Physics notes · Chapter 10 of 11
Magnetism & Electromagnetism
15 sections, 72 flashcards. Open a card to check your answer, or revise them in the app with spaced revision.
What Is a Magnet?
- A magnet attracts iron and a few similar metals; hung freely, it points roughly north-south. Magnetism is this invisible pull, acting without touching.
- Magnetic materials: iron, nickel, cobalt and steel. Non-magnetic: wood, plastic, paper, rubber, glass.
Check yourself
Define a magnet
An object that attracts iron and a few similar metals.
Which way does a freely hung magnet point?
Roughly north-south.
Name the magnetic materials
Iron, nickel, cobalt and steel.
Name non-magnetic materials
Wood, plastic, paper, rubber, glass.
Surprising non-magnetic metals
Copper, gold and aluminium are non-magnetic though they are metals.
What is magnetism?
The invisible pull that acts without touching the object.
Discovery and Types of Magnets
- The first magnets were natural rocks, lodestone, made of the iron mineral magnetite; the names come from Magnesia, where such rocks were found.
- The horseshoe shape brings both poles close, making the pull stronger.
- Types of Magnets
| Magnet type | Shape |
|---|---|
| Bar magnet | Straight bar |
| Horseshoe magnet | U-shaped |
| Cylindrical magnet | Round rod |
| Ball-ended magnet | Rounded ends |
Check yourself
What were the first magnets?
Natural rocks called lodestone, found in nature.
Lodestone is made of…
The iron mineral magnetite, which attracts iron.
Where do the names magnet/magnetite come from?
Magnesia, where such rocks were found.
Why is the horseshoe shape used?
It brings both poles close, making the pull stronger.
Poles of a Magnet
- The poles are where the pull is strongest (usually the ends), where iron filings cluster thickly.
- The end that turns to the Earth's north is the North pole (N), the other the South pole (S). Every magnet has two, whatever its shape.
Cut a magnet and you get two complete magnets, each with its own N and S.
Check yourself
Define the poles of a magnet
The two places (usually the ends) where the pull is strongest — iron filings cluster thickly there.
North pole vs South pole
The end turning toward Earth's north is the North pole (N); the end facing south is the South pole (S).
How many poles does every magnet have?
Always two — one N and one S, whatever its shape.
Has a magnetic monopole ever been found?
No — a single lone pole has never been found.
What happens if you cut a magnet?
You get two complete new magnets, each with its own N and S.
Law of Magnetic Poles
- Like poles repel; unlike poles attract: N-N and S-S push apart, N-S pull together.
- Magnetic force is a non-contact force, acting across a gap like gravity.
- An iron nail is only ever attracted, never repelled, so it is not a true magnet.
- Law of Magnetic Poles
| Poles facing | Result |
|---|---|
| N - N | Repel |
| S - S | Repel |
| N - S | Attract |
Check yourself
State the basic law of poles
Like poles repel; unlike poles attract. N-N and S-S push apart; N-S pulls together.
Is magnetic force a contact force?
No — it is a non-contact force, acting across a gap like gravity.
What is the sure test of magnetism?
Repulsion, not attraction.
Why is an iron nail not a true magnet?
It is only ever attracted, never repelled.
Making a Magnet and the Compass
- Single-touch method: stroke an iron bar with one pole, in one direction only, about 30–40 times.
- A compass is a small magnetised needle pivoted to spin freely; it settles north-south, showing the way.
Check yourself
Describe the single-touch method
Stroke an iron bar with one pole, in one direction only, about 30-40 times.
Why not rub back and forth?
Rubbing back and forth would undo the effect.
Define a compass
A tiny magnetised needle pivoted to spin freely.
Where does the compass needle settle?
In the north-south direction, helping find the way.
Magnetic Field and Field Lines
- The magnetic field is the region where a magnet's force can be felt.
- Field lines run from N to S outside the magnet and from S to N inside, so each is a closed loop. Crowded lines mean a strong field (near the poles); spread lines a weak one.
- A compass needle lines up along the field line at any point.
Check yourself
Define the magnetic field
The region where a magnet's force can be felt.
Direction of field lines outside and inside a magnet
Outside: North pole to South pole. Inside: S to N, so each line is a complete closed loop.
Can field lines cross?
Never.
Crowded vs spread field lines
Crowded = strong field (near poles); spread = weak field.
How does a compass needle behave in a field?
It always lines up along the field line at any point.
Earth as a Magnet
- The Earth behaves like a giant bar magnet, with a very weak field of only about 0.25 to 0.65 gauss.
- Magnetic declination: the angle between geographic north and magnetic north.
Check yourself
Earth behaves like…
A giant bar magnet with its own field.
Where is Earth's magnetic South pole?
At Earth's geographic North — which is why a compass N points north.
Strength of Earth's magnetic field
Very weak, only about 0.25 to 0.65 gauss.
Define magnetic declination
The angle between geographic and magnetic north.
Unit of Magnetic Field
- Field strength is magnetic flux density, symbol B.
- SI unit: the tesla (T), after Nikola Tesla; 1 T = 1 weber per square metre (Wb/m²). CGS unit: the gauss (G), after C. F. Gauss.
One tesla exerts 1 newton on 1 metre of wire carrying 1 ampere: N/(A·m).
- Units of Magnetic Field
| Quantity | Unit | Note |
|---|---|---|
| Magnetic field (B) | tesla (T) | SI unit |
| Magnetic field (B) | gauss (G) | CGS, 1 T = 10⁴ G |
| 1 tesla equals | 1 Wb/m² | weber per m² |
Check yourself
What is field strength called?
Magnetic flux density, with symbol B.
SI unit of magnetic field
The tesla (T), named after Nikola Tesla.
CGS unit of magnetic field
The gauss (G), named after C. F. Gauss.
1 tesla in weber terms
1 tesla = 1 weber per square metre (Wb/m²).
Conversion to memorise
1 tesla = 10,000 gauss (10⁴ G).
Define one tesla via force
The field that exerts 1 newton on 1 metre of wire carrying 1 ampere — i.e. N/(A·m).
Magnetic Materials (Classification)
- Diamagnetic materials are weakly repelled (copper, bismuth, water); paramagnetic ones weakly attracted (aluminium, platinum), because their unpaired electrons align with the field; ferromagnetic ones strongly attracted (iron, nickel, cobalt).
- A soft magnet (soft iron) loses its magnetism easily; a hard magnet (steel) keeps it.
- Magnetic Materials
| Type | Behaviour | Example |
|---|---|---|
| Diamagnetic | Weakly repelled | Copper, bismuth |
| Paramagnetic | Weakly attracted | Aluminium, platinum |
| Ferromagnetic | Strongly attracted | Iron, nickel, cobalt |
Check yourself
Define diamagnetic materials
Weakly repelled by a magnet — e.g. copper, bismuth, water.
Define paramagnetic materials
Weakly attracted by a magnet — e.g. aluminium, platinum.
Define ferromagnetic materials
Strongly attracted — e.g. iron, nickel, cobalt.
What causes paramagnetism?
Unpaired electrons that align with the field.
Soft vs hard magnet
A soft magnet (soft iron) loses magnetism easily; a hard magnet (steel) keeps it.
Electromagnetism and the Electromagnet
- Oersted: a current-carrying wire produces a magnetic field around it. Coiled into a solenoid, the wire makes a strong, bar-magnet-like field.
- An electromagnet is a soft-iron core wrapped with a current-carrying coil; more turns and more current make it stronger.
- Uses: electric bells, cranes, relays and loudspeakers.
Check yourself
Oersted's discovery
A current-carrying wire produces a magnetic field around it.
What is a solenoid?
A coiled wire that makes the field strong and bar-magnet-like.
Define an electromagnet
A soft-iron core wrapped with a current-carrying coil.
What increases an electromagnet's strength?
More turns and more current.
Key advantage of an electromagnet
It works only while current flows, so it can be switched on and off.
Uses of electromagnets
Electric bells, cranes, relays and loudspeakers.
Motor Effect and Defining the Tesla
- A current-carrying conductor in a magnetic field feels a force: the principle of the electric motor, which turns electrical energy into mechanical (rotational) energy.
- The direction of the force is given by Fleming's left-hand rule (the motor rule).
Check yourself
What happens to a current-carrying conductor in a magnetic field?
It experiences a force — the working principle of the electric motor.
What does an electric motor convert?
Electrical energy into mechanical (rotational) energy.
Which rule gives the direction of this force?
Fleming's left-hand rule (the motor rule).
Electromagnetic Induction and the Dynamo
- Electromagnetic induction (Faraday, 1831): a changing magnetic field induces a current in a coil. The direction comes from Fleming's right-hand rule (the generator rule).
- A dynamo or generator turns mechanical energy into electrical energy; an AC generator makes alternating current with a coil rotating in a field.
Check yourself
What does a dynamo / generator convert?
Mechanical energy into electrical energy.
What does an AC generator produce?
Alternating current, by a coil rotating in a field.
Define electromagnetic induction
A changing magnetic field induces a current in a coil (Faraday, 1831).
Which rule gives the induced current direction?
Fleming's right-hand rule (the generator rule).
Frequency of Indian mains AC
50 hertz.
Transformer
- A transformer changes the voltage of alternating current (AC only) by mutual electromagnetic induction between two coils.
- Step-up: more secondary turns, higher voltage. Step-down: fewer secondary turns, lower voltage.
Transformers do not work on DC: DC gives no changing field.
- Transformers
| Transformer | Voltage | Secondary turns |
|---|---|---|
| Step-up | Increases | More than primary |
| Step-down | Decreases | Fewer than primary |
Check yourself
What does a transformer change?
The voltage of alternating current (AC only).
What principle does it work on?
Mutual electromagnetic induction between two coils.
Step-up vs step-down transformer
Step-up increases voltage (more secondary turns); step-down decreases voltage (fewer secondary turns).
What type is a mobile phone charger?
A step-down transformer (plus rectifier).
Do transformers work on DC?
No — DC gives no changing field.
Electromagnetic Waves and Spectrum
- Electromagnetic waves carry energy as combined electric and magnetic fields, need no medium, and all travel at the speed of light, 3 × 10⁸ m/s.
- Radio waves: broadcasting and mobile communication. X-rays: medical imaging of bones.
- EM Waves and Their Common Uses
| EM wave | Common use |
|---|---|
| Radio waves | TV, radio, mobile |
| Microwaves | Ovens, radar |
| Infrared | TV remote, heating |
| X-rays | Bone imaging |
| Gamma rays | Cancer therapy |
Check yourself
What do electromagnetic waves carry?
Energy as combined electric and magnetic fields.
Speed of all EM waves in vacuum
The speed of light, 3 × 10⁸ m/s.
Do EM waves need a medium?
No — they can travel through empty space.
What are infrared rays used for?
TV remote controls (and heating).
What are radio waves used for?
Broadcasting and mobile communication.
What are X-rays used for?
Medical imaging of bones.
Applications: MRI and Maglev
- MRI (Magnetic Resonance Imaging) scans the body with strong magnetic fields and radio waves, imaging soft tissues, the brain and organs without X-rays.
- Maglev trains float and move by magnetic levitation.
- Magnets also work in loudspeakers, microphones and hard disks; magnetic strips store data on ATM and credit cards.
Check yourself
What does MRI stand for and use?
Magnetic Resonance Imaging — scans the body using strong magnetic fields and radio waves.
What does MRI image best?
Soft tissues, brain and organs, without X-rays.
How do Maglev trains work?
They float and move using powerful magnetic levitation.
Other everyday uses of magnets
Loudspeakers, microphones and hard disks; magnetic strips store data on ATM and credit cards.
All 11 chapters of Physics notes
- Units, Measurement & Physical Quantities11 sections
- Motion, Laws of Motion & Forces13 sections
- Gravitation11 sections
- Work, Power, Energy & Conservation13 sections
- Properties of Matter & Fluids12 sections
- Heat & Thermodynamics12 sections
- Waves & Sound11 sections
- Light & Optics15 sections
- Current Electricity13 sections
- Magnetism & Electromagnetism15 sections
- Modern & Nuclear Physics16 sections