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

12th Standard

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Physics

Time : 00:25:00 Hrs
Total Marks : 25
25 x 1 = 25
1. If voltage applied on a capacitor is increased from V to 2V, choose the correct conclusion

(a)

Q remains the same, C is doubled

(b)

Q is doubled, C doubled

(c)

C remains same, Q doubled

(d)

Both Q and C remain same

2. Two charged bodies of charges +q, -3q are brought in contact and separated. The charges prossed by each body after separation.

(a)

-q,-q

(b)

+q,-2q

(c)

-q, +3q

(d)

q, q

3. A carbon resistor of (47 ± 4.7 ) k Ω to be marked with rings of diff erent colours for its identifi cation. Th e colour code sequence will be

(a)

Yellow – Green – Violet – Gold

(b)

Yellow – Violet – Orange – Silver

(c)

Violet – Yellow – Orange – Silver

(d)

Green – Orange – Violet - Gold

4. The internal resistance of a 2.1 V cell which gives a current of 0.2 A through a resistance of 10 Ω is

(a)

0.2 Ω

(b)

0.5 Ω

(c)

0.8 Ω

(d)

1.0 Ω

5. The temperature coefficient of resistance is positive__________.

(a)

for metals

(b)

for insulators

(c)

(a) and (b)

(d)

all the above

6. A flat dielectric disc of radius R carries an excess charge on its surface. The surface charge density is σ. The disc rotates about an axis perpendicular to its plane passing through the center with angular velocity ω. Find the magnitude of the torque on the disc if it is placed in a uniform magnetic field whose strength is B which is directed perpendicular to the axis of rotation

(a)

$\frac { 1 }{ 4 } \sigma \omega \pi BR$

(b)

$\frac { 1 }{ 4 } \sigma \omega \pi B{ R }^{ 2 }$

(c)

$\frac { 1 }{ 4 } \sigma \omega \pi B{ R }^{ 3 }$

(d)

$\frac { 1 }{ 4 } \sigma \omega \pi B{ R }^{ 4 }$

7. An α-particle enters a magnetic field of 1T with a velocity 106 m/s in a direction perpendicular to the field. The force on α-particle is

(a)

1.6 x 10-13N

(b)

6.4  x 10-13N

(c)

4.8  x 10-13N

(d)

3.2  x 10-13N

8. In a transformer, eddy current loss is minimized by using

(a)

(b)

(c)

shell type core

(d)

thick copper wire

9. The relation between velocity of light, permeability and permittivity of free space in________.

(a)

${ C }^{ 2 }=\frac { 1 }{ { \mu }_{ 0 }{ \varepsilon }_{ 0 } }$

(b)

$C=\frac { 1 }{ { \mu }_{ 0 }{ \varepsilon }_{ 0 } }$

(c)

${ \mu }_{ 0 }={ \varepsilon }_{ 0 }c$

(d)

${ \varepsilon }_{ 0 }={ \mu }_{ 0 }c$

10. Radio frequency waves are produced by ______.

(a)

(b)

(c)

accelerated charges

(d)

11. ________ radiations are used to study atomic structure.

(a)

UV

(b)

IR

(c)

X-rays

(d)

Gamma

12. A plane mirror produces a magnification of

(a)

-1

(b)

+ 1

(c)

Zero

(d)

Between 0 and + ∞

13. In an experiment of find the focal length of a concave mirror a graph is drawn between the magnitudes of u and V. the graph looks like

(a)

(b)

(c)

(d)

14. An object 5 cm tall is placed 1m from concave spherical mirror which has a radius of curvature of 20 cm. The size of the image is

(a)

0.11 cm

(b)

0.50 cm

(c)

0.55 cm

(d)

0.60 cm

15. When the intensity of a light source is increased

(a)

the number of photons emitted by the source in unit time increases

(b)

more energetic photons are emitted

(c)

faster photons are emitted

(d)

total energy of the photons emitted per unit time decreases.

16. 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

17. The minimum potential given to the anode for which the photoelectric current becomes zero is called _______ potential.

(a)

cut-off

(b)

threshold

(c)

saturation

(d)

active

18. The anode of a simple photo emissive cell is _______

(a)

glass tube

(b)

quartz crystal

(c)

platinum wire

(d)

iron rod

19. ______ are used to study virus and bacteria.

(a)

Lenses

(b)

Mirrors

(c)

Electron microscope

(d)

Telescopes

20. Force is the rate of change of ________

(a)

mass

(b)

acceleration

(c)

momentum

(d)

velocity

21. The wavelength of de-Broglie waves doesn't depend on ________

(a)

mass

(b)

velocity

(c)

momentum

(d)

change

22. 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)

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

23. When hydrogen atom is in its first excited level, its radius is __________ of the Bohr radius.

(a)

twice

(b)

same

(c)

half

(d)

four times

24. Cathode rays are nothing but ____________

(a)

electrons

(b)

protons

(c)

neutrons

(d)

hydrogen atoms

25. A pure semiconductor behaves slightly as a conductor of

(a)

low temperature

(b)

high temperature

(c)

room temperature

(d)

zero degree