#### 12th Standard Physics English Medium Free Online Test one mark questions with answer key 2020 - Part Five

12th Standard

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Physics

Time : 00:25:00 Hrs
Total Marks : 25
25 x 1 = 25
1. An electric dipole is placed at an alignment angle of 300 with an electric fi eld of 2 × 105 NC-1. It experiences a torque equal to 8 N m. Th e charge on the dipole if the dipole length is 1 cm is

(a)

4 mC

(b)

8 mC

(c)

5 mC

(d)

7 mC

2. An isolated metal sphere of radius 'r' is given a charge' q'. The potential energy of the sphere is

(a)

$\frac { { q }^{ 2 } }{ 4\pi { \varepsilon }_{ 0 }r }$

(b)

$\frac { { q }^{ } }{ 4\pi { \varepsilon }_{ 0 }r }$

(c)

$\frac { { q }^{ } }{ 8\pi { \varepsilon }_{ 0 }r }$

(d)

$\frac { { q }^{ 2 } }{ 8\pi { \varepsilon }_{ 0 }r }$

3. The expression for electric potentid difference is

(a)

$\int _{ R }^{ P }{ +\overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } }$

(b)

$\\ -\int _{ \infty }^{ P }{ +\overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } }$

(c)

$\\\int _{ \infty }^{ P }{ \overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } }$

(d)

$\int _{ R }^{ P }{ -\overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } }$

4. Torque experienced by a dipole in an electric field when it is placed parallel to the field is ______.

(a)

pE sin θ

(b)

-pE cosθ

(c)

infinity

(d)

zero

5. What is the current out of the battery?

(a)

1A

(b)

2A

(c)

3A

(d)

4A

6. For which of the following substances, resistance decreases with temperature?

(a)

copper

(b)

platinum

(c)

mercury

(d)

carbon

7. When three resistors are connected in series then the value of the effective resistance is

(a)

less than the individual resistance

(b)

greater than the individual resistance

(c)

greater than or equal to individual resistance

(d)

less than or equal to individual resistance

8. A wire of length l carries a current I along the Y direction and magnetic field is given by $\vec { B } =\frac { \beta }{ \sqrt { 3 } } =(\hat { i } +\hat { j } +\hat { k } )T.$ The magnitude of Lorentz force acting on the wire is

(a)

$\sqrt { \frac { 2 }{ \sqrt { 3 } } } \beta Il$

(b)

$\sqrt { \frac { 1 }{ \sqrt { 3 } } } \beta Il$

(c)

$\sqrt { 2 } \beta Il$

(d)

$\sqrt { \frac { 1 }{ 2 } } \beta Il$

9. A simple pendulum with charged bob is oscillating with time period T and let θ be the angular displacement. If the uniform magnetic field i switched ON in a direction perpendicular to the plane of oscillation then

(a)

time period will decrease but θ will remain constant

(b)

time period remain constant but θ will decrease

(c)

both T and θ will remain the same

(d)

both T and θ will decrease

10. A long struight conductor carrying a current lies along the axis of a ring. The conductor will exert a force on the ring if

(a)

carries a current

(b)

has uniformly distributed charge

(c)

has non-uniformly distributed

(d)

none of the above

11. Electric iron box and electric heater works on the principle of

(a)

heating effect of current

(b)

heating effect of voltage

(c)

heating effect of power

(d)

heating effect of emf

12. What is the current passing through a coil of radius of 8 cm having 50 turns, when a magnetic intensity at the centre of the coil is 125 A turns m-1?

(a)

0.1 ampere

(b)

0.4 ampere

(c)

1 ampere

(d)

4 ampere

13. A step-down transformer reduces the supply voltage from 220 V to 11 V and increase the current from 6 A to 100 A. Then its efficiency is

(a)

1.2

(b)

0.83

(c)

0.12

(d)

0.9

14. Eddy currents was first observed by

(a)

Foucault

(b)

Newton

(c)

(d)

Fleming

15. During the propagation of electromagnetic waves in a medium

(a)

electric energy density is double of the magnetic energy density

(b)

electric energy density is half of the magnetic energy density

(c)

electric energy density is equal to the magnetic energy density

(d)

both electric and magnetic energy densities are zero

16. A plane glass is placed over a various coloured letters (violet, green, yellow, red) The letter which appears to be raised more is,Two point white dots are 1 mm apart on a black paper. They are viewed by eye of pupil diameter 3 mm approximately. The maximum distance at which these dots can be resolved by the eye is, [take wavelength of light, λ = 500 nm]

(a)

1 m

(b)

5 m

(c)

3 m

(d)

6 m

17. When a metallic surface is illuminated with radiation of wavelength λ, the stopping potential is V. If the same surface is illuminated with radiation of wavelength 2λ, the stopping potential is $\frac{V}{4}$. The threshold wavelength for the metallic surface is

(a)

(b)

(c)

$\frac{5}{2}λ$

(d)

18. Gases begin to conduct electricity at low pressure because

(a)

at low pressure gases turn to plasma

(b)

colliding electrons can acquire higher K.E due to increased mean free path leading to ionisation of atoms

(c)

atoms breakup into electrons and protons

(d)

the electrons in atoms can move freely at low pressure

19. If the potential difference is increased by 9 times, the de-Broglie wavelength becomes ________ times the original.

(a)

$\frac{1}{3}$

(b)

3

(c)

9

(d)

$\frac{1}{9}$

20. The electric potential between a proton and an electron is given by $V={ V }_{ 0 }in\left( \frac { r }{ { r }_{ 0 } } \right)$, where r0 is a constant. Assume that Bohr atom model is applicable of nth orbit rn with the principal quantum number n is

(a)

${ r }_{ n }\alpha \frac { 1 }{ n }$

(b)

${ r }_{ n }\alpha n$

(c)

${ r }_{ n }\alpha n\frac { 1 }{ { n }^{ 2 } }$

(d)

${ r }_{ n }\alpha { n }^{ 2 }$

21. A current gain for, a transistor working as CB amplifier is 0.90. If emitter current is 10 mA, then base current is

(a)

1mA

(b)

2 mA

(c)

0.1 mA

(d)

0.2 mA

22. If half of the a.c. signal is rectified it is a _______

(a)

half wave

(b)

full wave

(c)

voltage

(d)

bridge

23. The variation of frequency of carrier wave with respect to the amplitude of the modulating signal is called _______.

(a)

Amplitude modulation

(b)

Frequency modulation

(c)

Phase modulation

(d)

Pulse width modulation

24. The maximum range of ground or surface wave propagation depends on

(a)

power of transmitter only

(b)

the frequency of the radio waves only

(c)

none of them

(d)

both of them

25. __________ type of modulation costless.

(a)

Frequency

(b)

Time

(c)

Amplitude

(d)

Phase