Magnets and magnetic fields

Welcome to the World of Magnets!

In this chapter, we are going to explore the invisible forces that make your fridge magnets stick, help birds navigate across the globe, and power the electric motors in everything from your toothbrush to Tesla cars. Magnetism is closely linked to electricity, and understanding one helps you master the other. Don't worry if it feels a bit "mystical" at first—even though we can't see magnetic fields, we can definitely see what they do!

1. The Basics: Poles and Attraction

Every magnet has two ends called poles: a North pole and a South pole. Even if you snap a magnet in half, you'll just end up with two smaller magnets, each with its own North and South pole!

The Golden Rule of Magnetism

Magnets follow a very simple rule when they get near each other:
1. Like poles repel: North pushes away North; South pushes away South.
2. Opposite poles attract: North pulls toward South.

Analogy: Think of it like two identical ends of a battery or the same sides of a Velcro strip that won't stick together. "Likes" just don't want to be roommates!

Quick Review: Identifying Poles

Common Mistake: Students sometimes think magnets attract all metals. This is not true! Only ferromagnetic materials like iron, steel, nickel, and cobalt are attracted to magnets.

Key Takeaway: North attracts South; North repels North.

2. Permanent vs. Induced Magnets

Not all magnets are the same. Some are "always on," while others need a bit of help.

Permanent Magnets

A permanent magnet produces its own magnetic field all the time. It doesn't turn off. A bar magnet or a horseshoe magnet is a perfect example.

Induced Magnets

An induced magnet is a material that becomes a magnet only when it is placed in a magnetic field.
• When you remove the permanent magnet, the induced magnet usually loses most or all of its magnetism quickly.
• Important point: The force between an induced magnet and a permanent magnet is always attractive.

Example: If you rub a permanent magnet against a paperclip, that paperclip can briefly pick up other paperclips. The paperclip has become an induced magnet!

Key Takeaway: Permanent magnets are "always on"; induced magnets only work when near a permanent one.

3. Mapping the Invisible: Magnetic Fields

A magnetic field is the region around a magnet where a force acts on another magnet or a magnetic material.

Magnetic Field Lines

We use "field lines" to visualize the invisible field. There are three rules for drawing them:
1. They always go from North to South.
2. They never cross each other.
3. The closer the lines are, the stronger the field.

Memory Aid: "North to South is the route!"

The Dipping Compass and Earth's Field

A compass contains a tiny bar magnet that points toward the Earth's magnetic North pole. This is proof that the core of the Earth is magnetic! A dipping compass can also show the "dip" of the field, showing that the field lines aren't just flat—they curve into the Earth.

Did you know? The Earth's magnetic North pole is actually a "magnetic South pole" because it attracts the North end of our compasses!

Key Takeaway: Field lines flow North to South. Cluttered lines mean a strong field.

4. Magnetic Fields and Current

When an electric current flows through a wire, it creates a magnetic field around that wire. This is the foundation of electromagnetism.

The Right-Hand Grip Rule

To find the direction of the magnetic field around a straight wire:
1. Point your right thumb in the direction of the current.
2. Curl your fingers as if grabbing the wire.
3. The direction your fingers curl is the direction of the magnetic field circles.

Strength of the Field

The field is stronger if:
• You increase the current.
• You are closer to the wire.

Key Takeaway: Electricity + Wire = Magnetism. Use your right hand to find the direction.

5. Solenoids: Powering Up

A solenoid is just a long coil of wire. By coiling the wire, you bundle the magnetic fields together, making them much stronger.

Inside the Solenoid

The field inside a solenoid is strong and uniform (it goes straight through). Outside, the field looks exactly like a bar magnet.

How to make an Electromagnet stronger:

1. Increase the current.
2. Add more turns (loops) of wire.
3. Place an iron core inside the coil (this becomes an induced magnet and boosts the strength).

Key Takeaway: A solenoid is a "super-wire" coil. Add an iron core to make it a powerful electromagnet.

6. The Motor Effect

When you put a wire carrying a current inside a magnetic field, the wire feels a force. This is called the Motor Effect. The wire literally jumps!

Fleming's Left-Hand Rule

This is the most important trick to learn for your exam! Use your left hand and hold your thumb, first finger, and second finger at right angles to each other.

• First Finger = Field (North to South).
• Second Finger = Current (Positive to Negative).
• Thumb = Motion (The direction the wire will move).

Memory Aid: Use "FBI" — Force (Thumb), B-Field (Index), I-Current (Middle).

Calculating the Force

To find the size of the force, use this formula:
\( Force (N) = magnetic flux density (T) \times current (A) \times length (m) \)
In symbols: \( F = BIl \)

• B is the magnetic flux density, measured in Tesla (T). It’s basically the "strength" of the magnet.

Key Takeaway: Use your left hand to find the direction of movement. Use \( F = BIl \) to find the strength of the push.

7. Electric Motors

An electric motor uses the Motor Effect to create rotation.
1. A coil of wire is placed in a magnetic field.
2. When current flows, one side of the coil is pushed up and the other side is pushed down (based on Fleming's Left-Hand Rule).
3. This creates a spinning motion.

Don't worry if this seems tricky: You don't need to know the complex internal structure of a motor for this specific section, just that the interaction between the magnet and the current causes the rotation!

Key Takeaway: Motors turn electrical energy into kinetic (moving) energy using magnetic forces.

Quick Review Checklist

• Do you know the difference between attraction and repulsion? (Section 1)
• Can you draw magnetic field lines from North to South? (Section 3)
• Can you use the Right-Hand Grip Rule for wires? (Section 4)
• Can you use Fleming's Left-Hand Rule for the "FBI" force? (Section 6)
• Do you know the \( F = BIl \) formula? (Section 6)