CORE Physics (9223) | Magnetism and electromagnetism

🌟 CORE Physics (9223): Study Notes on Magnetism and Electromagnetism 🌟

Hello future physicists! Ready to dive into a chapter that connects electricity and mysterious invisible forces? Magnetism and electromagnetism are fundamental concepts that explain everything from fridge magnets to massive power tools.

In this chapter, we will learn how magnets work, how electricity can *create* magnetism, and how those magnetic fields can, in turn, create movement. Don't worry if some parts seem like magic at first—we'll break them down step-by-step!

1. Permanent Magnets and Magnetic Fields

1.1 The Basics of Magnets

Every magnet has two ends called poles: the North-seeking pole (N) and the South-seeking pole (S).

• Rule 1: Attraction: Opposite poles attract (N attracts S).
• Rule 2: Repulsion: Like poles repel (N repels N, S repels S).

Analogy: Think of a relationship rule—opposites attract!

1.2 Magnetic Materials

Only certain metals are magnetic. The main magnetic materials are Iron, Cobalt, and Nickel (often remembered as FeCoNi, using their chemical symbols).

• Permanent Magnets: These magnets keep their magnetism all the time (e.g., speaker magnets, compass needles).
• Induced Magnets: These are materials (like a paperclip) that only become magnetic when they are placed near a permanent magnet. They lose their magnetism when the permanent magnet is removed.

1.3 Understanding the Magnetic Field

A magnetic field is the region around a magnet where another magnetic material or magnet experiences a force. It is an invisible force field.

• Direction: We always draw magnetic field lines showing the direction the North pole of a tiny compass would point.
• Key Rule: Field lines always go out of North and into South.
• Strength: The closer the field lines are to each other, the stronger the magnetic field is.

Quick Review: You can map a field using a compass. The compass needle aligns itself with the field lines.

2. Electromagnetism: Making Magnets with Electricity

This is where physics gets really exciting! In the 19th century, scientists discovered that moving charge (electric current) creates a magnetic field. This is called electromagnetism.

2.1 Field around a Straight Wire

When current flows through a straight wire, it creates a magnetic field that is circular around the wire.

To find the direction of this magnetic field, we use the Right-Hand Grip Rule (sometimes called the Corkscrew Rule):

1. Imagine gripping the wire with your right hand.
2. Point your thumb in the direction of the conventional current (Positive to Negative).
3. Your fingers will curl in the direction of the magnetic field lines.

2.2 The Solenoid and Electromagnets

A single straight wire creates a very weak magnetic field. To make it stronger, we coil the wire into a shape called a solenoid (a coil of wire).

• When current flows through a solenoid, the magnetic fields from each loop combine, creating a strong field that looks exactly like the field of a bar magnet (it has a clear North and South pole).
• A solenoid with a current running through it is called an electromagnet.

2.3 Increasing the Strength of an Electromagnet

The great thing about an electromagnet is that you can turn it on and off, and you can easily control its strength. You can increase the magnetic field strength by:

1. Increasing the Current: More current = stronger field.
2. Increasing the Number of Turns: More loops of wire = stronger field.
3. Adding an Iron Core: Inserting a piece of soft iron inside the solenoid massively concentrates and strengthens the field.

Real World Example: Electromagnets are used in large scrapyard cranes to lift heavy metal objects, which they can drop simply by switching off the current!

Key Takeaway: Electricity creates magnetism. The strength of an electromagnet depends on the current, the coils, and the core material.

3. The Motor Effect: Force on a Current

We know current creates a magnetic field. But what happens if you place a current-carrying wire inside the magnetic field of a permanent magnet?

3.1 The Motor Effect Principle

If a wire carrying an electric current cuts across an external magnetic field, it experiences a force. This force is what causes movement in electric motors.

Important Condition: The wire must be at 90 degrees (perpendicular) to the field lines for the force to be maximum.

The factors that increase the size of this force are:

• Increasing the current (I).
• Using a stronger magnetic field (B).

3.2 Determining the Direction of the Force

The direction of the force (the movement) is always perpendicular to both the field and the current. To work out which way the wire will move, we use Fleming’s Left-Hand Rule.

Don't worry if this seems tricky at first! You need to use your left hand and stretch out your thumb, forefinger, and middle finger so they are all at right angles to each other.

Fleming’s Left-Hand Rule Mnemonic

1. Thumb (F): Represents the Force (Movement).
2. Forefinger (B): Represents the B-Field (Magnetic Field Direction: N to S).
3. Middle Finger (I): Represents the I-Current (Direction of Conventional Current: + to -).

Memory Aid: Think of the sequence F-B-I (Force, Field, Current).

3.3 How the Simple Electric Motor Works

The motor effect is used in simple electric motors. A motor contains a coil of wire (armature) placed inside a strong permanent magnetic field.

Step-by-Step Motor Operation:

1. Current flows into the coil.
2. Because the current flows in opposite directions on the two sides of the coil (Side A up, Side B down), the motor effect creates opposite forces (one side pushed up, one side pushed down).
3. These opposite forces cause the coil to rotate.
4. A device called a commutator (and brushes) is used to reverse the direction of the current in the coil every half turn, ensuring the force continues to push the coil in the same direction, keeping the motor spinning constantly.

Did you know? This principle is used not only in fans and drills but also in the voice coils of loudspeakers, making the cone vibrate to produce sound!

Key Takeaway: Magnetism and electricity interact to create a force (the motor effect). Fleming’s Left-Hand Rule predicts the direction of this force.

Summary Review Box for Magnetism and Electromagnetism

Key Terms to Master:

• Permanent Magnet: Keeps magnetism always.
• Induced Magnet: Temporarily magnetic when near a magnet.
• Magnetic Field Lines: Go from North (N) to South (S).
• Electromagnetism: Current creates a magnetic field.
• Solenoid: A coil of wire that intensifies the field.
• Motor Effect: A force experienced by a current-carrying wire in a magnetic field.

Rules to Remember:

1. Attraction/Repulsion: Like poles repel, unlike poles attract.
2. Field Direction (Wire): Use the Right-Hand Grip Rule (Thumb = Current, Fingers = Field).
3. Force Direction (Motor): Use Fleming’s Left-Hand Rule (F-B-I).