MDCAT Physics Medium Questions

Practice MDCAT Physics Medium MCQs with explanations — PMC, ETEA, NUMS entry test preparation.

MDCAT Physics Medium Questions

  1. Question 1

    Q1. A capacitor in an AC circuit has a reactance given by Xc = 1 / (2πfC). What happens to Xc when frequency is doubled?

    • A) It doubles
    • B) It halves
    • C) It remains the same
    • D) It becomes zero

    Answer: It halves

    Explanation: Xc is inversely proportional to frequency; when frequency doubles, Xc halves. 'It doubles' is the opposite of the correct behavior.

  2. Question 2

    Q2. An inductor is connected to an AC source. If the frequency of the source is doubled, the reactance of the inductor

    • A) Remains the same
    • B) Doubles
    • C) Halves
    • D) Becomes zero

    Answer: Doubles

    Explanation: Inductive reactance is directly proportional to frequency; when frequency doubles, reactance doubles. 'It halves' is the opposite behavior.

  3. Question 3

    Q3. The average power dissipated in an AC circuit is maximum when the power factor is

    • A) 0
    • B) 0.5
    • C) 1
    • D) -1

    Answer: 1

    Explanation: Maximum power dissipation occurs when power factor = 1 (i.e., cosφ = 1); '0' means no real power is dissipated.

  4. Question 4

    Q4. The resonance frequency of an LC circuit is given by

    • A) 1 / (2π√LC)
    • B) 1 / (2πLC)
    • C) √(L/C)
    • D) √(C/L)

    Answer: 1 / (2π√LC)

    Explanation: Resonance frequency is given by 1 / (2π√LC); '1 / (2πLC)' is an incorrect formula, missing the square root.

  5. Question 5

    Q5. The quality factor (Q) of an LC circuit is a measure of its

    • A) Bandwidth
    • B) Resonance frequency
    • C) Selectivity
    • D) Power factor

    Answer: Selectivity

    Explanation: Q factor measures the selectivity or sharpness of resonance; 'Bandwidth' is related to Q, but is not what Q directly measures.

  6. Question 6

    Q6. The impedance of a series RLC circuit at resonance is

    • A) R
    • B) Xl
    • C) Xc
    • D) √(R^2 + (Xl - Xc)^2)

    Answer: R

    Explanation: At resonance, Xl = Xc, so impedance = R; '√(R^2 + (Xl - Xc)^2)' is the general formula for impedance, not at resonance.

  7. Question 7

    Q7. A choke coil is used to control current in an AC circuit. It is preferred over a resistor because it

    • A) Increases the current
    • B) Decreases the current without power loss
    • C) Increases the power factor
    • D) Decreases the impedance

    Answer: Decreases the current without power loss

    Explanation: A choke coil (inductor) controls current without significant power loss; 'Increases the current' is the opposite effect.

  8. Question 8

    Q8. The current in an AC circuit is wattless when the circuit contains only

    • A) Resistance
    • B) Inductance or capacitance
    • C) Impedance
    • D) Conductance

    Answer: Inductance or capacitance

    Explanation: Wattless current occurs in purely inductive or capacitive circuits; 'Resistance' does not cause wattless current.

  9. Question 9

    Q9. The efficiency of a transformer is defined as the ratio of

    • A) Output power to input power
    • B) Input power to output power
    • C) Output voltage to input voltage
    • D) Input voltage to output voltage

    Answer: Output power to input power

    Explanation: Efficiency is the ratio of output power to input power; 'Input power to output power' is the inverse ratio.

  10. Question 10

    Q10. A capacitor is connected to an AC source with a frequency of 50 Hz. If the capacitance is 10 μF, what is the capacitive reactance?

    • A) 159 Ω
    • B) 318 Ω
    • C) 637 Ω
    • D) 1000 Ω

    Answer: 318 Ω

    Explanation: Xc = 1 / (2πfC) = 1 / (2π * 50 Hz * 10e-6 F) = 318 Ω. Option A is incorrect because it is half the correct value.

  11. Question 11

    Q11. A series RLC circuit has R = 100 Ω, L = 0.1 H, and C = 10 μF. If the frequency of the AC source is 50 Hz, what is the impedance?

    • A) 100 Ω
    • B) 150 Ω
    • C) 200 Ω
    • D) 250 Ω

    Answer: 150 Ω

    Explanation: First, calculate Xl and Xc. Xl = 31.4 Ω, Xc = 318 Ω. Then, Z = sqrt(R^2 + (Xl - Xc)^2) = sqrt(100^2 + (31.4 - 318)^2) = 150 Ω (approx). Option A is incorrect because it neglects reactance.

  12. Question 12

    Q12. What is the resonant frequency of a series RLC circuit with L = 0.1 H and C = 10 μF?

    • A) 50 Hz
    • B) 100 Hz
    • C) 159 Hz
    • D) 318 Hz

    Answer: 159 Hz

    Explanation: f_res = 1 / (2π sqrt(LC)) = 1 / (2π sqrt(0.1 H * 10e-6 F)) = 159 Hz. Option A is incorrect because it is not the calculated resonant frequency.

  13. Question 13

    Q13. A step-down transformer has a primary voltage of 220 V and a secondary voltage of 110 V. If the primary current is 1 A, what is the secondary current?

    • A) 1 A
    • B) 2 A
    • C) 3 A
    • D) 4 A

    Answer: 2 A

    Explanation: For an ideal transformer, I_primary * V_primary = I_secondary * V_secondary. So, I_secondary = I_primary * (V_primary / V_secondary) = 1 A * (220 V / 110 V) = 2 A. Option A is incorrect because it assumes equal current in primary and secondary.

  14. Question 14

    Q14. The power factor of an AC circuit is 0.8. If the true power is 800 W, what is the apparent power?

    • A) 640 W
    • B) 800 W
    • C) 1000 W
    • D) 1200 W

    Answer: 1000 W

    Explanation: Power factor = True Power / Apparent Power. So, Apparent Power = True Power / Power Factor = 800 W / 0.8 = 1000 W. Option A is incorrect because it results from multiplying instead of dividing.

  15. Question 15

    Q15. The capacitive reactance of a capacitor at 100 Hz is 100 Ω. What is its capacitive reactance at 50 Hz?

    • A) 50 Ω
    • B) 100 Ω
    • C) 200 Ω
    • D) 400 Ω

    Answer: 200 Ω

    Explanation: Xc is inversely proportional to frequency. So, at 50 Hz, Xc = 100 Ω * (100 Hz / 50 Hz) = 200 Ω. Option A is incorrect because it assumes Xc is directly proportional to frequency.

  16. Question 16

    Q16. In an AC circuit, the true power is 800 W and the apparent power is 1000 VA. What is the power factor?

    • A) 0.4
    • B) 0.6
    • C) 0.8
    • D) 1

    Answer: 0.8

    Explanation: Power factor = True Power / Apparent Power = 800 W / 1000 VA = 0.8. Option A is incorrect because it is less than the correct value.

  17. Question 17

    Q17. A capacitor has a capacitive reactance of 100 Ω at 50 Hz. What is its capacitance?

    • A) 10 μF
    • B) 20 μF
    • C) 31.8 μF
    • D) 63.6 μF

    Answer: 31.8 μF

    Explanation: C = 1 / (2πfXc) = 1 / (2π * 50 Hz * 100 Ω) = 31.8 μF. Option A is incorrect because it is less than the correct value.

  18. Question 18

    Q18. An AC source with a frequency of 50 Hz is connected to a series RLC circuit. If L = 0.1 H and C = 100 μF, what is the resonant frequency?

    • A) 25 Hz
    • B) 50 Hz
    • C) 100 Hz
    • D) 159 Hz

    Answer: 50 Hz

    Explanation: f_res = 1 / (2π sqrt(LC)) = 1 / (2π sqrt(0.1 H * 100e-6 F)) = 50 Hz (approx). Option A is incorrect because it is not the calculated resonant frequency.

  19. Question 19

    Q19. A capacitor is connected to an AC source. What happens to its capacitive reactance when the frequency is doubled?

    • A) It becomes half
    • B) It becomes double
    • C) It remains the same
    • D) It becomes zero

    Answer: It becomes half

    Explanation: Capacitive reactance is inversely proportional to frequency; doubling frequency halves reactance. Option B is incorrect because reactance decreases, not increases.

  20. Question 20

    Q20. What is the impedance of a series RLC circuit at resonance?

    • A) R
    • B) X_L
    • C) X_C
    • D) √(R^2 + (X_L - X_C)^2)

    Answer: R

    Explanation: At resonance, X_L = X_C, so impedance equals resistance R. Option D is the general impedance formula, not specific to resonance.

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