil and magnet attract. Place their interaction-force directions and explain whether a stationary unchanged arrangement proves continuous induced current.
Answer and explanation

Force on the coil points toward the magnet; force on the magnet points toward the coil. They are equal/opposite and act on different bodies. Attraction was supplied as an assumption, not inferred from absent pole/current labels. An unchanged linked flux does not continuously induce emf; current also needs the appropriate conducting path and energy account.

Read source labels, scope, axes and intentional omissions.

Use the complete local explanation to draw a justified mechanism or calculate a supported quantity.

State what is observed, what is assumed and what evidence remains needed.

Response criteria

  • Interpret the source labels, units and scope accurately.
  • Explain the relevant mechanism with a labeled model or supported calculation.
  • Identify a meaningful assumption, uncertainty or missing comparison.

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