Magnetism & Electromagnetism
Magnets, electromagnets, motors and electromagnetic induction
A magnet has a north and south pole. Like poles repel, unlike attract. The area around a magnet where it exerts force is a magnetic field — shown by field lines from north to south.
Electromagnets are made by wrapping a coil of wire around an iron core and passing current through it. Strength increases with more turns, more current, or a soft iron core.
The Motor Effect: A current-carrying wire in a magnetic field experiences a force. Direction found using **Fleming's Left-Hand Rule** — thuMb = Motion, First finger = Field, seCond finger = Current.
Electromagnetic Induction: Moving a wire through a magnetic field (or changing the field around a coil) induces a voltage. This is how **generators** work — mechanical energy → electrical energy.
Transformers change voltage levels using two coils around an iron core. Step-up transformers increase voltage (more turns on secondary). Step-down decrease it. Formula: Vp/Vs = Np/Ns.
Key Points to Remember
- 1Like poles repel, unlike attract
- 2Fleming's Left-Hand Rule for motor effect
- 3Electromagnetic induction generates voltage
- 4Transformers: Vp/Vs = Np/Ns
Pakistan Example
Electric Motors in Karachi's Fans and Factories
Every ceiling fan in Pakistan uses the motor effect — current in a coil inside a magnetic field creates rotation. Pakistan's national grid uses step-up transformers at power stations (Tarbela, Mangla) to transmit at 500 kV, then step-down transformers near homes bring it to 220V.