Electricity and Magnetism

Subject: General Science — Physics | Topic: Electricity and Magnetism Exam: AP Group 2 (APPSC) | Sources: NCERT Class X, Lucent's GS

Introduction

Electricity and magnetism is one of the most formula-rich and high-yield topics in APPSC science. Expect 2-3 questions, often on Ohm's Law, series/parallel circuits, Fleming's rules, and domestic electricity. Mastering the key formulas guarantees marks.

Electric Charge and Current

• Electric charge: positive (proton) and negative (electron). Like charges repel, unlike attract.
• SI unit of charge: Coulomb (C). Charge of one electron = 1.6 × 10⁻¹⁹ C.
• Electric current (I): Rate of flow of charge. I = Q/t. SI unit: Ampere (A).
• Conventional current: positive to negative (opposite to electron flow).
• Conductors: copper, silver, aluminium. Insulators: rubber, plastic, wood, glass.

Electric Potential and Ohm's Law

Potential Difference: V = W/Q. 1 Volt = 1 Joule / 1 Coulomb.

Ohm's Law: V = IR (at constant temperature)

Resistance (R): Opposition to current flow. SI unit: Ohm (Ω).

R = ρ × L / A (ρ = resistivity, L = length, A = cross-section area)

• Silver has the lowest resistivity (best conductor), followed by copper.

Series and Parallel Circuits

Household wiring is in parallel — each appliance gets full voltage and operates independently.

Kirchhoff's Current Law (KCL): Current entering a junction = current leaving. Kirchhoff's Voltage Law (KVL): Sum of potential differences around any closed loop = 0.

Electric Power and Energy

• Fuse wire: Alloy of tin and lead, low melting point, connected in series. Melts on overload.

Magnetism

• Every magnet has North and South poles. Like poles repel, unlike attract.
• Magnetic field lines: emerge from North, enter South (outside the magnet).
• Earth is a giant magnet. Geographic North ≈ Magnetic South.
• A freely suspended magnet always points North-South.

Electromagnetism

Oersted's Experiment (1820): Electric current produces a magnetic field (compass deflects near current-carrying wire).

Right-Hand Thumb Rule: Thumb = direction of current, curled fingers = direction of magnetic field.

Solenoid: Coil of many turns behaving like a bar magnet. With soft iron core → electromagnet (temporary, strength increases with current and turns).

Motors, Generators, and Transformers

• Faraday's Law: Changing magnetic flux induces EMF. EMF = −dΦ/dt.
• Lenz's Law: Induced current opposes the change that caused it.
• Step-up transformer: more secondary turns → higher voltage.
• Step-down transformer: fewer secondary turns → lower voltage.
• Transformers work only with AC (DC does not produce changing flux).

Domestic Electricity

• India: 220V, 50 Hz AC supply.
• Three wires: Live (red/brown), Neutral (black/blue), Earth (green).
• Earth wire: safety wire preventing electric shock.
• Short circuit: Live and neutral touch directly → excessive current. Protected by fuse or MCB.

Likely Exam Questions

1. Ohm's Law states: Ans: V = IR (Voltage = Current × Resistance)

2. Household wiring uses which type of circuit? Ans: Parallel circuit

3. 1 kWh equals how many Joules? Ans: 3.6 × 10⁶ Joules

4. Fleming's Left-Hand Rule is used for: Ans: Electric motor (finding direction of force on current-carrying conductor in magnetic field)

5. The frequency of AC supply in India is: Ans: 50 Hz

6. Transformers work only with: Ans: AC (Alternating Current)

7. The metal with the lowest resistivity is: Ans: Silver

8. Oersted's experiment demonstrated that: Ans: Electric current produces a magnetic field

9. A fuse wire is made of an alloy of: Ans: Tin and lead (low melting point)

10. The SI unit of resistance is: Ans: Ohm (Ω)