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

12th Standard

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Physics

Time : 00:25:00 Hrs
Total Marks : 25
25 x 1 = 25
1. An electric field $\vec { E } =10x\hat { i }$ exists in a certain region of space. Then the potential difference V = Vo – VA, where Vo is the potential at the origin and VA is the potential at x = 2 m is:

(a)

10 J

(b)

-20 J

(c)

+20 J

(d)

-10 J

2. Charge per unit volume is called

(a)

Linear charge density (λ)

(b)

Surface charge density (σ)

(c)

Volume charge density (ρ)

(d)

Electric flux

3. The negative gradient of potential is

(a)

torque

(b)

electric current

(c)

electric field intensity

(d)

electric force

4. A charge QμC is placed at the center of a cube. The flux coming out from any surface will be

(a)

$\frac { Q }{ { 24\varepsilon }_{ 0 } }$

(b)

$\\ \frac { Q }{ { 8\varepsilon }_{ 0 } }$

(c)

$\frac { Q }{ { 6\varepsilon }_{ 0 } } \times { 10 }^{ -6 }$

(d)

$\frac { Q }{ { 6\varepsilon }_{ 0 } } \times { 10 }^{ -3 }$

5. Surface density of charge on a sphere of radius R in terms of electric intensity E at a distance r in free space is

(a)

${ \varepsilon }_{ 0 }E{ \left( \frac { R }{ r } \right) }^{ 2 }$

(b)

$\frac { { \varepsilon }_{ 0 }ER }{ { r }^{ 2 } }$

(c)

${ \varepsilon }_{ 0 }E{ \left( \frac { r }{ R } \right) }^{ 2 }$

(d)

${ e }_{ 0 }E\frac { r }{ { R }^{ 2 } }$

6. Gauss law is another form of ________.

(a)

Newton's law

(b)

Kepler's law

(c)

Ohm's law

(d)

Coulomb's law

7. Two wires of A and B with circular cross section made up of the same material with equal lengths. Suppose RA = 3 RB, then what is the ratio of radius of wire A to that of B?

(a)

3

(b)

$\sqrt3$

(c)

$\frac{1}{\sqrt3}$

(d)

$\frac{1}{3}$

8. Kirchhoff's I law is a consequence of__________

(a)

law of conservation of energy

(b)

law of conservation of charges

(c)

law of conservation of currents

(d)

law of conservation of voltages

9. Th ree wires of equal lengths are bent in the form of loops. One of the loops is circle, another is a semi-circle and the third one is a square. Th ey are placed in a uniform magnetic fi eld and same electric current is passed through them. Which of the following loop confi guration will experience greater torque ?

(a)

circle

(b)

semi-circle

(c)

square

(d)

all of them

10. Lorentz force generally refers to force experienced by a charge due to combined action of

(a)

magnetic fields

(b)

electric fields

(c)

electric, magnetic & gravitational fields

(d)

electric and magnetic fields

11. In an electrical circuit, R, L, C and AC voltage source are all connected in series. When L is removed from the circuit, the phase difference between the voltage and current in the circuit is $\frac{\pi}{3}$. Instead, if C is removed from the circuit, the phase difference is again  $\frac{\pi}{3}$.  The power factor of the circuit is

(a)

1/2

(b)

1/$\sqrt2$

(c)

1

(d)

$\sqrt3$/2

12. Which current- time graph represents the steady movement of short magnet through a coil XY?

(a)

(b)

(c)

(d)

13. Which of the following is NOT true for electromagnetic waves?

(a)

it transport energy

(b)

it transport momentum

(c)

it transport angular momentum

(d)

in vacuum, it travels with different speeds which depend on their frequency

14. A magnetic field is produced by

(a)

a changing electric field

(b)

a moving charge

(c)

both of them

(d)

none of them

15. A ray of light travelling in a transparent medium of refractive index n falls, on a surface separating the medium from air at an angle of incidents of 45o. The ray can undergo total internal reflection for the following n,

(a)

n = 1.25

(b)

n = 1.33

(c)

n = 1.4

(d)

n= 1.5

16. A point source of light B is placed at a distance L in front of the centre of a mirror of width d hung vertically on a wall. A man walks is front of the mirror along a line parallel to the mirror at a distance 2 L front it as shown. The greatest distance over  which he can see the image of the light source in the mirror is

(a)

d/2

(b)

d

(c)

2d

(d)

3d

17. In photoelectric emission, a radiation whose frequency is 4 times threshold frequency of a certain metal is incident on the metal. Th en the maximum possible velocity of the emitted electron will be

(a)

$\sqrt { \frac { { hv }_{ 0 } }{ m } }$

(b)

$\sqrt { \frac { { 6hv }_{ 0 } }{ m } }$

(c)

$2\sqrt { \frac { { hv }_{ 0 } }{ m } }$

(d)

$\sqrt { \frac { { hv }_{ 0 } }{2m } }$

18. Two photons. each of energy 2.5 eV are simultaneously incident on the metal surface. If the work function of the metal is 4.5 eV then from the surface of the metal

(a)

one electron will be emitted

(b)

two electrons will be emitted

(c)

more than two electrons will be emitted

(d)

not a single electron will be emitted

19. Electron microscope operates in ______ media.

(a)

UV

(b)

X-ray

(c)

complete vacuum

(d)

air

20. mirror of focal length of f and the maximum and minimum distance of

(a)

1

(b)

2

(c)

3

(d)

4

21. 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)

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

22. Millikan's oil drop experiment established that

(a)

electric charge depends on velocity

(b)

electric charge is quantised

(c)

electron has wave nature

(d)

electron has particle nature

23. Qutput characteristics of an NPN, transistor in CE mode is drawn between ___________

(a)

IC and VBE

(b)

IE and VCE

(c)

IB and VCE

(d)

IC and VCE

24. The internationally accepted frequency deviation for the purpose of FM broadcasts

(a)

75 kHz

(b)

68 kHz

(c)

80 kHz

(d)

70 kHz

25. Optical fiber communication is generally prefered over general communication system because

(a)

it has signal security

(b)

it is more efficient

(c)

it cannot be jammed as easily as radio waves

(d)

all of the above