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Chemistry | GRADE 11 | QCAA Board
QCAA Physics (Australia) – Grade 12 Electromagnetism
Magnetic Fields – Comprehensive Study Notes
QCAA Physics – Grade 12
1. Concept of a Magnetic Field
Definition
A magnetic field is a region in space where a magnetic force can be experienced by moving charges, electric currents, or magnetic materials.
It is a vector field, meaning it has both magnitude and direction at every point in space.
The symbol used for magnetic field is:
\( \mathbf{B} \)
The SI unit of magnetic field is:
\( \text{Tesla (T)} \)
Key Idea
A magnetic field is produced by:
-
Moving electric charges
-
Electric currents
-
Permanent magnets
-
Changing electric fields
This principle was first demonstrated by Hans Christian Ørsted (1820) when he observed that a current-carrying wire deflects a compass needle.
Magnetic Field vs Electric Field
| Electric Field | Magnetic Field |
|---|---|
| Produced by electric charges | Produced by moving charges |
| Acts on any charge | Acts only on moving charges |
| Symbol: E | Symbol: B |
| Unit: N/C or V/m | Unit: Tesla (T) |
Direction of Magnetic Field
The direction of the magnetic field at a point is defined as:
The direction in which the north pole of a compass needle points.
2. Magnetic Field Lines
Magnetic field lines are imaginary lines used to represent magnetic fields.
They show:
• Direction of the magnetic field
• Strength of the magnetic field
Properties of Magnetic Field Lines
-
Field lines form closed loops.
-
Outside a magnet, they go from North → South.
-
Inside the magnet, they go from South → North.
-
Field lines never intersect.
-
The closer the lines, the stronger the field.
-
Field lines are denser near the poles of a magnet.
3. Magnetic Field Due to a Moving Electric Charge
A moving electric charge produces a magnetic field around it.
If a charge moves with velocity v, the magnetic field forms circular loops around the path of motion.
Important Principle
Moving charges generate magnetic fields.
This is the fundamental origin of all magnetism.
Direction of Magnetic Field (Right-Hand Rule)
To determine the direction of the magnetic field:
Right Hand Rule
-
Point the thumb of your right hand in the direction of motion of the positive charge.
-
Your curled fingers show the direction of the magnetic field lines.
For a negative charge, the direction is opposite.
Diagram – Magnetic Field Around a Moving Charge
4. Magnetic Field Due to Electric Current
Electric current is simply many moving charges.
Therefore, a current-carrying conductor produces a magnetic field around it.
This is the basis of electromagnetism.
Magnetic Field Around a Straight Current-Carrying Wire
The magnetic field forms concentric circular loops around the wire.
Right-Hand Grip Rule
-
Hold the wire with your right hand.
-
Thumb → direction of current
-
Curled fingers → direction of magnetic field.
Diagram – Magnetic Field Around Wire
Field lines form circles around the wire.
Magnetic Field Strength Around a Wire
Magnetic field depends on:
• Current in the wire
• Distance from the wire
\( B \propto I \)
\( B \propto \dfrac{1}{r} \)
Combined relation:
\( B = \dfrac{\mu_0 I}{2\pi r} \)
Where:
( B ) = magnetic field (Tesla)
( I ) = current (A)
( r ) =
distance from wire (m)
\( \mu_0 \) = permeability of free space
\( \mu_0 = 4\pi \times 10^{-7} \) TmA-1
5. Magnetic Field Due to a Circular Current Loop
When current flows in a circular loop, the magnetic field lines resemble those of a bar magnet.
The centre of the loop has a strong uniform magnetic field.
Diagram
The loop behaves like a magnetic dipole.
6. Magnetic Field of a Solenoid
A solenoid is a long coil of wire carrying current.
It produces a uniform magnetic field inside the coil.
Magnetic Field Pattern
Inside the solenoid:
• Field lines are parallel
• Field is uniform
Outside the solenoid:
• Field resembles a bar magnet
Diagram
Inside field is strong and uniform.
7. Magnetic Field Due to a Permanent Magnet
A permanent magnet produces a magnetic field because of aligned magnetic domains inside the material.
Magnetic Field Pattern
Field lines:
• Start at North pole
• End at South pole
8. Comparison of Magnetic Fields
| Source | Shape of Magnetic Field |
|---|---|
| Moving charge | Circular loops |
| Straight current wire | Concentric circles |
| Circular loop | Dipole-like |
| Solenoid | Uniform inside |
| Bar magnet | Dipole field |
9. Key Concept Summary
Magnetic fields are produced by
✔ Moving charges
✔ Electric currents
✔ Permanent magnets
Magnetic field lines
• Show direction and strength
• Form closed loops
• Never intersect
Direction Rules
| Situation | Rule |
|---|---|
| Moving charge | Right-hand rule |
| Straight wire | Right-hand grip rule |
| Solenoid | Right-hand grip rule |
10. Real Life Applications
Magnetic fields are used in many technologies:
Electric Motors
Magnetic fields interact with currents to produce rotation.
Generators
Changing magnetic fields produce electric current.
MRI Scanners
Use strong magnetic fields to image the human body.
Compasses
Use Earth's magnetic field for navigation.
11. Exam-Style Concept Questions (QCAA style)
1. Why does a stationary charge not produce a magnetic field?
Because magnetic fields are produced only by moving charges.
2. Why do magnetic field lines never intersect?
Because at a point the magnetic field can have only one direction.
3. What happens to the magnetic field strength if the current doubles?
Magnetic field doubles.
\( B \propto I \)
4. What happens if distance from the wire increases?
Magnetic field decreases.
\( B \propto \frac{1}{r} \)
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