Step-by-step NCERT solutions for Chapter 12: Magnetic Effects of Electric Current.
A compass needle is a small magnet. When brought near a bar magnet, the magnetic field of the bar magnet exerts a force on the compass needle, causing it to deflect. The compass aligns itself along the direction of the magnetic field lines of the bar magnet.
Magnetic field lines emerge from the North pole and enter the South pole outside the magnet. Inside the magnet, they go from South to North, forming closed loops. They never intersect each other. The density of field lines indicates the strength of the magnetic field.
Properties of magnetic field lines:
• They are closed continuous curves
• They emerge from North pole and enter South pole
• They never intersect each other
• They are tangent to the direction of the magnetic field at any point
• The closer the lines, the stronger the magnetic field
• They are parallel near the centre of a magnet
Two magnetic field lines don't intersect because at the point of intersection, the compass needle would point in two different directions simultaneously, which is impossible. The resultant force at that point would have two different directions, which contradicts the definition of magnetic field direction.
When current passes through the circular loop in a magnetic field, the loop experiences a torque (turning force) and starts to rotate. This is the principle behind the electric motor. The two sides of the loop experience forces in opposite directions (by Fleming's Left Hand Rule), causing rotation.
A solenoid is a coil of many turns of insulated copper wire wound in the shape of a cylinder.
Properties:
• When current flows, it behaves like a bar magnet
• The magnetic field lines inside are uniform and parallel
• One end acts as North pole, the other as South pole
• Strength depends on number of turns and current
An electromagnet is a temporary magnet made by winding insulated copper wire around a soft iron core. When current passes through the coil, it becomes magnetic.
Uses:
• In electric bells
• In cranes to lift heavy magnetic materials
• In relays and switches
• In telephones
• In separating magnetic and non-magnetic materials
Fleming's Left Hand Rule states: Stretch the forefinger, middle finger, and thumb of your left hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field, the middle finger in the direction of current, then the thumb will point in the direction of the force on the conductor. It is used for electric motors.
Motor:
• Converts electrical energy to mechanical energy
• Uses Fleming's Left Hand Rule
• Uses DC power supply
• Has a split-ring commutator
Generator:
• Converts mechanical energy to electrical energy
• Uses Fleming's Right Hand Rule
• Produces AC or DC
• Has slip rings (AC) or split rings (DC)
Electromagnetic induction is the phenomenon of producing electric current by changing the magnetic field around a conductor. When a conductor moves through a magnetic field (or the magnetic field changes), an induced current is produced. This is the principle behind electric generators and transformers.
Advantages of AC:
• AC can be transmitted over long distances with less energy loss
• AC voltage can be easily stepped up or down using transformers
• AC is cheaper to generate than DC
• AC is more versatile in use
• DC cannot be easily transmitted over long distances
Short circuiting occurs when the live wire and neutral wire come in direct contact, causing a large current to flow, which can cause fire.
Safety measures:
• Fuses: Break the circuit when current exceeds safe limit
• MCB (Miniature Circuit Breaker): Automatically switches off
• Proper earthing of appliances
• Using insulated wires
• Not overloading single sockets
The earth wire provides a safety measure by conducting the current to the earth in case of any leakage from the metal body of an appliance. It prevents electric shock to users. The earth wire is connected to the metal body of the appliance and goes deep into the earth through a copper plate.