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Electricity and Magnetism — GK Questions with Answers

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36 questions
  1. The charge on a body is always a whole-number multiple of 1.6 × 10⁻¹⁹ C — this is called:
    1. conservation of charge
    2. quantisation of charge
    3. induction
    4. polarisation
    Show answer

    Answer: (B) quantisation of charge

    Charge is always q = ne, n a whole number — this is quantisation; conservation means the total charge stays constant.

  2. If the distance between two charges is halved, the force between them becomes —
    1. half
    2. double
    3. one-fourth
    4. four times
    Show answer

    Answer: (D) four times

    F ∝ 1/r²; halving the distance makes the force (2)² = 4 times.

  3. Why does salt dissolve easily in water?
    1. water has a very high dielectric constant (≈ 80)
    2. water has high density
    3. salt is light
    4. water is acidic
    Show answer

    Answer: (A) water has a very high dielectric constant (≈ 80)

    Because of the high dielectric constant, the attraction between Na⁺ and Cl⁻ in water falls about 80 times and the ions separate.

  4. Choose the wrong statement about electric field lines.
    1. they start from a positive charge
    2. they never cross
    3. they form closed loops
    4. they are perpendicular to a conductor’s surface
    Show answer

    Answer: (C) they form closed loops

    Electrostatic field lines do not form closed loops; magnetic field lines do.

  5. Why is it safe to sit inside a car during lightning?
    1. the car tyres are rubber
    2. a car is not tall
    3. an outside electric field does not reach inside a closed metal shell
    4. the car keeps moving
    Show answer

    Answer: (C) an outside electric field does not reach inside a closed metal shell

    This is electrostatic shielding (Faraday cage) — charge stays on the outer metal surface and the field inside is zero.

  6. Two capacitors of 2 μF and 3 μF are in series. Equivalent capacitance?
    1. 1.2 μF
    2. 5 μF
    3. 6 μF
    4. 0.83 μF
    Show answer

    Answer: (A) 1.2 μF

    In series 1/C = 1/2 + 1/3 = 5/6 → C = 1.2 μF; capacitor formulas are the reverse of resistor formulas.

  7. About how large is the drift velocity of electrons in a metal wire?
    1. 3 × 10⁸ m/s
    2. 10⁴ m/s
    3. 340 m/s
    4. 10⁻⁴ m/s
    Show answer

    Answer: (D) 10⁻⁴ m/s

    Drift velocity is very slow (≈ 10⁻⁴ m/s); a bulb glows at once because the electric field spreads at nearly the speed of light.

  8. The resistivity of a wire depends on —
    1. length
    2. material and temperature
    3. cross-section area
    4. applied voltage
    Show answer

    Answer: (B) material and temperature

    Resistivity is a property of the material; length and thickness change resistance, not resistivity.

  9. A 10 Ω wire is stretched to double its length. New resistance?
    1. 40 Ω
    2. 20 Ω
    3. 5 Ω
    4. 10 Ω
    Show answer

    Answer: (A) 40 Ω

    Volume stays the same, length doubles so area halves — R = ρL/A becomes four times, 40 Ω.

  10. When temperature rises, the resistance of a semiconductor —
    1. increases
    2. stays the same
    3. decreases
    4. becomes infinite
    Show answer

    Answer: (C) decreases

    In semiconductors more charge carriers become free on heating, so resistance falls (in metals it rises).

  1. The heating element of a heater is made of —
    1. copper
    2. nichrome
    3. tungsten
    4. aluminium
    Show answer

    Answer: (B) nichrome

    Nichrome has high resistivity and melting point and does not oxidise even when red hot.

  2. Resistances of 2 Ω, 3 Ω and 6 Ω are joined in parallel. Equivalent resistance?
    1. 11 Ω
    2. 0.5 Ω
    3. 6 Ω
    4. 1 Ω
    Show answer

    Answer: (D) 1 Ω

    1/R = 1/2 + 1/3 + 1/6 = 1, so R = 1 Ω — in parallel the equivalent is smaller than the smallest.

  3. Why are appliances in houses joined in parallel?
    1. to save electricity
    2. to use less wire
    3. to keep the current equal
    4. so every appliance gets full voltage and all can run separately
    Show answer

    Answer: (D) so every appliance gets full voltage and all can run separately

    In parallel every appliance has the same (230 V) potential difference and switching one off does not affect the others.

  4. Bulbs of 100 W and 60 W (both 220 V) are joined in series. Which glows brighter?
    1. 100 W
    2. 60 W
    3. both equal
    4. neither
    Show answer

    Answer: (B) 60 W

    In series the current is equal and the 60 W bulb has higher resistance, so P = I²R is larger for it.

  5. A battery of EMF 12 V and internal resistance 1 Ω is joined to 5 Ω. Terminal voltage?
    1. 12 V
    2. 2 V
    3. 10 V
    4. 6 V
    Show answer

    Answer: (C) 10 V

    I = 12/6 = 2 A, V = E − Ir = 12 − 2 = 10 V.

  6. Kirchhoff’s loop (voltage) law is based on —
    1. conservation of energy
    2. conservation of charge
    3. conservation of momentum
    4. conservation of mass
    Show answer

    Answer: (A) conservation of energy

    In a closed loop the sum of EMFs equals the sum of IR — this is conservation of energy; the junction law is based on conservation of charge.

  7. The EMF of a cell is measured most correctly by —
    1. voltmeter
    2. potentiometer
    3. ammeter
    4. galvanometer
    Show answer

    Answer: (B) potentiometer

    A potentiometer draws no current from the cell at balance, so it measures EMF correctly.

  8. 4 bulbs of 60 W burn 5 hours a day in a house. Units used in 30 days?
    1. 12
    2. 360
    3. 72
    4. 36
    Show answer

    Answer: (D) 36

    4 × 60 × 5 × 30 / 1000 = 36 kWh = 36 units.

  9. 1 kWh equals how many joules?
    1. 3.6 × 10⁶ J
    2. 3.6 × 10⁵ J
    3. 3.6 × 10³ J
    4. 1000 J
    Show answer

    Answer: (A) 3.6 × 10⁶ J

    1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J — this is 1 unit of electricity.

  10. How is a fuse joined in a circuit?
    1. in parallel in the neutral wire
    2. in the earth wire
    3. in series in the live wire
    4. parallel to the appliance
    Show answer

    Answer: (C) in series in the live wire

    A fuse goes in series in the live wire, so that it melts and breaks the whole circuit as soon as excess current flows.

  1. According to the new standard, what is the colour of the live wire in house wiring?
    1. blue
    2. green
    3. brown
    4. black
    Show answer

    Answer: (C) brown

    New standard: live brown (old red), neutral blue (old black), earth green/green-yellow.

  2. What is the main job of an MCB?
    1. breaking the circuit automatically on excess current (overload, short circuit)
    2. raising voltage
    3. changing AC to DC
    4. measuring power used
    Show answer

    Answer: (A) breaking the circuit automatically on excess current (overload, short circuit)

    An MCB breaks the circuit with a bimetallic strip on overload and an electromagnet on short circuit, and can be switched ON again.

  3. How many cells are there in a car battery (lead-acid)?
    1. 2
    2. 4
    3. 12
    4. 6
    Show answer

    Answer: (D) 6

    Each lead-acid cell gives ≈ 2 V; a 12 V battery has 6 cells in series.

  4. Who discovered the magnetic effect of current?
    1. Faraday
    2. Oersted
    3. Ohm
    4. Ampère
    Show answer

    Answer: (B) Oersted

    In 1820 Oersted saw a magnetic needle deflect near a current-carrying wire.

  5. Which rule gives the direction of force in an electric motor?
    1. Fleming’s left hand
    2. Fleming’s right hand
    3. Lenz’s law
    4. clock rule
    Show answer

    Answer: (A) Fleming’s left hand

    Fleming’s left hand for the motor (forefinger field, middle finger current, thumb force); right hand for the generator.

  6. Two parallel wires carrying current in the same direction —
    1. exert no force
    2. repel each other
    3. attract each other
    4. start rotating
    Show answer

    Answer: (C) attract each other

    Currents in the same direction attract and in opposite directions repel — the reverse of the rule for charges.

  7. What is done to convert a galvanometer into a voltmeter?
    1. low resistance in parallel
    2. high resistance in series
    3. low resistance in series
    4. high resistance in parallel
    Show answer

    Answer: (B) high resistance in series

    Adding a high resistance in series makes the total resistance of the voltmeter very large, so it draws almost no current from the circuit.

  8. What is the dip angle at the magnetic poles?
    1. 0°
    2. 45°
    3. 180°
    4. 90°
    Show answer

    Answer: (D) 90°

    At the magnetic poles the Earth’s field is vertical — dip 90°; at the magnetic equator 0°.

  9. Which of these is a diamagnetic material?
    1. iron
    2. aluminium
    3. cobalt
    4. bismuth
    Show answer

    Answer: (D) bismuth

    Bismuth is the strongest diamagnetic material; aluminium is paramagnetic, iron and cobalt ferromagnetic.

  10. The most suitable material for making a permanent magnet?
    1. soft iron
    2. steel
    3. copper
    4. aluminium
    Show answer

    Answer: (B) steel

    Steel has high retentivity and coercivity, so it does not lose magnetism easily; soft iron is for electromagnets.

  1. Lenz’s law is a result of —
    1. conservation of charge
    2. conservation of momentum
    3. conservation of energy
    4. conservation of mass
    Show answer

    Answer: (C) conservation of energy

    The induced current opposes its cause; if it helped, energy would be made without work — breaking conservation of energy.

  2. A transformer works on the principle of —
    1. mutual induction
    2. self induction
    3. Joule’s law
    4. Ohm’s law
    Show answer

    Answer: (A) mutual induction

    The changing current in the primary coil induces an EMF in the secondary coil — this is mutual induction.

  3. In a step-up transformer —
    1. the secondary has fewer turns, voltage rises
    2. the secondary has more turns, voltage rises and current falls
    3. both voltage and current rise
    4. frequency rises
    Show answer

    Answer: (B) the secondary has more turns, voltage rises and current falls

    Vₛ/Vₚ = Nₛ/Nₚ = Iₚ/Iₛ — with more turns in the secondary the voltage rises and the current falls in the same ratio.

  4. Why is the core of a transformer laminated (made of thin sheets)?
    1. to reduce weight
    2. to increase sound
    3. to increase magnetism
    4. to reduce loss due to eddy currents
    Show answer

    Answer: (D) to reduce loss due to eddy currents

    Insulating layers between thin sheets block the paths of eddy currents and reduce the waste of energy as heat.

  5. The rms voltage of household AC in India is 230 V. Its peak voltage is about —
    1. 325 V
    2. 230 V
    3. 163 V
    4. 460 V
    Show answer

    Answer: (A) 325 V

    V₀ = √2 × V_rms ≈ 1.414 × 230 ≈ 325 V.

  6. Which is the correct order of increasing frequency in the electromagnetic spectrum?
    1. X-ray, UV, visible, radio
    2. gamma, X, UV, visible
    3. radio, microwave, infrared, visible, UV, X, gamma
    4. visible, radio, gamma, X
    Show answer

    Answer: (C) radio, microwave, infrared, visible, UV, X, gamma

    Frequency is lowest for radio and highest for gamma rays; wavelength is the reverse.

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