Chapter 12: Magnetic Effects of Electric Current
12.1 Magnetic Field and Field Lines
12.1.1 Concepts and Properties of Magnetic Field
- Compass Needle: A small bar magnet whose ends point approximately north (north-seeking or north pole) and south (south-seeking or south pole). Like poles repel, and unlike poles attract.
- Magnetic Field: The region surrounding a magnet in which the force of the magnet can be detected.
- Magnetic Field Lines: The lines along which iron filings or a compass needle align, representing the direction and strength of the magnetic field.
- Properties of Field Lines: They emerge from the north pole and merge at the south pole outside the magnet. Inside, they flow from south to north, forming closed curves. They never intersect each other.
- Field Strength: Represented by the closeness of the field lines; the field is stronger where the lines are crowded.
12.2 Magnetic Field due to a Current-Carrying Conductor
12.2.1 Straight Conductor
- Oersted's Discovery: Hans Christian Oersted (1820) discovered that an electric current flowing through a metallic wire produces a magnetic effect, deflecting a nearby compass needle.
- Concentric Circles: The magnetic field lines around a straight current-carrying wire form concentric circles centered on the wire.
- Field Strength Factors: The magnitude of the magnetic field increases with an increase in current and decreases as the distance from the wire increases.
12.2.2 Right-Hand Thumb Rule
- Right-Hand Thumb Rule: A rule to find magnetic field direction. If you hold a current-carrying straight conductor in your right hand with the thumb pointing in the direction of the current, your wrapped fingers point in the direction of the magnetic field lines. Also called Maxwell's corkscrew rule.
12.2.3 Circular Loop
- Circular Loop Field: Every section of a current-carrying circular loop contributes to magnetic field lines in the same direction inside the loop. Concentric circles become larger away from the wire, appearing as straight lines at the center.
- Number of Turns: For a coil with n turns, the magnetic field is n times larger than that of a single turn because the fields of individual turns add up.
12.2.4 Solenoid
- Solenoid: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
- Solenoid Magnetic Field: The field pattern is identical to a bar magnet, with one end acting as a north pole and the other as a south pole.
- Uniform Field: The field lines inside the solenoid are parallel straight lines, indicating that the magnetic field is uniform at all points inside.
- Electromagnet: A temporary magnet formed by placing a core of magnetic material (like soft iron) inside a current-carrying solenoid.
12.3 Force on a Current-Carrying Conductor in a Magnetic Field
12.3.1 Amplitude and Direction of Force
- Ampere's Suggestion: Andre Marie Ampere suggested that if a current-carrying conductor exerts a force on a magnet, the magnet must exert an equal and opposite force on the conductor.
- Maximum Force: The displacement of the conductor is largest when the direction of the current is perpendicular to the magnetic field.
- Fleming's Left-Hand Rule: Stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular. If the forefinger points to the magnetic field and the middle finger points to the current, the thumb points in the direction of motion or force.
- Applications: Electric motors, electric generators, loudspeakers, microphones, and measuring instruments.
12.4 Domestic Electric Circuits
12.4.1 Supply and Wires
- Domestic Supply: Power is supplied at 220 V alternating current (AC) with a frequency of 50 Hz.
- Live Wire: Usually has red insulation; carries positive potential.
- Neutral Wire: Usually has black insulation; carries negative potential.
- Earth Wire: Has green insulation; connected to a metal plate deep in the ground as a safety measure to prevent severe electric shocks from appliances with metallic bodies.
12.4.2 Safety Devices and Hazards
- Short-Circuiting: Occurs when the live wire and neutral wire come into direct contact (due to damaged insulation or appliance fault), causing current to increase abruptly.
- Overloading: Occurs when too many appliances are connected to a single socket or due to an accidental hike in supply voltage.
- Electric Fuse: A critical safety device that melts due to Joule heating when excessive current flows, breaking the circuit and protecting appliances.