Welcome to Magnetism and Electromagnetism!
Ever wondered how your phone vibrates, how a scrap metal crane picks up a car, or even how a compass helps you find your way? It all comes down to magnetism and the way it interacts with electricity. Don’t worry if this seems a bit "invisible" and tricky at first—we are going to break it down into simple, manageable pieces.
This chapter is part of the "Interactions over small and large distances" section of your AQA Synergy course. We will explore how objects can push or pull each other without even touching!
1. The Basics: Magnets and Poles
Every magnet has two ends called poles: a North pole and a South pole. The magnetic forces are strongest at these poles.
Attraction and Repulsion
Magnets follow a simple rule of thumb:
• Like poles repel: Put two North poles (or two South poles) together, and they will push each other away.
• Unlike poles attract: Put a North pole near a South pole, and they will pull together.
This is a perfect example of a non-contact force.
Permanent vs. Induced Magnets
There are two main types of magnets you need to know:
1. Permanent Magnets: These produce their own magnetic field all the time (like a fridge magnet).
2. Induced Magnets: These are magnetic materials that become magnets only when they are placed in a magnetic field.
Important Point: Induced magnetism always causes a force of attraction. As soon as you move the material away from the permanent magnet, it loses most (or all) of its magnetism very quickly.
Quick Review: Think of an induced magnet like a "temporary shadow." It only exists when the "light" (the permanent magnet) is shining on it!
Key Takeaway: Like poles push away; opposite poles pull together. Induced magnets only work when they are near a permanent magnet.
2. Magnetic Fields
A magnetic field is the region around a magnet where a force acts on another magnet or a magnetic material.
Which materials are magnetic?
Not everything is magnetic! You only need to remember four: Iron, Steel, Cobalt, and Nickel.
Memory Aid: S.I.N.C. (Steel, Iron, Nickel, Cobalt).
Mapping the Field
Magnetic fields are invisible, but we can draw field lines to show where they are:
• The force between a magnet and a magnetic material is always one of attraction.
• The field is strongest at the poles.
• The direction of the field always goes from North to South.
Did you know? You can map these lines yourself! If you place a small magnetic compass near a bar magnet, the needle (which is a tiny magnet) will point in the direction of the field. By moving the compass and marking where it points, you can draw the whole pattern.
The Earth is a Giant Magnet
A compass works because the Earth has its own magnetic field! Scientists believe this is caused by movements in the liquid, iron-rich outer core deep inside our planet.
Interesting Fact: The Earth’s magnetic poles actually move and even flip over completely every few hundred thousand years!
Key Takeaway: Magnetic fields go from North to South and are strongest at the poles. The Earth's core makes the planet behave like a giant bar magnet.
3. Electromagnetism: Magnetism from Electricity
In 1820, a scientist discovered that when an electric current flows through a wire, it creates a magnetic field around that wire. This was a huge discovery because it meant we could turn magnetism on and off!
The Field around a Straight Wire
The magnetic field around a straight wire is shaped like a series of concentric circles (circles inside circles).
To find the direction of the field, use the Right-Hand Grip Rule:
1. Make a "thumbs up" gesture with your right hand.
2. Your thumb points in the direction of the current.
3. Your fingers curling around show the direction of the magnetic field.
Solenoids and Electromagnets
A single wire has a weak field. To make it stronger, we loop the wire into a coil called a solenoid.
• Inside a solenoid, the magnetic field is strong and uniform.
• Outside the solenoid, the field looks just like the field of a bar magnet.
• To make it even stronger, we add an iron core. This is now called an electromagnet.
Quick Review: Why use an electromagnet instead of a permanent magnet? Because you can turn it off, and you can change its strength by changing the current!
Key Takeaway: Current through a wire creates a magnetic field. Coiling the wire (solenoid) and adding an iron core creates a powerful electromagnet.
4. The Motor Effect (Higher Tier Only)
When you put a wire carrying a current inside a magnetic field (produced by permanent magnets), the two fields interact. This results in a force being exerted on the wire. This is called the motor effect.
Fleming’s Left-Hand Rule
Don't worry if this feels confusing—there is a simple trick to figure out which way the wire will move. 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 of the Force).
Calculating the Force
The size of the force depends on the strength of the magnetic field (magnetic flux density), the current, and the length of the wire.
The equation is:
\( Force = magnetic\ flux\ density \times current \times length \)
\( F = B \times I \times l \)
• F is force in newtons (N).
• B is magnetic flux density in tesla (T).
• I is current in amperes (A).
• l is length in metres (m).
Key Takeaway: The motor effect happens when a current-carrying wire is "pushed" by a magnetic field. Fleming's Left-Hand Rule helps you find the direction.
5. Electric Motors (Higher Tier Only)
We can use the motor effect to create rotation. A simple electric motor consists of a rectangular coil of wire that is free to turn in a magnetic field.
How it works:
1. Current flows through the coil.
2. The motor effect creates a force on each side of the coil (one side pushes up, the other pushes down).
3. This makes the coil spin.
4. The Split-Ring Commutator: This is a clever little device that reverses the direction of the current every half-turn. This keeps the coil spinning in the same direction.
Common Mistake: Forgetting the commutator! Without it, the coil would just flip back and forth like a pendulum instead of spinning in a full circle.
Key Takeaway: Electric motors turn electrical energy into kinetic energy (movement) using the motor effect and a commutator to keep things spinning.