- Magnet Basics
- Magnetic Field
- Earth & Compass
- Current & Magnetism
- Force on Charge
- Force on Wire
- Parallel Wires
- Applications
- Summary
Welcome back, dear student! In Unit 4, we studied electricity. Now in Unit 5, we will study a closely related topic: Magnetism. In your everyday life, you come across magnets in many places. The speakers on your phone, the fridge door latch, the compass used for direction, and the hard drive in a computer all use magnets. In this unit, we will learn what magnets are, how magnetic fields work, and how electricity can create magnets. Let us build a strong understanding step by step!
5.1 Magnet
Have you ever played with a small horseshoe magnet or a bar magnet as a child? If you bring it near a pin or a nail, the pin jumps up and sticks to it. But if you bring it near a piece of wood or plastic, nothing happens. Why? Because magnets only attract certain materials. Let me explain this deeply.
What is a Magnet?
A magnet is a piece of material that can attract certain metals like iron, steel, nickel, and cobalt. This attractive property is called magnetism. Materials that are attracted by a magnet are called magnetic materials. Materials that are not attracted are called non-magnetic materials.
| Magnetic Materials | Non-Magnetic Materials |
|---|---|
| Iron, Steel, Nickel, Cobalt | Wood, Plastic, Paper, Rubber, Glass |
Magnetic Poles
Every magnet has two ends where its magnetic force is strongest. These ends are called magnetic poles. If you take a bar magnet and dip it in iron filings, the filings cluster most thickly at the two ends. These two poles are called the North-seeking pole (N) and the South-seeking pole (S).
The names come from the fact that if you hang a magnet freely so it can rotate, the N-pole points approximately towards the geographic North of the Earth, and the S-pole points towards the geographic South.
Law of Magnetism
Just like electric charges, magnetic poles follow simple rules of attraction and repulsion.
1. Like poles repel each other: North repels North. South repels South.
2. Unlike poles attract each other: North attracts South. South attracts North.
Types of Magnets
Your textbook explains two main categories:
- Natural magnets: Found in nature. The most famous example is lodestone, which is a naturally magnetized form of iron ore. Ancient people discovered magnetism through lodestone.
- Artificial magnets: Made by humans. These can be temporary magnets (act as magnets only while near another magnet, like soft iron) or permanent magnets (keep their magnetism for a long time, like steel magnets). They come in many shapes: bar magnets, horseshoe magnets, ring magnets, and cylindrical magnets.
- Magnets attract only iron, steel, nickel, and cobalt
- Every magnet has two poles: North (N) and South (S)
- Like poles repel, unlike poles attract
- Magnetic monopoles do not exist (cannot isolate one pole)
- A freely suspended magnet aligns North-South
- Magnetic force is strongest at the poles, weakest at the centre
Practice Questions — Magnet Basics
Explanation: Every piece of a magnet, no matter how small, always has both a North pole and a South pole. Three pieces means 3 × 2 = 6 poles. This tests the rule that magnetic monopoles cannot exist.
Explanation: Magnets attract only iron, steel, nickel, and cobalt. Aluminium is NOT a magnetic material. Many students confuse aluminium with other metals. Always remember the four magnetic elements: Iron (Fe), Nickel (Ni), Cobalt (Co), and some alloys like steel.
Explanation: Like poles repel. North and North are like poles, so they repel. If it were North and South (unlike poles), they would attract.
5.2 Magnetic Field
We know that a magnet can attract a piece of iron without even touching it. How does the force travel across the empty space between them? The answer is the magnetic field. Just like an electric charge creates an electric field around it, a magnet creates a magnetic field around it.
A magnetic field is the region around a magnet where its magnetic force can be detected. Any magnetic material or another magnet placed in this region experiences a force.
Magnetic Field Lines
We represent the magnetic field using magnetic field lines. These are imaginary lines that show the direction and strength of the magnetic field. Your textbook describes their properties very clearly. Let me list every property you must know for the exam.
- Field lines start at the North pole and end at the South pole (outside the magnet)
- Inside the magnet, they go from South to North, forming closed loops
- Field lines never cross each other
- Where field lines are closer together, the field is stronger
- Where field lines are farther apart, the field is weaker
- They are smooth curves, not straight lines (except near the poles)
If a question shows two magnetic field lines crossing each other, that diagram is always wrong. Magnetic field lines never cross because at any single point in space, the magnetic field can only have one direction. If lines crossed, there would be two directions at one point, which is impossible.
Practice Questions — Magnetic Field
Explanation: The magnetic field is strongest where the field lines are closest together. Field lines are densest (most crowded) at the poles. This is why the poles attract magnetic materials most strongly. The centre of the magnet has the weakest field.
Explanation: At any single point in space, the magnetic field has exactly one direction (one tangent). If two field lines crossed at a point, there would be two different directions at that point, which is physically impossible. Option A is wrong because field lines are curves, not straight lines.
5.3 The Earth’s Magnetic Field and the Compass
Did you know that the Earth itself acts like a giant magnet? This is a fascinating fact and it explains why a compass works. Let me explain this carefully because the direction of the Earth’s magnetic poles often confuses students in exams.
Earth as a Giant Magnet
The Earth has a magnetic field around it, as if there is a huge bar magnet inside the Earth. But here is the tricky part: the geographic North pole of the Earth is actually a magnetic South pole! And the geographic South pole is a magnetic North pole.
Geographic North = Magnetic South (under the ground)
Geographic South = Magnetic North (under the ground)
The compass needle’s N-pole points to geographic North because it is attracted by the magnetic South pole buried there.
The Magnetic Compass
A compass is a simple instrument used to find direction. It consists of a small, lightweight magnetized needle that is balanced on a pivot so it can rotate freely. The N-pole of the needle always points towards geographic North (because it is attracted by the Earth’s magnetic South pole under the ground).
Practice Questions — Earth’s Magnetism
Explanation: Unlike poles attract. The compass needle’s North pole is attracted to geographic North, so the magnetic pole under the Earth at that location must be a South pole. If it were also a North pole, the needle would point away, not towards it!
Explanation: When near a strong magnet, the compass responds to the magnet’s field (not the Earth’s field). Like poles repel, so the needle’s N-pole is repelled by the bar magnet’s N-pole. The needle turns so its N-pole points away from the magnet’s N-pole.
5.4 Magnetic Field of a Current-Carrying Conductor
For a long time, people thought electricity and magnetism were completely separate things. But in 1820, a Danish scientist named Hans Christian Oersted made a discovery that changed physics forever. He found that an electric current can create a magnetic field! This discovery proved that electricity and magnetism are closely related.
Oersted’s Discovery
Oersted placed a compass needle near a wire carrying electric current. When the current was switched on, the needle deflected (moved away from North). When the current was switched off, the needle returned to its normal North-South position. This proved that a current-carrying conductor produces a magnetic field around it.
The Right-Hand Grip Rule
To find the direction of the magnetic field around a straight wire, we use the Right-Hand Grip Rule. Your textbook explains this clearly.
Imagine grasping the wire with your right hand so that your thumb points in the direction of the conventional current (from positive to negative). Then your curled fingers show the direction of the magnetic field lines around the wire.
Magnetic Field of a Solenoid
If you bend a straight wire into many circular loops (coils) and pass current through it, you get a solenoid. The magnetic field of a solenoid is much stronger than a straight wire because the fields of all the loops add together inside the coil.
A solenoid with an iron core inside it is called an electromagnet. The iron core greatly increases the strength of the magnetic field because iron is easily magnetized.
- Current: Increasing the current increases the field strength
- Number of turns: More turns of wire = stronger field
- Type of core: A soft iron core makes it much stronger than an air core
For a solenoid, curl the fingers of your right hand in the direction of the current flowing through the loops. Your thumb now points towards the North pole of the solenoid.
Practice Questions — Current and Magnetism
Explanation: Point your right thumb UP (direction of current). Your fingers curl from left to right at the top and from right to left at the bottom. On the right side of the wire, your fingers point UPWARDS. So the field at a point to the right of the wire is upwards.
Explanation: All three factors (current, turns, and core material) affect electromagnet strength. More current = stronger. More turns = stronger. Soft iron core = much stronger. Reversing current only changes the pole direction, not the strength. Plastic is non-magnetic, so it weakens the electromagnet.
5.5 Magnetic Force on a Moving Charge
We know that a magnet exerts a force on another magnet. But what happens if you put a single moving electric charge inside a magnetic field? The scientist Hendrik Lorentz answered this question. A magnetic field exerts a force on a moving charge, but there are specific conditions.
A magnetic field exerts a force on a charge ONLY if:
- The charge is moving (a stationary charge feels no magnetic force)
- The charge is moving NOT parallel to the magnetic field lines
If a charge moves parallel to the magnetic field lines (same direction or opposite direction), the magnetic force is ZERO. The maximum force occurs when the charge moves perpendicular (at 90 degrees) to the field.
$$ F = qvB\sin\theta $$
F = force (N) | q = charge (C) | v = speed of charge (m/s)
B = magnetic field strength (Tesla, T) | θ = angle between v and B
When θ = 90° (perpendicular), sin 90° = 1, so F = qvB (maximum force). When θ = 0° (parallel), sin 0° = 0, so F = 0 (no force). The SI unit of magnetic field strength B is the Tesla (T).
Fleming’s Left-Hand Rule
To find the direction of the force on a positive charge moving in a magnetic field, we use Fleming’s Left-Hand Rule.
Hold your left hand with the thumb, index finger, and middle finger all at right angles to each other:
- Index finger (Forefinger) → direction of the Magnetic field (B)
- Middle finger → direction of the Current or Velocity (v)
- Thumb → direction of the Force (F)
Practice Questions — Force on a Moving Charge
Explanation: When a charge moves parallel to the magnetic field, the angle θ = 0°. Since sin 0° = 0, the force F = qvB × 0 = 0. A charge must have a velocity component perpendicular to the field to experience a force. This is one of the most frequently tested concepts!
Explanation: Index finger (B) points into the page. Middle finger (v, current direction for positive charge) points to the right. Your thumb points UPWARDS. So the force is upwards. If it were a negative charge (electron), the force would be downwards (you would point middle finger in opposite direction to velocity).
5.6 Magnetic Force on a Current-Carrying Wire
A wire carrying current contains many moving charges (electrons). Since each moving charge experiences a force in a magnetic field, the entire wire also experiences a force. This is the principle behind electric motors!
$$ F = BIL\sin\theta $$
F = force (N) | B = magnetic field strength (T)
I = current (A) | L = length of wire in the field (m) | θ = angle between wire and field
This formula comes directly from the charge formula. Since I = Q/t and v = L/t for the charges in the wire, F = qvB becomes F = BIL when perpendicular (θ = 90°). The direction of the force is again found using Fleming’s Left-Hand Rule (middle finger = direction of current).
- Maximum force when wire is perpendicular to field (θ = 90°)
- Zero force when wire is parallel to field (θ = 0°)
- Direction of force: use Fleming’s Left-Hand Rule
- Length L is only the part of the wire INSIDE the magnetic field
Practice Questions — Force on a Wire
Explanation: Since the wire is perpendicular, sin 90° = 1, so F = BIL = 0.4 × 3 × 0.5 = 0.6 N. When perpendicular, you do not need to multiply by sine. Just use BIL directly.
Explanation: When parallel, θ = 0°, and sin 0° = 0. So F = BIL × 0 = 0. A current-carrying wire parallel to a magnetic field experiences no force at all.
5.7 Magnetic Force Between Two Parallel Current-Carrying Wires
This topic surprises many students. We know that each current-carrying wire creates its own magnetic field. So when two wires are placed near each other, each wire sits inside the other wire’s magnetic field. This means each wire exerts a force on the other!
Two parallel wires carrying currents in the SAME direction ATTRACT each other.
Two parallel wires carrying currents in OPPOSITE directions REPEL each other.
This is the OPPOSITE of what happens with static charges (like charges repel). With wires, SAME direction = attract, OPPOSITE direction = repel.
- Charges: Like charges REPEL, unlike charges ATTRACT
- Wires: Same direction currents ATTRACT, opposite direction currents REPEL
- Many students mix these up in exams. Be careful!
Practice Questions — Parallel Wires
Explanation: Same direction currents = attraction. This is a fundamental rule. Remember the trick: it is the opposite of the charge rule. Like charges repel, but same-direction currents attract.
Explanation: Opposite direction currents = repulsion. The wires push each other away. This is why in household wiring, two wires carrying current in opposite directions must be secured firmly to prevent them from pushing apart.
5.8 Applications of Magnetism
Magnetism is not just a textbook topic. It is used in countless real-life applications. Your textbook discusses several of them. Understanding these applications helps you see how physics connects to the real world and often appears in exam questions.
| Application | How Magnetism is Used |
|---|---|
| Electric Motor | A current-carrying coil in a magnetic field experiences a force (F = BIL). This force rotates the coil, converting electrical energy to mechanical energy. |
| Electric Generator | When a coil is rotated in a magnetic field, an EMF (voltage) is induced. Converts mechanical energy to electrical energy. |
| Electromagnet | Used in cranes to lift heavy iron/steel scrap. Used in electric bells, relays, and magnetic locks. The magnet can be turned on and off by switching the current. |
| MRI Scanner | Uses very strong magnetic fields and radio waves to create detailed images of the inside of the human body. No harmful X-rays involved. |
| Magnetic Levitation | Trains (like the Maglev train) float above the track using magnetic repulsion. This eliminates friction and allows very high speeds. |
| Speakers and Earphones | A varying current in a coil near a permanent magnet causes the coil to vibrate. This vibration creates sound waves. |
| Compass | A small permanent magnet that aligns with the Earth’s magnetic field to show direction. |
Notice that many of these applications combine magnetism with electricity (which you learned in Unit 4). This connection between electricity and magnetism is called electromagnetism, and it is one of the most important areas of modern physics.
Practice Questions — Applications
Explanation: A motor uses F = BIL to push a current-carrying coil in a magnetic field, causing rotation. This converts electrical energy to mechanical energy (motion). A generator does the reverse: it converts mechanical energy to electrical energy.
Explanation: An electromagnet is only magnetic when current flows. When you switch off the current, it releases the scrap metal. A permanent magnet is always magnetic and cannot be “turned off,” so you could not release the metal easily.
Complete Unit Summary — Exam Preparation
Excellent work, dear student! You have completed Unit 5 on Magnetism. This unit connects closely with Unit 4 (Electricity). Many concepts here, like Oersted’s discovery and the motor, show how electricity creates magnetism. Here is everything you must remember for your exam.
Force on a moving charge: $$ F = qvB\sin\theta $$
Force on a current-carrying wire: $$ F = BIL\sin\theta $$
Maximum force (perpendicular): $$ F_{max} = qvB = BIL $$
Zero force (parallel): $$ \theta = 0^\circ \Rightarrow F = 0 $$
- Magnets attract iron, steel, nickel, and cobalt only
- Like poles repel, unlike poles attract
- Magnetic monopoles cannot exist (every piece has both poles)
- Magnetic field lines: N to S outside, S to N inside, never cross
- Closer field lines = stronger field
- Geographic North = Magnetic South (underground)
- Oersted discovered that current creates a magnetic field
- Right-Hand Grip Rule: thumb = current, fingers = field direction
- Electromagnet strength depends on current, turns, and core material
- Magnetic force acts ONLY on moving charges (not stationary)
- Force is zero when charge moves parallel to field
- Force is maximum when charge moves perpendicular to field
- Fleming’s Left-Hand Rule: Index = B, Middle = I/v, Thumb = F
- Same-direction currents ATTRACT, opposite-direction currents REPEL
- SI unit of B (magnetic field) = Tesla (T)
✓ Earth’s poles trap: Geographic North = Magnetic South. Never mix them up!
✓ Parallel wires trap: Same direction = ATTRACT (opposite of charges!)
✓ Zero force conditions: Stationary charge OR parallel motion = zero force
✓ Fleming’s rule: Always use LEFT hand for force direction (right hand is for field direction)
✓ Monopole rule: You can never break a magnet to get one pole. Every piece has two poles
✓ Field lines: If a diagram shows crossing lines, the diagram is WRONG
✓ Electromagnet: To make it stronger, increase current, add turns, use iron core
✓ Motor vs. Generator: Motor = electrical to mechanical. Generator = mechanical to electrical
Unit 5 on Magnetism, together with Unit 4 on Electricity, forms the foundation of electromagnetism, which you will study in much more detail in Grades 11 and 12. Understanding the Right-Hand Grip Rule and Fleming’s Left-Hand Rule now will save you a lot of confusion later. Practise drawing the field lines and applying the hand rules until they feel natural. Good luck with your exam, dear student!