Deck 6: Capacitance, and Magnetic Flux

Question 1

Decide whether each of these statements is True (T) or False
A transformer has a step-down voltage ratio of 5. When the primary coil alternating voltage is 240 V and a
Load dissipating 40 W is connected across the secondary coil then, neglecting transformer power losses:
(i) The primary current is 0.167 A.
(ii) The secondary current is 0.028 A.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Answer 1

B

Question 2

Decide whether each of these statements is True (T) or False
A coil of 100 turns is wound round the core of a magnetic circuit of length 0.5 m. When the coil carries a
Current of 5 A:
(i) The m.m.f. is 500 ampere-turns.
(ii) The magnetic field strength in the circuit is 1000 A/m.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Answer 2

A

Question 3

Decide whether each of these statements is True (T) or False
The reluctance of a magnetic circuit is doubled if:
(i) Its cross-sectional area is doubled.
(ii) Its length is doubled.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Answer 3

C

Question 4

Decide whether each of these statements is True (T) or False
A hard magnetic material when compared with a soft magnetic material has:
(i) A higher remanence.
(ii) A higher coercivity.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Question 5

An inductor has an inductance of 200 mH. The back e.m.f. produced when the current through it changes at 2 A/s is:

A) 0.2 V
B) 0.4 V
C) 2 V
D) 4 V

Question 6

Decide whether each of these statements is True (T) or False
For capacitors of 8 μF and 4 μF:
(i) When they are connected in series their combined capacitance is 12 μF.
(ii) When they are connected in parallel their combined capacitance is 2.7 μF.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Question 7

Decide whether each of these statements is True (T) or False
The capacitance of a parallel plate capacitor is doubled if:
(i) The separation of the plates is doubled.
(ii) The area of the plates is doubled.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Question 8

Decide whether each of these statements is True (T) or False
The energy stored by a capacitor is doubled if:
(i) The potential difference across the capacitor is doubled.
(ii) The charge on the capacitor plates is doubled.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Question 9

Decide whether each of these statements is True (T) or False
A 20 μF capacitor is charged from a 10 V supply and then an uncharged 10 μF capacitor is connected in
Parallel with it.
(i) The combined capacitance is 30 μF.
(ii) The potential difference across the connected pair becomes 6.7 V.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F