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General Science: Magnetic Effects of Electric Current
📅 Date: 30 December 2025 (Tuesday)
This chapter is the foundation of modern machinery (fans, motors, generators). For competitive exams, the questions here are less about calculations and more about Rules (Left Hand/Right Hand), Device Principles (Motor/Generator), and Domestic Safety (Fuse/Earthing).
1. The Discovery (Oersted's Experiment)
The Scientist: Hans Christian Oersted (1820).
The Experiment: He noticed that a compass needle deflected when placed near a wire carrying electric current.
Conclusion: Electricity produces Magnetism (Electromagnetism).
2. Magnetic Field Lines (Visualizing the Invisible)
A magnet is surrounded by a "field" where its force is felt.
- Direction (Outside Magnet): North Pole → South Pole.
- Direction (Inside Magnet): South Pole → North Pole. (They form closed loops).
- Strength: The closer the lines, the stronger the magnetic field.
Two magnetic field lines never cross each other. If they did, the compass would have to point in two directions at once at the intersection point, which is impossible.
The Right-Hand Thumb Rule
Used to find the direction of the magnetic field around a straight wire.
Action: Imagine holding the wire in your right hand.
Thumb: Points in direction of Current.
Fingers: Wrap around in direction of Magnetic Field.
3. The Solenoid and Electromagnet
Solenoid: A coil of many circular turns of insulated copper wire (looks like a spring).
Behavior: When current passes through it, it behaves exactly like a Bar Magnet (One end North, one end South).
Electromagnet: If you put a piece of Soft Iron inside a solenoid, the magnetic field becomes very strong.
Nature: Temporary Magnet (Magnetism is lost when current is switched off).
Uses: Cranes (lifting heavy loads), Electric Bells.
4. Force on a Current-Carrying Conductor (The Motor Principle)
If you put a wire carrying current inside an external magnetic field, the wire experiences a Force (it moves).
Max Force Condition: Current direction is Perpendicular (90°) to the Magnetic Field.
Fleming’s Left-Hand Rule (For Motors)
Stretch your thumb, forefinger, and middle finger mutually perpendicular.
- Forefinger: Field (Magnetic).
- Center Finger: Current.
- Thumb: Thrust (Force/Motion).
Mnemonic: Father (Field), Mother (Motion), Child (Current). Or just remember: Motor = Motion = Left Hand.
The Electric Motor
Conversion: Electrical Energy → Mechanical Energy.
Key Component: Split Ring Commutator.
Function: It reverses the direction of current in the coil every half rotation to keep the motor spinning in the same direction.
5. Electromagnetic Induction (The Generator Principle)
The Scientist: Michael Faraday.
The Concept: If you move a magnet near a coil (or change the magnetic field), a current is induced in the coil.
Conversion: Mechanical Energy → Electrical Energy.
Fleming’s Right-Hand Rule (For Generators)
Used to find the direction of Induced Current.
- Forefinger: Field.
- Thumb: Motion (Thrust).
- Middle Finger: Induced Current.
- AC (Alternating Current): Direction changes periodically. Uses Slip Rings.
- DC (Direct Current): Direction is constant. Uses Split Ring Commutator.
6. Domestic Electric Circuits
This section is critical for practical/general knowledge questions.
A. The Wiring Color Code
- Live Wire (Positive): Red insulation. (Carries current, usually 220V).
- Neutral Wire (Negative): Black insulation. (Returns current, 0V).
- Earth Wire: Green insulation. (Safety wire connected to the ground).
Mentor's Note: In new international standards, Live is Brown, Neutral is Blue, and Earth is Green-Yellow. However, Indian exams often stick to Red/Black/Green.
B. Key Safety Features
- Electric Fuse: Placed on the Live Wire. Protects against Short Circuits (Live touches Neutral) and Overloading.
- Earthing (The Green Wire): Connected to the metallic body of appliances (Iron, Fridge). If current leaks, it flows to the earth instead of giving you a shock.
C. India's Power Supply Specs (Memorize)
Voltage: 220V.
Frequency: 50 Hz (Current changes direction 100 times a second).
7. Mentor’s Final Drill (Exam-Ready Questions)
A: 50 Hz.
Q: Which rule is used to find the direction of force in an Electric Motor?
A: Fleming’s Left-Hand Rule.
Q: What is the core material of an electromagnet?
A: Soft Iron. (Steel is not used because it retains magnetism permanently).
Q: In domestic circuits, what is the color of the Earth wire?
A: Green.
Q: Who discovered Electromagnetic Induction?
A: Michael Faraday.
Q: To avoid risk of electric shock, which wire is connected to the metal body of an appliance?
A: Earth Wire.
Study Strategy: The Hand Confusion is the biggest trap. Remember: Want to create Motion? (Motor) → Left Hand. Want to create Current? (Generator) → Right Hand.