Which of the following devices may be used to step up the voltage in a d.c. circuit?
a step-up transformer
a d.c. generator
a wattmeter
an induction coil
Correct answer is D
An induction coil is a transformer for producing high-voltage alternating current from a low-voltage direct current, consisting essentially of two concentric coils with a common soft-iron core, a primary coil with relatively few windings of heavy wire, and a secondary coil with many turns of fine wire. Excitation of the primary coil by rapidly interrupted or variable current induces high voltage in the secondary coil.
A 40-W instrument has a resistance of 90\(\Omega\). On what voltage should it be operated normally?
60V
150V
225V
3600V
Correct answer is A
P = \(\frac{V}{R}\)
V = \(\sqrt{PR}\)
V = \(\sqrt{40 \times 90}\)
= 60V
9 x 10-13m
9 x 10-7m
9 x 10-3m
9 x 107m
Correct answer is B
C = \(\frac {\varepsilon_o\ A}{d}\)
10-2 x 10-6 = \(\frac {9 \times 10 ^{-12}}{d}\) x 10-4
= 9 x 10-7m
200C
50C
12C
0.002C
Correct answer is C
Q = it
= 100 x 10-3 x 2 x 60
= 12C
0.5 \(\Omega\)
1.0\(\Omega\)
2.0\(\Omega\)
4.0\(\Omega\)
Correct answer is C
E = 4 + 2r and E = 2 + 3r
therefore, r = 2\(\Omega\)