Electrical and magnetic phenomena

You rub a balloon against your wool sweater 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 $\dots$."

You rub two balloons with the same wool 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 wool cloth.

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.

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

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 the cable could not be made of a single material.

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.

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 hang two cork balls covered in aluminum 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?

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 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.

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 of paper 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.

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.

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

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.

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.

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 objects?
2. b) Compare magnetic attraction with gravitational attraction: state one similarity and one important difference.