1995 - JAMB Physics Past Questions and Answers - page 4
31
If a resistance is halved in value and the potential difference across it is tripled, then the ratio of the new current to the old one is
A
1 : 6
B
1 : 3
C
2 : 1
D
6 : 1
correct option: d
\(\frac{V_1}{V_2}\) = \(\frac{I_1R_1}{I_2R_2}\)
\(\frac{I_1}{I_2}\)V2 = 3V1 and R1 = 2R2
\(\frac{V_1}{3V_1}\) = \(\frac{I_1 \times\ 2R_2}{I_2\times R_2}\)
\(\frac{I_1}{I_2}\) = \(\frac{6}{1}\)
Users' Answers & Comments\(\frac{I_1}{I_2}\)V2 = 3V1 and R1 = 2R2
\(\frac{V_1}{3V_1}\) = \(\frac{I_1 \times\ 2R_2}{I_2\times R_2}\)
\(\frac{I_1}{I_2}\) = \(\frac{6}{1}\)
32
A 12V battery has an internal resistance of 0.5W.If a cable of 1.0W resistance is connected across the two terminals of the battery, the current drawn from the battery is
A
16.0A
B
8.0A
C
0.8A
D
0.4A
correct option: b
I = \(\frac{E.m.f}{R + r}\)
= \(\frac{12}{1 + 0.5}\)
I = 8A
Users' Answers & Comments= \(\frac{12}{1 + 0.5}\)
I = 8A
33
The purpose of a dielectric material in a parallel plate capacitor is to
A
increase its capacitance
B
decrease its capacitance
C
insulate the plate from each other
D
increase the magnetic field between the plates
correct option: c
Users' Answers & Comments34
An electric current of 2 amperes flows in a heating coil of resistance 50 ohms for 3 minutes 20 seconds. Determine the heat produced
A
0.5kJ
B
8.0kJ
C
20.0kJ
D
40.0kJ
correct option: d
Heat energy = Elect. energy
heat energy= I2RT
= 2 X 2 X 50 X 200
= 40000J
to convert to kJ, divide J by 1000
hence, \(\frac{40000}{1000}\) = 40kJ
Users' Answers & Commentsheat energy= I2RT
= 2 X 2 X 50 X 200
= 40000J
to convert to kJ, divide J by 1000
hence, \(\frac{40000}{1000}\) = 40kJ
35
If the maximum voltage across a 100ohms resistor is 20V, then the maximum power it can dissipate is
A
5.00W
B
4.00W
C
2.00W
D
25W
36
Which of the following is required to convert a milliammeter to an ammeter?
A
A high resistance in parallel
B
A high resistance in series
C
A low resistance in parallel
D
A low resistance in series
correct option: c
Users' Answers & Comments37
The primary winding of a transformer has 400 turns and its secondary has 100 turns. If a source of e.m.f. of 12V is applied to the primary, the secondary e.m.f. will be
A
3V
B
6V
C
24V
D
48V
correct option: a
\(\frac{V_s}{V_p}\) = \(\frac{N_s}{N_p}\) = \(\frac{I_p}{I_s}\)
\(\frac{100}{400}\) = \(\frac{V_S}{12}\)
V = 3V
Users' Answers & Comments\(\frac{100}{400}\) = \(\frac{V_S}{12}\)
V = 3V
38
A light of energy 5eV falls on a metal and the electrons with a maximum kinetic energy of 2eV are ejected. The work function of the metal is
A
0.4eV
B
2.5eV
C
3.0eV
D
7.0eV
39
The half-life of a radio-active element is 9 days, what fraction of atoms has decayed in 36 days?
A
\( \frac{1}{16} \)
B
\( \frac{1}{4} \)
C
\( \frac{1}{2} \)
D
\( \frac{15}{16} \)
correct option: d
Users' Answers & Comments40
An alternating source is connected in series to a capacitor of capacity reactance \( 10^3 \sqrt{3} \) and a resistor of resistance 103 Ohms. The Impedance of the circuit is...
A
\( 2000 \sqrt{3} \) Ohms
B
2000 Ohms
C
\( 1000 \sqrt{3} \) Ohms
D
1000 Ohms
correct option: b
Users' Answers & Comments