Chapter 12: Magnetic Effects Of Electric Current

Magnetic Field: The region surrounding a magnet in which the force of the magnet can be detected. It is a vector quantity having both magnitude and direction.
Magnetic Field Lines: Imaginary continuous curves used to represent the magnetic field.
- Outside the magnet, lines emerge from the North pole and enter the South pole.
- Inside the magnet, the direction of field lines is from the South pole to the North pole, forming closed continuous loops.
- The relative strength of the field is indicated by the closeness of the field lines (crowded lines represent a stronger field).
- No two field lines intersect each other. If they did, a compass placed at the intersection would point in two directions at the same time, which is impossible.

Oersted's Discovery: An electric current flowing through a metallic wire produces a magnetic field around it.
Straight Conductor:
- Field lines form concentric circles centered on the wire.
- Magnetic field strength is directly proportional to the current ( $B \propto I$ ) and inversely proportional to the distance from the wire ( $B \propto \frac{1}{r}$ ).
- Right-Hand Thumb Rule: Imagine holding a current-carrying straight conductor in your right hand such that your thumb points in the direction of the current. Then your fingers will curl around the conductor in the direction of the magnetic field lines.
Circular Loop:
- Every point of the wire produces concentric circular field lines that become larger and appear as straight lines near the center of the loop.
- The magnetic field produced is directly proportional to the number of turns ( $n$ ) in the coil because the current in each turn flows in the same direction and adds up.
Solenoid:
- A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
- The field pattern inside a solenoid is uniform and parallel, resembling that of a bar magnet.
- Electromagnet: A strong magnetic field produced inside a solenoid can be used to magnetize a magnetic material, such as soft iron, placed inside the coil.

A current-carrying conductor placed in a magnetic field experiences a mechanical force.
The direction of force reverses if the direction of current or magnetic field is reversed.
The force is greatest when the direction of current is perpendicular to the direction of the magnetic field ( $\theta = 90^\circ$ ).
Fleming's Left-Hand Rule:
- Stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular to each other.
- Forefinger: Points in the direction of the magnetic field.
- Middle Finger: Points in the direction of current.
- Thumb: Points in the direction of motion or force acting on the conductor.

Direct Current (DC): Current that flows in one direction only. Sources include dry cells and lead-acid accumulators.
Alternating Current (AC): Current that periodically reverses its direction of flow at regular intervals.
- In India, AC changes direction every $\frac{1}{100}\,\text{s}$ , which corresponds to a frequency of $50\,\text{Hz}$ .
- Advantage of AC over DC: AC can be transmitted over long distances with minimal loss of electrical power.

Electric power is supplied through mains via three distinct wires:
- Live Wire (Phase): Red insulation, maintained at a potential of $220\,\text{V}$ .
- Neutral Wire: Black insulation, maintained at a potential of $0\,\text{V}$ .
- Earth Wire: Green insulation, connected to a metal plate buried deep inside the earth.
Safety Measures:
- Earthing: Provides a low-resistance conducting path for leakage current to prevent electric shocks when metallic appliances develop a fault.
- Electric Fuse: A safety device containing a wire with a low melting point that breaks the circuit if the current exceeds a safe limit.
- Short-Circuiting: Occurs when live and neutral wires come in direct contact due to worn insulation, causing an abrupt rise in current.
- Overloading: Occurs when the total current drawn by appliances exceeds the current capacity of the circuit, caused by connecting multiple heavy appliances to a single socket.