Electric & Magnetic Forces — Practice · Grade 8 Science

Chapter: Forces

What this practice covers

By the end of Electric & Magnetic Forces you should be able to:

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

Vocabulary used: field, repel, controlled variable, model, evidence, system boundary, assumption, electric charge, polarization, current, coil, electromagnet, compass.

Problem 1

Two north magnetic poles brought together will…
  1. attract
  2. repel
  3. lose all magnetism
  4. always cancel gravity
Answer and explanation

repel

Like poles repel.

Problem 2

Which factor should change in a fair test of magnet separation?
  1. The magnet each trial
  2. The clip each trial
  3. Only the separation
  4. The surface each trial
Answer and explanation

Only the separation

Changing only separation isolates its effect.

Problem 3

Must objects touch for a force to act? Give an example.
Answer and explanation

No; a magnet exerts a force on an iron clip across a gap.

Problem 4

A charged comb attracts small neutral paper pieces. Explain a model without assuming the paper gains a permanent charge. Explain the reasoning in your own words.
Answer and explanation

Charge separation, or polarization, can produce a net attraction even while the paper remains neutral.

The comb's field can slightly rearrange charges in the paper.

Opposite charge closer to the comb can experience a stronger attraction than the farther like charge's repulsion.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 5

Two negative charges are placed near each other. Predict the electric interaction. Show the reasoning, not just the conclusion.
Hints

Distinguish net charge from uneven distribution of charge.

Answer and explanation

They repel: like charges repel in the simple electrostatic model.

Problem 6

A compass responds without touching a magnet. What model explains this?
Hints

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

Answer and explanation

The magnet produces a field at the compass; the compass's magnetic needle experiences a turning effect.

Problem 7

Why not change the test magnet size between positions?
Answer and explanation

That would change another cause of force, confounding the intended distance comparison.

That would change another cause of force, confounding the intended distance comparison.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 8

Illustrative tests use the same magnets and orientation with separations 1, 2 and 3 cm. Force magnitudes are 12, 6 and 3 arbitrary units. A paper barrier at 1 cm still gives 12 units. Identify evidence for an interaction across a gap, describe the distance trend and explain why the barrier does not prove a universal field law.
Answer and explanation

The objects affect one another without direct contact and remain coupled across the supplied paper barrier. The measured force decreases with separation in this range. These illustrative records do not establish a universal inverse-square rule for all magnet geometries or show that every barrier behaves like paper. A field is a model of the spatial influence, not a material stream of air.

The objects affect one another without direct contact and remain coupled across the supplied paper barrier. The measured force decreases with separation in this range. These illustrative records do not establish a universal inverse-square rule for all magnet geometries or show that every barrier behaves like paper. A field is a model of the spatial influence, not a material stream of air.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 9

Use the coil records to ask an investigable question about each changed factor. Then draw a repelling-magnet energy model and contrast the work needed for separating an attractive pair.
Hints

Use the illustrated study titled Field strength and stored energy. Keep the observations, model and conditions separate.

Answer and explanation

At fixed 0.2 A, changing turns from 10 to 20 raises the illustrative lift count 3→5; at fixed 20 turns, current 0.2→0.3 A changes it 5→7. These proxy outcomes support questions and a bounded trend, not a direct calibrated force law. Moving repelling magnets closer requires input work; separating attracting magnets can require input work instead. Energy is associated with the pair’s arrangement and can transfer into later motion.

At fixed 0.2 A, changing turns from 10 to 20 raises the illustrative lift count 3→5; at fixed 20 turns, current 0.2→0.3 A changes it 5→7. These proxy outcomes support questions and a bounded trend, not a direct calibrated force law. Moving repelling magnets closer requires input work; separating attracting magnets can require input work instead. Energy is associated with the pair’s arrangement and can transfer into later motion.

Response criteria

  • Isolate turns/current using the proper records.
  • Distinguish proxy measurement from force.
  • Model potential energy, orientation and work.

Problem 10

An electric-field arrow at a location points upward. Predict force direction on a positive test charge and on a negative test charge. Explain why both cannot simply follow the same arrow.
Answer and explanation

Positive: upward. Negative: downward. Field direction uses the positive-test-charge convention.

Identify the definition of electric-field direction.

Reverse the force direction for a negative test charge.

Response criteria

  • Name both directions.
  • Explain the sign convention.
  • Distinguish field from a path.

Problem 11

A negatively charged balloon attracts neutral paper. Explain the charge shift, why the paper can stay neutral, and why attraction does not prove net positive charge.
Answer and explanation

Bound electron clouds shift slightly, leaving a relatively positive near side and negative far side. Equal total amounts remain; closer attraction can exceed farther repulsion. The paper need not have net positive charge.

Use the polarization figure rather than inventing charge creation.

Separate local charge redistribution from total net charge.

Response criteria

  • Describe the near and far sides.
  • Keep total paper charge zero.
  • Explain the unequal-distance effect.

Problem 12

Use the closed-loop and cut-magnet figures. Give the field direction outside and inside a bar magnet, then predict the poles on each cut piece.
Answer and explanation

Outside N to S; inside S to N. Each cut piece has both N and S poles.

Trace a complete loop.

Identify both poles on each smaller magnet.

Response criteria

  • Give both directions.
  • Keep both poles on each piece.
  • Distinguish magnetic poles from electric charge.

Problem 13

Explain how changing current can make a speaker produce sound. Name the coil, fixed magnet, cone and surrounding air.
Answer and explanation

Changing coil current changes the magnetic interaction and force on the coil. The attached cone vibrates and disturbs air, producing sound waves.

Follow the supplied cutaway connections.

Explain a mechanism between each pair of parts.

Response criteria

  • Connect current to magnetic force.
  • Connect coil to cone motion.
  • Connect repeated cone motion to air vibration.

Problem 14

The supplied clip counts are 3/3/4 at 10 turns, 5/5/6 at 20 turns, and 6/7/6 at 30 turns. Find the means and state a supported conclusion and a limit.
Answer and explanation

Means approximately 3.33, 5.33 and 6.33 clips. Lifting performance increases in this tested range with comparable current and setup. The response varies and is a proxy, not force in newtons or proof of proportionality.

Add each three-trial set and divide by three.

Compare means and retain the individual variation.

Response criteria

  • Calculate all three means.
  • Hold current and setup comparable.
  • State proxy and range limits.

Problem 15

Electric field direction: use a positive test chargeElectric field direction: use a positive test chargePositive sourceNegative source+−Field arrow = force direction for a small POSITIVE test charge.A negative test charge feels electric force in the opposite direction.
The blue arrows show electric-field direction, defined by the force on a small positive test charge. They point away from a positive source and toward a negative source. A negative test charge feels force opposite the field arrow. Lines represent a model at many positions, not strings or paths an object must follow. The drawing shows selected directions in a plane, not every field direction in three dimensions.
For this unit’s illustrated case, what does a field drawing help explain?
Answer and explanation

It represents where and in what direction a source can influence suitable test objects. It does not show physical strings or prove identical force at every location.

A magnet can attract a suitable object before touching it. Compare the same objects at different distances. Do not assume every shiny metal is strongly attracted, because magnetic response depends on the material.

It represents where and in what direction a source can influence suitable test objects. It does not show physical strings or prove identical force at every location.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 16

A small negative charge is placed where an electric-field arrow points left. Predict the electric force direction. Then explain why an attracted neutral paper piece need not acquire a net charge.
Answer and explanation

The negative charge feels force right, opposite the positive-test-charge convention. A neutral paper piece can polarize while keeping balanced total positive and negative charge.

Use the field convention before choosing the test-charge force.

Distinguish charge redistribution from transfer that changes net charge.

The negative charge feels force right, opposite the positive-test-charge convention. A neutral paper piece can polarize while keeping balanced total positive and negative charge.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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