Electricity and magnetism

Written by Dr. Hamza Farooq (Physics)

ECAT · Physics · 7 questions

Name: ____________________ Date: ____________

Two 4 microfarad capacitors are connected in series. What is the equivalent capacitance?
- A 2 microfarad
- B 8 microfarad
- C 4 microfarad
- D 16 microfarad
How much energy is stored in a 2 microfarad capacitor charged to 10 V?
- A 1 millijoule
- B 200 microjoule
- C 20 microjoule
- D 100 microjoule
If the distance between two point charges is doubled, the force between them becomes:
- A Double
- B Four times
- C One quarter
- D Half
A wire of length 0.4 m carries 2 A at right angles to a magnetic field of 0.5 T. What force acts on it?
- A 4 N
- B 0.2 N
- C 1.6 N
- D 0.4 N
At resonance in a series RLC circuit, the impedance is:
- A Zero
- B Maximum
- C Minimum and equal to R
- D Equal to the reactance of the inductor
What is the time constant of an RC circuit with R = 1 kilohm and C = 1 microfarad?
- A 1 s
- B 1 ms
- C 1 microsecond
- D 1000 s
The electric field of a point charge varies with distance r as:
- A 1 / r²
- B r²
- C r
- D 1 / r

Answer key

1. A, 2. D, 3. C, 4. D, 5. C, 6. B, 7. A

In series, 1/C = 1/4 + 1/4 = 1/2, so C = 2 microfarad.
Energy = 1/2 C V² = 0.5 x 2 x 10⁻⁶ x 100 = 100 microjoule.
Coulomb's law follows an inverse square law, so doubling the distance divides the force by 4.
F = B I L = 0.5 x 2 x 0.4 = 0.4 N.
At resonance the inductive and capacitive reactances cancel, leaving only the resistance, so the current is largest.
Time constant = RC = 1000 x 10⁻⁶ = 10⁻³ s, which is 1 ms.
E = kq / r², so it falls with the square of the distance.