Welcome to Magnetism and Electromagnetism!

Hello future Physicists! Don't worry if the words "electromagnetism" sound complicated – this chapter is fascinating and deals with forces you use every single day, from your phone's speakers to powerful cranes.
We will explore how magnets work, how electricity can create magnetism, and how we can use those magnetic fields to make things move (the basis of all motors!). Get ready to connect these two powerful forces!

1. Permanent and Induced Magnetism: The Basics

1.1 Magnetic Materials and Poles

Not all materials are attracted to magnets. Only a few special metals are magnetic.
Key Magnetic Materials (FISC):

  • Ferrite (or just Iron)
  • Iron
  • Steel
  • Cobalt
  • Nickel

Every magnet, no matter its shape, has two ends called poles: the North pole (N) and the South pole (S).

Rule of Attraction and Repulsion:

  • Like poles repel: N repels N; S repels S.
  • Opposite poles attract: N attracts S.
Analogy: Think of magnets like friendship: people who are too similar sometimes clash (repel), but opposites attract!

1.2 Permanent vs. Induced Magnets

Permanent Magnets

These are magnets made of materials like steel (which is hard to magnetise but stays magnetic for a long time).

  • They have a constant magnetic field.
  • They do not need to be near another magnet or electricity to work.
  • Example: Bar magnets, fridge magnets.

Induced Magnets

When a magnetic material (like soft iron) is brought near a permanent magnet, it temporarily becomes a magnet itself. This is called induced magnetism.

  • The induced pole closest to the permanent magnet is always the opposite pole (to ensure attraction).
  • The magnetism is temporary; it disappears as soon as the permanent magnet is removed.
Example: If you touch a steel paper clip to a strong magnet, the paper clip temporarily becomes magnetic and can pick up another paper clip.

Quick Review Box: Magnet Basics

  • FISC materials are magnetic.
  • Opposite poles attract.
  • Induced magnets are always attracted to the permanent magnet that caused them.

2. Magnetic Fields: The Invisible Force

2.1 What is a Magnetic Field?

A magnetic field is the area around a magnet where a magnetic force can be experienced. It is an invisible force field that we represent using magnetic field lines.

2.2 Drawing Magnetic Field Lines

Field lines follow strict rules:

  • They always travel from the North (N) pole to the South (S) pole outside the magnet.
  • They are continuous loops (inside the magnet, they travel S to N).
  • They never cross each other.
  • The closer the lines are to each other, the stronger the magnetic field is (usually strongest at the poles).

Did you know? The Earth itself is a giant magnet! Its magnetic field helps protect us from harmful particles from the Sun.

2.3 Plotting Magnetic Fields with a Compass

A plotting compass is a small compass used to map out the shape and direction of a magnetic field. The needle of the compass always points in the direction of the field line.

Step-by-Step: Plotting a Field
  1. Place a bar magnet on a piece of paper.
  2. Place the compass near the North pole. Mark where the North end of the compass needle points (usually with a small dot).
  3. Move the compass so its tail end is now on the dot you just marked.
  4. Mark the new position of the needle's head.
  5. Repeat this process, creating a chain of dots, until you reach the South pole.
  6. Draw a smooth line connecting the dots, adding an arrow pointing from N to S to show direction.

Key Takeaway: Magnetic field lines show both the strength (density) and direction (N to S) of the magnetic force.

3. Electromagnetism: Electricity Creates Magnetism

3.1 Magnetic Field Around a Current-Carrying Wire

In 1820, Hans Christian Oersted discovered that whenever an electric current flows through a wire, a magnetic field is created around that wire. This is the foundation of electromagnetism.

For a straight wire, the field lines are concentric circles (like ripples in a pond) around the wire.

Determining Field Direction: The Right-Hand Grip Rule

This simple rule helps you remember the direction of the magnetic field (B) given the direction of the current (I).

  1. Grip the wire with your right hand.
  2. Your thumb must point in the direction of the current (I) (from positive to negative).
  3. Your curled fingers show the direction of the magnetic field lines (B).

Memory Aid: RHR (Right Hand Rule) is for finding the Field direction around a Current.

3.2 The Solenoid and Electromagnets

A solenoid is a long coil of wire. When current flows through it, the magnetic fields from each loop combine, creating a powerful field that looks exactly like the field of a bar magnet.

An electromagnet is a solenoid with a piece of soft iron inserted inside the coil (the core).

How to Increase Electromagnet Strength (VITAL points):
  1. Increase the Current (I): More electricity flowing means a stronger field.
  2. Increase the Number of Turns: More loops in the coil means the small fields add up more times.
  3. Use a Soft Iron Core: Soft iron is easily magnetised (induced magnetism) and greatly strengthens the overall magnetic field. (Note: Soft iron is preferred over steel because it quickly loses its magnetism when the current is switched off, which is essential for temporary lifting magnets!)

Key Takeaway: Electricity and magnetism are fundamentally linked. We use coils (solenoids) to create controllable and strong electromagnets.

4. The Motor Effect: Making Things Move

4.1 Principle of the Motor Effect

If you place a current-carrying wire inside an existing magnetic field (created by a permanent magnet), the wire experiences a force. This force is often strong enough to make the wire move. This is known as the Motor Effect.

Why does this happen? The magnetic field created by the current in the wire interacts with the external magnetic field, resulting in a pushing or pulling force.

4.2 Determining Force Direction: Fleming's Left-Hand Rule (LHR)

This rule is essential for determining the direction of the movement (force). You must use your LEFT HAND.

Step-by-Step: Using Fleming's LHR

Hold out your left hand so your thumb, forefinger, and middle finger are all at right angles (90 degrees) to each other:

  1. Thumb (F): Represents the Force or movement (Motion).
  2. Forefinger (B): Represents the external B field (Field direction, N to S).
  3. Middle Finger (I): Represents the I current direction (Conventional Current, + to –).

Memory Aid: F.B.I. (Force, B field, I current). Don't confuse this rule (LHR) with the Right-Hand Grip Rule (RHR) from Section 3.1!

4.3 Factors Affecting the Magnitude of the Force

The size of the force (F) experienced by the conductor depends on:

  • Current (I): Higher current = Stronger Force.
  • Magnetic Field Strength (B): Stronger external magnet = Stronger Force.
  • Length (L): The longer the wire placed inside the field, the stronger the Force.
This relationship is often summarised as \(F \propto BIL\), where B is the magnetic flux density (field strength).

Common Mistake Alert! Always check which hand to use. RHR for finding the field around a wire. LHR for finding the force on a wire when placed in an external field.

5. Applications of Electromagnetism and the Motor Effect

5.1 The Simple D.C. Motor

A D.C. motor uses the motor effect to convert electrical energy into kinetic (movement) energy.

The components are:

  • A coil (armature) placed between two strong permanent magnets.
  • A commutator (a split ring).
  • Brushes (to supply current to the commutator).

How it Works (The Concept):
  1. Current flows into the coil.
  2. Due to the Motor Effect (LHR), one side of the coil experiences an upward force, and the opposite side experiences a downward force.
  3. These forces cause the coil to rotate.
  4. When the coil reaches the vertical position, the commutator swaps the connections, reversing the direction of the current in the coil.
  5. Reversing the current means the forces now point in the opposite direction, which keeps the rotation continuous (it stops the coil from swinging back).

5.2 Loudspeakers

Loudspeakers rely on the motor effect to produce sound.

  1. A signal (varying electrical current) from an amplifier is sent into a coil of wire attached to a cone (or diaphragm).
  2. This coil is placed over the pole of a powerful permanent magnet.
  3. The varying current in the coil creates a continually changing force (Motor Effect).
  4. The changing force causes the coil and the cone to vibrate back and forth rapidly.
  5. This vibration creates pressure waves in the air, which we hear as sound.

Final Key Takeaway: Magnetism and electricity are inseparable. We use the field created by one force to create motion or sound through the other force!

You have completed the Magnetism and Electromagnetism chapter. Great work!