Rocket arrives with a compass from a drawer. The needle swings and settles pointing one way.
"Nothing is touching it," he says. "So something is pulling it. A field."
Raven turns the compass. The needle swings back to the same direction. "What do you notice? It always finds the same way."
"Earth is the magnet," Rocket says. "A giant one. And the needle is the test object."
Nova hovers above the compass. "You have used a field every day of this course," she says. "Can you name it?"
Rocket looks at the ball by the door. "Gravity. Every drop test. Every bounce. Every throw that came back down."
Raven writes it in the Field Log. "Fields everywhere, and we never saw one."
A physics text explains that Earth acts like a very large bar magnet. A compass needle is a small magnet that lines up with Earth's field.
The end of the needle that points toward the north is called the north pole of the needle.
Magnets have a second power. Only certain materials, such as iron, cobalt and nickel, show strong magnetic effects. Scientists call them ferromagnetic.
That is why a fridge magnet sticks to the iron in a refrigerator door but not to a wooden table.
Electric current can also make a magnet. Early in the 1800s a Danish scientist found that a compass needle was deflected by a wire carrying current.
Magnets made with current are called electromagnets. They are used for everything from a crane lifting scrapped cars to medical imaging machines.
Static electricity is everywhere too. Rubbing creates the spark you feel after walking across a wool carpet. Static cling comes from clothes rubbing in a dryer.
And gravity? The field around Earth pulls on every ball at the Field. Dropped, kicked or thrown, every one came back down.
| Everyday thing | Which field | Touch needed? |
|---|---|---|
| A compass needle swinging north | magnetic (Earth's) | no |
| A sock clinging in the dryer | electric (charge from rubbing) | no |
| A crane lifting scrap iron | magnetic (an electromagnet) | no |
| A dropped ball falling | gravitational (Earth's) | no |
| Week review | True or false? |
|---|---|
| Magnets and charged objects can push or pull across a gap. | ? |
| Like poles attract and unlike poles repel. | ? |
| The force between two charges weakens as they move apart. | ? |
| A field is a map of the force around an object. | ? |
| Gravity can repel as well as attract. | ? |
Strong week, physicist. Tomorrow is Game Day: a family magnet maze and a tape strip race.