Physics

Capacitors and Capacitance

189 Questions

Capacitors and capacitance are crucial topics in physics, covering the storage of electric charge and energy. These concepts explore series and parallel combinations, dielectric materials, and capacitive reactance. Questions frequently appear in various competitive engineering and medical entrance exams.

Series and parallel capacitorsParallel plate capacitorsCapacitive reactance formulasDielectrics and permittivitySpherical capacitors

Capacitors and Capacitance Questions

Multiple choice general knowledge science & technology

CAPACITANCE

  1. Ampere

  2. Farad

  3. Watt

  4. Henry
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The SI unit of capacitance is the Farad (F), named after Michael Faraday. Ampere is the unit of current, Watt is the unit of power, and Henry is the unit of inductance.

Multiple choice general knowledge

How does a capacitor react to AC?

  1. As the frequency of the applied AC increases, the reactance increases

  2. As the amplitude of the applied AC increases, the reactance increases

  3. As the frequency of the applied AC increases, the reactance decreases

  4. As the amplitude of the applied AC increases, the reactance decreases
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Capacitive reactance is inversely proportional to frequency (Xc = 1/(2πfC)). Higher frequency means the capacitor offers less opposition to current flow. The reactance depends only on frequency and capacitance, not on amplitude.

Multiple choice

The figure shows the high-frequency capacitance-voltage (C-V) characteristics of a Metal/SiO2 /silicon (MOS) capacitor having an area of 1 $\times$10-4 cm2. Assume that the permittivities ($\epsilon_0 \epsilon_r$) of silicon and SiO2 are 1 $\times$ 10-12 F / cm and 3.5 $\times$10-13 F / cm respectively.

Consider the following statements about the C-V characteristics plot: S1: The MOS capacitor has an n-type substrate. S2: If positive charges are introduced in the oxide, the C-V plot will shift to the left. Which of the following is true?

  1. Both S1 and S2 are true.

  2. S1 is true and S2 is false.

  3. S1 is false and S2 is true.

  4. Both S1 and S2 are false.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Multiple choice

In the following figure, C1 and C2 are ideal capacitors. C1 has been charged to 12 V before the ideal switch S is closed at t = 0. The current i(t) for all t is

  1. zero

  2. a step function

  3. an exponentially decaying function

  4. an impulse function

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

When two ideal capacitors are suddenly connected by closing a switch, charge redistribishes almost instantaneously. This produces an impulse (Dirac delta) current at the switching instant t=0, after which the current is zero as the capacitors reach a new equilibrium. The impulse represents the theoretically infinite current that would flow for an infinitesimal time to equalize voltages.

Multiple choice

The circuit shown in the figure is used to charge the capacitor C alternately from two current sources as indicated. The switches S1 and S2 are mechanically coupled and connected as follows: For 2nT $\le$ t $\le$ (2n + 1) T, (n = 0, 1, 2, ...) S1 to P1 and S2 to P2 For (2n + 1) T $\le$ t $\le$ (2n + 2) T, (n = 0, 1, 2, ...) S1 to Q1 and S2 to Q2

Assume that the capacitor has zero initial charge. Given that u (t) is a unit step function, the voltage vc (t) across the capacitor is given by
  1. $\displaystyle \sum_{n=1}^\infty (-1)^n \ tu \ (t - nT)$
  2. u (t) + 2$\displaystyle \sum_{n=1}^\infty (-1)^n \ u \ (t - nT)$
  3. t u (t) + 2$\displaystyle \sum_{n=1}^\infty (-1)^n \ u \ (t - nT)$(t - nT)
  4. $\displaystyle \sum_{n=1}^\infty [0.5 - e^{-(t-2nT)} + 0.5 e^{-(t-2nT)} ]$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation