Case Study:

During a school annual function in Delhi, a photographer uses a digital camera to capture moments on stage. When the lighting in the hall becomes dim, the photographer turns on the camera flash. Each time a picture is taken, a bright burst of light illuminates the stage for a fraction of a second.

This flash is produced using a capacitor, an electronic component capable of storing electrical energy. Inside the camera, the battery slowly charges a large electrolytic capacitor to a high voltage through a charging circuit. Once charged, the capacitor stores energy in the electric field between its plates. 

When the shutter button is pressed, the stored energy is suddenly released through a xenon flash tube, producing an intense burst of light that lasts only a few milliseconds. The rapid discharge of the capacitor provides a very large current that a normal battery cannot supply instantly. 

Typical flash capacitors may have capacitance values between 80 μF and 240 μF and operate at voltages around 180-330 V, allowing them to store enough energy for a bright photographic flash.

The energy stored in a capacitor is given by the formula

E = 1/2*CV².

where C is the capacitance and V is the voltage across the capacitor.

For example, a 100 μF capacitor charged to 330 V can store several joules of energy and release it in about 1 millisecond, producing a very powerful flash of light. 

After the flash occurs, the capacitor begins charging again from the battery so that the camera is ready for the next photograph. This practical application demonstrates how capacitors store electrical energy and deliver it rapidly when required.

Questions

Section A - MCQs

1. In a camera flash circuit, the capacitor is mainly used to:

A. Increase resistance in the circuit
B. Store electrical energy and release it quickly
C. Reduce voltage of the battery
D. Convert AC into DC

2. The energy stored in a capacitor depends on:

A. Resistance and current
B. Capacitance and voltage
C. Temperature and pressure
D. Magnetic field only

3. When the flash button is pressed, the capacitor:

A. Slowly charges again
B. Discharges rapidly through the flash tube
C. Stops storing energy permanently
D. Converts energy into sound

4. The formula for energy stored in a capacitor is:

A. E = IV
B. E = mc²
C. E = 1/2*CV²
D. E = IR

Section B - Short Answer Questions

1. Why is a capacitor used in a camera flash instead of using the battery directly?

2. Explain the process that occurs in the camera flash when the shutter button is pressed.

3. Write the formula for energy stored in a capacitor and explain the meaning of each term.

Section C - Long Answer Question

1. A camera flash uses a capacitor of 100 μF charged to 300 V.

a) Calculate the energy stored in the capacitor.
b) Explain how this stored energy produces a bright flash.
c) What would happen to the flash brightness if the capacitance were doubled while voltage remained the same?

Answer Key

MCQ Answers

1. B - The capacitor stores energy and releases it quickly.
2. B - Energy depends on capacitance and voltage.
3. B - The capacitor discharges rapidly through the flash tube.
4. C - E =1/2*CV².

Short Answer Solutions

1. A battery provides energy slowly, but a camera flash requires a large amount of energy in a very short time. A capacitor stores energy gradually and releases it instantly to produce the flash.

2. When the shutter button is pressed, the capacitor discharges through the flash tube. The high current ionizes the gas inside the tube and produces a bright burst of light.

3.

E = 1/2*CV²

Where:

• E = energy stored in the capacitor
• C = capacitance of the capacitor
• V = voltage across the capacitor

Long Answer Solution

a) Energy stored

Given:
C = 100 μF = 100 * 10^-6 F
V = 300 V

E = 1/2*CV²
E = 1/2 * (100* 10^-6) * (300)²
E = 0.5 * 100 * 10^-6 * 90000
E = 4.5  J

Energy stored = 4.5 Joules

b) This stored energy is released quickly through the flash tube, producing a bright burst of light.

c) If capacitance doubles, stored energy also increases since (E ∝ C), so the flash becomes brighter.