# Multiple Choice Questions on Electric Current

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1. One coulomb of electrical charge is contributed by how many electrons?

Electrical charge of one electron is - 1.6 X 10 - 19 coulomb, hence one coulomb implies
1 / | - 1.6 X 10 - 19 | or 0.625 X 1019 number of electrons.

2. 5 X 1016 electrons pass across the section of a conductor in 1 minutes and 20 seconds. The current flowing is

The charge of an electron is - 1.6 X 10-19 coulomb. Therefore total negative charge passes across the section of a conductor in 1 minute and 20 seconds is
5 X 1016 X 1.6 X 10-19 = 8 X 10 - 3. Therefore, charge passes across the section in one second, is 8 X 10 - 3 coulomb/80 second = 10 - 4 coulomb/second (or Amp) = 0.1 mA.

3. An electric current of 6 A is same as

Current is the rate of charge transferred per second. A current of 6 ampere implies 6 coulomb charge transferred through a cross section of conductor per second. Therefore 6 ampere = 6 coulomb/second.

4. Ampere-second is the unit of

Electrical current is nothing but rate of flow of charge per second. Therefore, ampere = coulomb/second hence coulomb = ampere × second or ampere-second.

5. The current in a circuit follows the relation i = 100sinωt. If frequency is 25 Hz how long will it take for the current to rise to 50 A ?

Let, at t second the instantaneous value of current i = 50A.
∴ 50 = 100sinωt
or, sinωt = 1/2
or, ωt = π⁄6
Now, ω = 2π.f = 2πX25 = 50π, [Since frequency f = 25 Hz.]
Therefore, t = π ⁄ (50X6Xπ) = 1 ⁄ 300 second = 3.33 ms.

6. The equation of a current is given by i = Imsin2ωt. The frequency of the current in Hz is

The general equation of a current wave is i = Imsinωt = Imsin2πf.t .....(1)
Where, f is the frequency of the current wave. Here, the given equation is
i = Imsin2ωt......(2)
Comparing, (1) & (2) we get, 2ωt = 2πft or, ω = π.f or, f = ω ⁄ π

7. The equation of alternating current is i = 42.4sin628t. Then the average value of current is

The relation between average value and peak value of a sinusoidal waveform is given by
$\frac{2I_m}{\pi}\; = \;I_0 \;\Rightarrow\;I_0\; =\; 0.636\times I_m$
Here, Im = 42.4 A Therefore, I0 = 0.636X42.4 = 26.97 A

8. If 1 A current is flowing through a series circuit having 100 resistors of each having resistance of 1 Ω. What will be the current in the circuit where, these 100 resistors are connected in parallel?

Let , the applied voltage across the circuit be V. Therefore, current of
1A = V/(100 X 1 Ω)=100 V
Now when these 100 resistors of each 1 Ω will be connected in parallel, the equivalent resistance of combined circuit will be 1/100=0.01 ohm
Now, the current will be V/R=100/0.01=100×100= 104 A.

9. In the figure shown, what will be the current passing through 2 Ω resistor ?

According to current division law, required current,
$I\;=\;1A\times \frac{1}{1+(2+1)}\;=\;0.25A$

10. A copper conductor of one square millimetre can safely carry a current of

It is rating of copper conductor. For 1 sq mm safe current is 10 A. In case of 1.5 sq mm the safe current is 15 A.

11. For carrying an electric current of 75 A an aluminium conductor should have a minimum cross-section of

The current carrying capacity of aluminium conductor is near about 3 A/mm2.

12. A copper wire of length l and diameter d has potential difference V applied across its two ends. The drift velocity is vd. If the diameter of the wire is made d/2, then the drift velocity becomes

The drift velocity is a basic property of conductor material and hence it does not depend upon the length or diameter of the conductor.

13. Mass of a proton is how many times greater than mass of an electron?

Mass of a proton is 1840 times greater than mass of an electron.

14. An electric current of 6 A is same as

Current is rate of charge transferred per unit time. A current of 6 ampere implies 6 coulomb charges transferred through a cross section of conductor per second. Therefore, 6 amp = 6 coulomb/second.

15. On which factors does the severity of electric shock depends ?

The severity and effects of an electrical shock depends on a number of factors, such as the pathway through the body, the amount of current, the length of time of the exposure, whether the skin is wet or dry, magnitude of voltage, and type of supply ac or dc.

16. Five coulomb of electrical charge is contributed by how many electrons?

Electrical charge of one electron is -1.6 X 10 - 19, hence five coulomb implies
5 / | - 1.6 X 10 - 19 | or 3.125 X 1019 numbers of electrons.

17. The transient current in a series AC circuit is given by I(s)=(s+1) ⁄ (s2+s+1). Find the initial current

$i(0+)\;=\;\lim_{s \to \infty} sI(s)\; or,\;\lim_{s \to \infty} \frac{s(s+1)}{s^2+s+1}\; =\; 1\;A$

18. Alternating current is found most suitable for

Alternating current can provide any desired combination of voltage and current by means of transformer. So, resistance welding can be suitably controlled using alternating current.

19. The rms value of a half wave rectified symmetrical square wave current of 2A is given by

Vrms = Vm/√ 2 = 2/√ 2 = 1.414 A.

20. How many coulombs of charge flows through a circuit carrying 5A in 5 min?

The current means the rate of charge transfer per second. That means current I = Q/t Here, I = 5A, and t = 5 minutes = 5 x 60 = 300 sec. Therefore, total charge flows during 5 minutes is 5 x 300 = 1500 coulomb.