What is the difference between AC and DC?

DC (Direct Current): current flows in one direction; produced by batteries, solar cells, DC generators. Voltage constant. AC (Alternating Current): current alternates direction periodically; produced by AC generators (alternators); India supply: 230V, 50 Hz. AC used for power transmission (easy to step up/down with transformers); DC used for electronics, charging.

Samacheer Kalvi 12th Basic Electrical Engineering Exercise Solutions

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Basic Electrical Engineering — Exercise Solutions

Key Concepts Quick Recap

Concept	Formula/Value
Ohm's Law	V = IR (V=volts, I=amperes, R=ohms)
Power	P = VI = I²R = V²/R (watts)
Energy	E = Pt (watt-hours or joules)
Kirchhoff's Current Law (KCL)	Sum of currents entering = sum leaving a node
Kirchhoff's Voltage Law (KVL)	Sum of voltages around a closed loop = 0
Series resistance	R_total = R₁ + R₂ + R₃
Parallel resistance	1/R_total = 1/R₁ + 1/R₂ + 1/R₃
Transformer ratio	V₁/V₂ = N₁/N₂ = I₂/I₁
Indian AC supply	230V, 50 Hz

I. Choose the Best Answer (MCQs)

1. According to Ohm's Law, if voltage is doubled and resistance remains same, current will: (a) Halve (b) Double (c) Remain same (d) Quadruple

Answer(b) Double I = V/R. If V doubles, I doubles (direct proportionality).

2. Three resistors of 3Ω, 6Ω, and 9Ω are connected in parallel. The equivalent resistance is: (a) 18Ω (b) 9Ω (c) About 1.64Ω (d) 6Ω

Answer(c) About 1.64Ω 1/R = 1/3 + 1/6 + 1/9 = 6/18 + 3/18 + 2/18 = 11/18. R = 18/11 ≈ 1.64Ω.

3. A transformer with 200 primary turns and 1000 secondary turns is a: (a) Step-down transformer (b) Step-up transformer (c) Isolation transformer (d) Auto-transformer

Answer(b) Step-up transformer N₂/N₁ = 1000/200 = 5 (step-up ratio). V₂ = V₁ × 5 (voltage stepped up). Step-down: N₂ < N₁.

4. The SI unit of electrical resistance is: (a) Ampere (b) Volt (c) Ohm (Ω) (d) Watt

Answer(c) Ohm (Ω)

5. In a DC motor, electrical energy is converted to: (a) Light energy (b) Chemical energy (c) Mechanical energy (d) Thermal energy

Answer(c) Mechanical energy Motor: converts electrical → mechanical energy (Fleming's left-hand rule). Generator: converts mechanical → electrical energy (Fleming's right-hand rule).

6. Fuse wire is made of: (a) Copper (b) Aluminium (c) Tin-lead alloy (low melting point) (d) Steel

Answer(c) Tin-lead alloy (low melting point) Fuse: safety device; tin-lead alloy melts when excess current flows → breaks circuit → protects equipment. MCB (Miniature Circuit Breaker) is the modern equivalent.

7. The frequency of AC supply in India is: (a) 25 Hz (b) 50 Hz (c) 60 Hz (d) 100 Hz

Answer(b) 50 Hz India: 230V (line voltage), 50 Hz. USA/Canada: 120V, 60 Hz.

II. Short Answer Questions

Q1. State and explain Kirchhoff's Laws.

Answer

KCL (Kirchhoff's Current Law): At any junction in a circuit, the algebraic sum of currents is zero. Currents entering = currents leaving. Based on conservation of charge.

Example: If 5A and 3A enter a node, and I₃ leaves: 5 + 3 = I₃ → I₃ = 8A.

KVL (Kirchhoff's Voltage Law): In any closed loop, the algebraic sum of all EMFs and voltage drops is zero. Based on conservation of energy.

Useful for: complex circuits with multiple loops and voltage sources.

Q2. What is a transformer and how does it work?

Answer

Transformer: static electrical device that transfers electrical energy between circuits using electromagnetic induction. Changes voltage and current levels; same frequency.

Principle: Mutual induction — changing current in primary coil → changing magnetic flux → induced EMF in secondary coil.

Turn ratio: V₁/V₂ = N₁/N₂ (for ideal transformer; efficiency ~95-99%)

Types:

Step-up: N₂ > N₁ → V₂ > V₁ (power transmission at high voltage, low current → less loss).
Step-down: N₂ < N₁ → V₂ < V₁ (household use — 11kV → 230V).

Exam Checklist

Ohm's Law: V = IR; Power: P = VI = I²R.
Series: resistances add; Parallel: reciprocals add.
KCL: currents at junction; KVL: voltages in loop.
Transformer: V₁/V₂ = N₁/N₂; step-up: N₂ > N₁.
Motor: electrical → mechanical; Generator: mechanical → electrical.
India AC: 230V, 50 Hz.
Fuse: protects against overcurrent (tin-lead, low melting point).

Frequently Asked Questions

Ohm's Law: V = IR. The current (I) through a conductor is directly proportional to the voltage (V) across it and inversely proportional to its resistance (R), at constant temperature. Limitations: Not applicable to non-ohmic devices (diodes, transistors, electrolytes), at very high temperatures (resistance of metals increases non-linearly), or in vacuum tubes.