# 27 Lecture

## PHY101

### Midterm & Final Term Short Notes

## Capacitors and Currents

Capacitors and Currents are an important topic in the field of physics, particularly in the study of electricity and electronics.

**Important Mcq's**

Midterm & Finalterm Prepration

Past papers included

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**A capacitor of capacitance 2 microfarads is charged to 100 V. The energy stored in the capacitor is:**

a) 1 J

b) 2 J

c) 4 J

d) 10 J

Answer: c) 4 J

Explanation: The energy stored in a capacitor is given by the formula E = 1/2 CV^2, where C is the capacitance and V is the voltage across the capacitor. Substituting the given values, we get E = 1/2 * 2 * 10^-6 * (100)^2 = 4 J.

**Which of the following materials would be the best choice for making a capacitor with a high capacitance?**

a) Air

b) Glass

c) Paper

d) Aluminum foil

Answer: d) Aluminum foil

Explanation: The capacitance of a capacitor depends on the area of the plates, the distance between them, and the dielectric constant of the material between them. Aluminum foil has a high surface area and can be rolled up to form a capacitor with a large area. It is also a good conductor, which is important for minimizing resistance and maximizing capacitance.

**A capacitor is connected in series with a resistor and a battery. What happens to the voltage across the capacitor as time passes?**

a) It decreases exponentially.

b) It increases linearly.

c) It remains constant.

d) It oscillates.

Answer: a) It decreases exponentially.

Explanation: In a series circuit, the same current flows through all the components, so the voltage across the capacitor and the resistor must add up to the voltage of the battery. As the capacitor charges up, the voltage across it increases, but the voltage across the resistor decreases. The rate of change of the voltage across the capacitor is proportional to the current flowing through the circuit and the capacitance of the capacitor. This leads to an exponential decrease in the voltage across the capacitor as it charges up.

**What is the time constant of a circuit consisting of a 10 microfarad capacitor and a 1 kilohm resistor?**

a) 1 microsecond

b) 10 microseconds

c) 100 microseconds

d) 1 millisecond

Answer: b) 10 microseconds

Explanation: The time constant of an RC circuit is equal to the product of the resistance and the capacitance, ? = RC. Substituting the given values, we get ? = 10 * 10^-6 * 10^3 = 10 * 10^-3 = 10 microseconds.

**A capacitor is charged up to a voltage of 12 V and then disconnected from the battery. If the capacitance is 2 microfarads, how much charge is stored on the capacitor?**

a) 6 microcoulombs

b) 12 microcoulombs

c) 24 microcoulombs

d) 48 microcoulombs

Answer: b) 12 microcoulombs

Explanation: The charge stored on a capacitor is given by the formula Q = CV, where C is the capacitance and V is the voltage across the capacitor. Substituting the given values, we get Q = 2 * 10^-6 * 12 = 24 * 10^-6 = 12 microcoulombs.

**Which of the following is true about the current in a capacitor?**

a) The current is always zero.

b) The current is always positive.

c) The current can be positive or negative.

d) The current is independent of the voltage across the capacitor.

Answer: a) The current is always zero.

Explanation: In a DC circuit, a capacitor acts as an open circuit, so

**Subjective Short Notes**

Midterm & Finalterm Prepration

Past papers included

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**What is the definition of capacitance?**

**Answer:** Capacitance is the ability of a capacitor to store electric charge per unit voltage.

**Define the time constant of a capacitor.**

**Answer: **The time constant of a capacitor is the time taken by the capacitor to discharge to 63.2% of its initial charge or voltage.

**What is the energy stored in a capacitor?**

**Answer: T**he energy stored in a capacitor is given by the formula: E = 1/2 CV^2, where E is the energy stored, C is the capacitance, and V is the voltage across the capacitor.

**What happens when a capacitor is connected in series with a resistor and a battery?**

**Answer: **A capacitor connected in series with a resistor and a battery forms a simple RC circuit, where the capacitor charges and discharges through the resistor.

**Define the current in a capacitor.**

**Answer: **The current in a capacitor is the rate of change of charge stored in the capacitor with respect to time.

**What is the unit of capacitance?**

**Answer:** The unit of capacitance is farad (F).

**How can you increase the capacitance of a capacitor?**

**Answer: **The capacitance of a capacitor can be increased by increasing the area of the plates, decreasing the distance between the plates, or by using a material with a high dielectric constant between the plates.

**What is the charge on a capacitor when it is fully charged?**

**Answer:** The charge on a capacitor when it is fully charged is equal to the product of its capacitance and the voltage across it.

**What is the difference between a capacitor and a battery?**

**Answer: **A capacitor stores energy in an electric field, while a battery stores energy in a chemical form.

**What is the formula for the time constant of a capacitor in an RC circuit?**

**Answer: **The formula for the time constant of a capacitor in an RC circuit is given by the product of the resistance and the capacitance, i.e., ? = RC.