Learning Objectives
- Understand the magnetic field due to a current-carrying conductor
- Learn about the force on a current-carrying conductor in a magnetic field
- Understand the principle of electric motor and electric generator
- Learn about domestic electric circuits and safety measures
Key Concepts
Magnetic Field and Field Lines
A magnet creates a magnetic field around it. Magnetic field lines emerge from the north pole and merge at the south pole (outside the magnet). They are closed curves, never cross each other, and are closer where the field is stronger.
Magnetic Field due to Current-Carrying Conductor
Oersted's experiment: A current-carrying conductor produces a magnetic field around it (compass needle deflects near a current-carrying wire).
Straight wire: Concentric circular field lines around the wire. Direction given by Right-Hand Thumb Rule: Thumb points in current direction; curled fingers give field direction.
Circular loop (coil): Field lines are straight at the centre. The face of the coil acts like a magnet.
Solenoid: A coil of many turns. Produces a uniform magnetic field inside, similar to a bar magnet. An electromagnet is a solenoid with a soft iron core.
Force on a Current-Carrying Conductor in a Magnetic Field
A current-carrying conductor placed in a magnetic field experiences a force. Direction given by Fleming's Left-Hand Rule: Forefinger = magnetic Field (B), Middle finger = Current (I), Thumb = Force/Motion (F). All three are mutually perpendicular.
Force is maximum when the conductor is perpendicular to the field and zero when parallel.
Electric Motor
A device that converts electrical energy into mechanical energy (rotational motion). Uses Fleming's Left-Hand Rule. Key component: split-ring commutator reverses the current direction every half rotation to maintain continuous rotation.
Electromagnetic Induction
Faraday's law: When the magnetic field through a coil changes, an electromotive force (EMF) is induced, causing current to flow if the circuit is closed.
Fleming's Right-Hand Rule gives the direction of induced current: Forefinger = Field, Thumb = Motion (of conductor), Middle finger = Induced current.
Electric Generator
Converts mechanical energy into electrical energy using electromagnetic induction.
- AC Generator: Uses slip rings; produces alternating current. Output reverses direction periodically.
- DC Generator: Uses split-ring commutator; produces direct current.
In India: AC supply at 220 V, 50 Hz.
Domestic Electric Circuits
Three wires enter the house: Live (red), Neutral (black), Earth (green). Potential difference = 220 V. Appliances are connected in parallel (each gets full voltage). Safety devices: fuse, MCB (miniature circuit breaker), earthing.
Short circuit: Live and neutral wires touch directly, causing excessive current flow. Fuse melts and breaks the circuit.
Summary
A current-carrying conductor produces a magnetic field (Oersted's discovery). A conductor in a magnetic field experiences a force (motor principle). Changing magnetic fields induce current (generator principle). Fleming's left-hand and right-hand rules give force and induced current directions respectively. Domestic circuits use parallel connections with safety devices.
Important Terms
- Electromagnetic Induction
- Generation of current due to change in magnetic field through a coil
- Solenoid
- A long coil of wire that produces a uniform magnetic field inside when carrying current
- Commutator
- A device that reverses the direction of current in the coil of a motor/generator
- Fuse
- A safety device that melts and breaks the circuit when excessive current flows
Quick Revision
- Right-Hand Thumb Rule: current direction → magnetic field direction around a wire
- Fleming's Left-Hand Rule: Field, Current → Force (motor)
- Fleming's Right-Hand Rule: Field, Motion → Induced current (generator)
- Motor: electrical → mechanical; Generator: mechanical → electrical
- Domestic supply in India: 220 V, 50 Hz AC; appliances in parallel; fuse/MCB for safety