Magnetic Field and Field Lines
A magnetic field is a region around a magnetic material or a moving electric charge within which magnetic force is exerted.
Characteristics of Magnetic Field Lines:
- Originate from North Pole and terminate at South Pole outside the magnet (inside: South to North).
- Form continuous closed loops.
- Degree of closeness represents magnetic field strength (crowded near poles).
- Two field lines never intersect because at the point of intersection, a compass needle would have to point simultaneously in two different directions, which is physically impossible.
Magnetic Field Due to Current Configurations
- Straight Current-Carrying Conductor: Field lines are concentric circles centered on the wire.
- Right-Hand Thumb Rule: Imagine holding the wire in your right hand with thumb pointed along current; curled fingers indicate magnetic field line direction.
- Current through a Solenoid:
A long cylindrical coil of insulated copper wire containing many turns behaves exactly like a bar magnet. Inside the solenoid, field lines are parallel straight lines, indicating a uniform magnetic field.
Force on a Current-Carrying Conductor
When a current-carrying conductor is placed in an external magnetic field, it experiences a mechanical force:
$$F = BIl\sin\theta$$
Fleming's Left-Hand Rule (Electric Motors):
Stretch thumb, forefinger, and middle finger of left hand mutually perpendicular:
- Forefinger: Magnetic Field ($B$)
- Middle Finger: Current ($I$)
- Thumb: Motion / Force ($F$)
Domestic Electric Circuits
Domestic power is supplied through three primary wires:
- Live Wire (Red / Brown): High potential ($220\text{ V}$).
- Neutral Wire (Black / Blue): Zero potential ($0\text{ V}$).
- Earth Wire (Green / Yellow): Connected to a metal plate buried deep underground near the building. Provides a low-resistance path for leakage current to prevent electric shocks.
Short Circuit vs Overloading:
- Short Circuit: Live and neutral wires touch directly due to damaged insulation; circuit resistance drops near zero, and gigantic current flows, generating fire hazards.
- Electric Fuse: A safety device containing an alloy wire of low melting point that melts and breaks the circuit whenever current exceeds rated capacity.
