Exercises: Electrical and magnetic phenomena

You rub a balloon against your woollen jumper and, when you bring it close to the wall, it sticks to it for a while, as shown in the diagram.

Explain what happened to the balloon when it was rubbed and why it sticks to the wall.

Complete the sentence and give an example of each case: "Two electric charges with the same sign $\dots$ and two charges with opposite signs $\dots$."

Classify these objects as conductors or insulators and justify your choice:

1. a) A steel spoon.
2. b) An eraser.
3. c) A copper wire.
4. d) A cork stopper.
5. e) A coin.

Household electrical cables have copper on the inside and a plastic casing on the outside, as shown in the cross-section in the diagram.

Explain the role played by each material and why they could not be made of a single material.

You bring two magnets together by their north poles. Do they attract or repel each other? What if you bring the north pole of one close to the south pole of the other? State the general rule for magnetic poles.

We scatter iron filings over a piece of paper with a magnet underneath. Describe the pattern formed by the filings and explain what this pattern shows us about the magnet.

Why does a compass needle always point approximately towards the north? What does Earth itself have to do with this behaviour?

Name two everyday devices or objects that work using magnets (at home, in the car, or at school) and explain the role played by the magnet in each one.

After combing your hair with a plastic comb on a dry day, the comb attracts small pieces of paper even though the paper is not charged. Explain how the comb became charged and why it can attract light objects without touching them.

You rub two balloons with the same woollen cloth and, when you release them close to each other, they move apart.

1. a) What type of charge have the two balloons acquired: same sign or opposite signs?
2. b) Why do they repel each other?
3. c) Predict what will happen if you bring one of the balloons close to the woollen cloth.

An electrician's screwdriver has a metal tip and a thick plastic handle. Relate each part to its electrical behaviour (conductor or insulator) and explain why this combination protects the user.

A magnet moves a paperclip without touching it, as shown in the diagram.

1. a) What do we call forces that act without contact between bodies?
2. b) Compare magnetic attraction with gravitational attraction: state one similarity and one important difference.

You bring a magnet close to a compass and the needle deflects from its usual position.

1. a) Why does the needle move?
2. b) What does this indicate about why the compass points north when there are no magnets nearby?

Cards with a magnetic strip, such as some hotel key cards, can stop working if kept next to a powerful magnet. Explain what the magnetic strip stores and what may have happened to it while next to the magnet.

You snap a bar magnet in half and a classmate says you are left with one piece that only has a north pole and another that only has a south pole. Is your classmate right? Justify what actually happens.

We place two magnets underneath a sheet with iron filings: first facing a north pole to a south pole, and then facing two north poles. Describe the differences you would expect to see in the filings' pattern in each case and explain what they indicate about the forces between the magnets.

You hang two cork balls covered in aluminium foil from threads and touch them with a plastic rod that was previously rubbed: the balls move apart.

1. a) Why do they repel each other?
2. b) What would happen if you only touched one of them with the rod?
3. c) Knowing that a glass rod rubbed with silk becomes positively charged, how would you check the sign of the charge on your balls?

Design a simple experiment to find out whether a wooden ruler is a conductor or an insulator using only a battery, a light bulb, and two wires. Describe the setup, what you would observe in each case, and what you would conclude.

An electric motor contains magnets and a coil (loops of wire) through which electric current flows, as shown in the diagram.

Explain qualitatively how the rotational motion is produced and name two household appliances in your home that run on an electric motor.

A loudspeaker converts incoming electric current into sound thanks to a magnet and a coil attached to a paper cone. Order and explain the chain of phenomena from the moment the current arrives until we hear the music.

Earth behaves like a huge magnet.

1. a) What everyday evidence proves this?
2. b) Predict what would happen to compasses and navigation if this magnetism suddenly disappeared.
3. c) The magnetic north pole does not coincide exactly with the geographic North Pole: explain the consequence of this when navigating with a compass.

During a storm, the base of the clouds becomes negatively charged and the ground is left positively charged, as shown in the diagram.

1. a) Explain why lightning eventually strikes between the cloud and the ground.
2. b) Relate lightning to the charging experiments we do in class: name one similarity and one difference.

You have three metal bars that look identical on the outside: two are magnets and the third is just unmagnetised iron. Using only the bars themselves, without a compass or any other objects, explain step by step how to find out which of the three is not a magnet.

On a dry day, you rub a plastic pen against your jumper sleeve and manage to deflect a thin stream of tap water without touching it.

1. a) Explain the phenomenon.
2. b) Predict whether it will work just as well on a very humid day and explain why.
3. c) Would it work the same with a metal rod held in your hand? Justify your answer by thinking about conductors and insulators.

Topic challenge. How do invisible forces compete in your home? Become a researcher and prepare a scientific report solving this challenge:

1. Gravitational force: Drop an eraser from $h = 0,8\text{ m}$, $1,2\text{ m}$, and $1,6\text{ m}$ (uncertainty of $\pm 0,5\text{ cm}$). Time each height three times with your phone (uncertainty of $\pm 0,1\text{ s}$). Construct a table with mean times and uncertainties, and plot the graph of $h$ versus $t^2$ to estimate the acceleration $g$.

2. Electric and magnetic forces: Rub a plastic pen with wool and measure with a ruler the maximum distance at which it attracts paper confetti. Then, measure at what distance a fridge magnet manages to move a metal paperclip.

3. Report and analysis: Compare the range and characteristics of the three forces in a final table. Explain with reasoning how a small electrostatic charge can overcome the gravity of the entire planet to lift the paper.

Assessment criteria:

• Precision in data collection, correct treatment of uncertainties, and rigorous preparation of table and graph.

• Clear understanding and differentiation of gravitational, electrical, and magnetic phenomena.

• Argumentative quality and scientific rigour in the report's conclusions.