1) Magnet
A magnet is a material or object that attracts magnetic substances such as iron, nickel, and cobalt. It has two poles, called the north pole and south pole.
A magnet is used in a compass, electric motor, generator, loudspeaker, and many devices of daily life.
Properties of a magnet
- It attracts magnetic materials.
- Like poles repel each other.
- Unlike poles attract each other.
- Poles always occur in pairs.
- When freely suspended, a magnet points in the north-south direction.
- The magnetic force is maximum near the poles.
- A magnet can influence objects without touching them.
Like poles → repel
Unlike poles → attract
Freely suspended magnet → north-south direction
2) Magnetic field
The region around a magnet or a current-carrying conductor where magnetic force can be felt is called the magnetic field. It is a vector quantity because it has both magnitude and direction.
Magnetic field lines
Magnetic field lines are imaginary lines used to represent the magnetic field. Outside a magnet, they go from the north pole to the south pole. Inside the magnet, they go from the south pole to the north pole.
- Field lines never intersect each other.
- The closer the lines, the stronger the magnetic field.
- At the poles, field lines are crowded, so the field is strongest there.
How to find magnetic field lines
Magnetic field lines can be found by using iron filings, a compass needle, or by drawing them using the direction rules. When iron filings are sprinkled around a magnet, they arrange themselves in a pattern that shows the field lines.
3) Bar magnet and solenoid
A current-carrying solenoid behaves like a bar magnet. A solenoid is a long coil of insulated wire wound in the form of a cylinder. When current flows through it, it develops a north pole at one end and a south pole at the other.
Similarities of bar magnet and solenoid
- Both have two poles: north and south.
- Both show a magnetic field pattern similar to a bar magnet.
- Both have the strongest magnetic field at the ends.
- Both obey the same idea of attraction and repulsion.
- Both can be used to study magnetic field lines.
Difference between bar magnet and solenoid
| Bar magnet | Solenoid |
|---|---|
| Permanent magnet | Temporary magnet when current flows |
| Fixed poles | Poles depend on current direction |
| Cannot easily change strength | Strength can be changed by current |
| Used as a natural permanent magnet | Used in electromagnets and devices |
4) Direction rules
Right hand thumb rule
This rule is used to determine the direction of the magnetic field around a straight current-carrying conductor. Hold the conductor in your right hand so that the thumb points in the direction of current. The curled fingers then show the direction of the magnetic field lines.
It is very useful when drawing the circular magnetic field around a wire. If the current direction changes, the direction of magnetic field also changes.
How to use the right hand thumb rule
- Grip the wire with your right hand.
- Point the thumb in the direction of current.
- Wrap the fingers around the wire naturally.
- The curl of the fingers gives the direction of magnetic field lines.
Thumb → Current direction
Curled fingers → Magnetic field direction
Fleming’s right hand rule
Fleming’s right hand rule is used to determine the direction of induced current in an electric generator. Stretch the thumb, forefinger, and middle finger of the right hand so that they are mutually perpendicular to each other.
The forefinger shows the magnetic field, the thumb shows the motion of the conductor, and the middle finger shows the induced current. This rule is used in electromagnetic induction.
How to use Fleming’s right hand rule
- Stretch the thumb, forefinger, and middle finger of the right hand mutually perpendicular.
- Forefinger gives the direction of magnetic field.
- Thumb gives the direction of motion of the conductor.
- Middle finger gives the direction of induced current.
Forefinger → Magnetic field
Thumb → Motion of conductor
Middle finger → Induced current
Fleming’s left hand rule
Fleming’s left hand rule is used in electric motors to determine the direction of force on a current-carrying conductor placed in a magnetic field. Stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular to one another.
In this rule, the forefinger shows magnetic field, the middle finger shows current, and the thumb shows force or motion. This rule is very important in motor questions.
How to use Fleming’s left hand rule
- Stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular.
- Forefinger gives magnetic field direction.
- Middle finger gives current direction.
- Thumb gives direction of force or motion.
5) North pole and south pole by current direction
In a solenoid, the direction of current decides the magnetic poles. If the current appears anticlockwise when seen from one end, that end becomes the north pole. If the current appears clockwise from one end, that end becomes the south pole.
6) Electric motor
An electric motor converts electrical energy into mechanical energy. It works on the principle that a current-carrying conductor placed in a magnetic field experiences a force.
Main parts of an electric motor
| Part | Function |
|---|---|
| Armature or coil | Rotates in the magnetic field |
| Magnet | Provides the magnetic field |
| Split-ring commutator | Reverses current after every half turn |
| Brushes | Supply current to the rotating coil |
| Axle | Transfers rotational motion |
Working of electric motor
When current passes through the coil, the two sides of the coil experience forces in opposite directions. These forces create a turning effect called torque. The split-ring commutator reverses the current after half a rotation, so the coil keeps rotating continuously in the same direction.
Force on conductor: F = BIL sinθ
For θ = 90°, F = BIL
7) Electric generator
An electric generator converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction.
Main parts of a generator
| Part | Function |
|---|---|
| Armature or coil | Rotates in the magnetic field |
| Magnet | Provides magnetic field |
| Slip rings | Help in transferring induced current |
| Brushes | Collect current from slip rings |
| Shaft | Rotates the coil |
Working of generator
When a coil rotates in a magnetic field, the magnetic flux linked with the coil changes. This changing flux induces an electric current in the coil. The direction of induced current is given by Fleming’s right hand rule.
8) Transformer
A transformer is used to increase or decrease AC voltage. It works on the principle of mutual induction and is used in power transmission.
Parts of transformer
| Part | Why it is used |
|---|---|
| Primary coil | Receives AC input |
| Secondary coil | Gives output voltage |
| Soft iron core | Provides path for magnetic flux |
| Laminated sheets | Reduce eddy current loss |
| Insulated copper wire | Good conductivity and avoids short circuit |
Transformer formulas
Vs / Vp = Ns / Np
Is / Ip = Np / Ns
VpIp = VsIs
Types of transformer
- Step-up transformer: Ns > Np, so voltage increases.
- Step-down transformer: Ns < Np, so voltage decreases.
9) Important formulas and rules
| Topic | Formula / Rule |
|---|---|
| Force on conductor | F = BIL sinθ |
| Maximum force | F = BIL |
| Transformer voltage | Vs / Vp = Ns / Np |
| Transformer current | Is / Ip = Np / Ns |
| Anticlockwise current at one end | North pole |
| Clockwise current at one end | South pole |
10) Quick revision
- A magnet has two poles: north and south.
- Like poles repel and unlike poles attract.
- Magnetic field lines never intersect.
- A solenoid behaves like a bar magnet.
- Right hand thumb rule gives magnetic field direction around a wire.
- Fleming’s right hand rule gives induced current direction in generator.
- Fleming’s left hand rule gives force direction in motor.
- Motor converts electrical energy to mechanical energy.
- Generator converts mechanical energy to electrical energy.
- Transformer works only on AC.