#### 12th Standard Physics English Medium Free Online Test 1 mark questions 2020 - Part Eight

12th Standard

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Physics

Time : 00:25:00 Hrs
Total Marks : 25
25 x 1 = 25
1. Three capacitors are connected in triangle as shown in the figure. The equivalent capacitance between the points A and C is

(a)

1μF

(b)

2μF

(c)

3μF

(d)

$\frac{1}{4}$μF

2. What will happen if two conducting spheres are separately charged and then brought in contact?

(a)

Total charge on the two spheres is conserved

(b)

The total energy is conserved

(c)

Both charge and energy are conserved

(d)

The final potential is the mean of the original potentials.

3. During charging a capacitor variation of potential V of the capacitor with time t as shown a

(a)

(b)

(c)

(d)

4. The total dipole moment per unit volume of the dielectric is________.

(a)

induction

(b)

charge distribution

(c)

polarisation

(d)

quantisation

5. The repulsive force between two like charges of 1 coulomb each separated by a distance of 1 m in vacuum is equal to :

(a)

9 x 109 N

(b)

109 N

(c)

9 X 10-9 N

(d)

9 N

6. Near a charged object A another earthed object B is placed. Charges are induced on B. If B is removed. After that the charge on A ________.

(a)

remains the same

(b)

decreases

(c)

increases

(d)

becomes zero

7. In a large building, there are 15 bulbs of 40W, 5 bulbs of 100W, 5 fans of 80W and 1 heater of 1kW are connected. The voltage of electric mains is 220V. The minimum capacity of the main fuse of the building will be

(a)

14 A

(b)

8 A

(c)

10 A

(d)

12 A

8. An electron moves straight inside a charged parallel plate capacitor of uniform charge density σ. Th e time taken by the electron to cross the parallel plate capacitor when the plates of the capacitor are kept under constant magnetic fi eld of induction $\vec { B }$is

(a)

${ \varepsilon }_{ ° }\frac { eIB }{ \sigma }$

(b)

${ \varepsilon }_{ ° }\frac { IB }{ \sigma {I} }$

(c)

${ \varepsilon }_{ ° }\frac { IB }{ {e}\sigma }$

(d)

${ \varepsilon }_{ ° }\frac { IB }{ \sigma }$

9. Two short bar magnets have magnetic moments 1.20 Am2 and 1.00 Am2 respectively. They are kept on a horizontal table parallel to each other with their north poles pointing towards the south. They have a common magnetic equator and are separated by a distance of 20.0 cm. The value of the resultant horizontal magnetic induction at the mid-point O of the line joining their centers is (Horizontal components of Earth’s magnetic induction is 3.6 x 10-5 Wb m-2 )

(a)

3.60 × 10-5 Wb m-2

(b)

3.5 × 10-5 Wb m-2

(c)

2.56 × 10-4 Wb m-2

(d)

2.2 × 10-4 Wb m-2

10. A thin semi-circular conducting ring (PQR) of radius r is falling with its plane vertical in a horizontal magnetic fi eld B, as shown in the fi gure.

The potential diff erence developed across the ring when its speed v, is

(a)

Zero

(b)

$\frac { { Bv\pi { r }^{ 2 } } }{ 2 }$and P is at higher potential

(c)

πrBv and R is at higher potential

(d)

2rBvand R is at higher potential

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. A power of 11,000 W is transmitted at 220 V. The current through line wires is

(a)

50 A

(b)

5 A

(c)

500 A

(d)

0.5 A

13. 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

14. Cellular phones use radio frequencies in

(a)

AM band

(b)

UHF band

(c)

FM band

(d)

upto 54 MHz

15. The absorption spectra are characteristic of the __________.

(a)

source

(b)

absorbing substance

(c)

time of travel from source to screen

(d)

spectrometer used to study the spectrum

16. A plane glass is placed over a various coloured letters (violet, green, yellow, red) The letter which appears to be raised more is,

(a)

red

(b)

yellow

(c)

green

(d)

violet

17. Two plane mirrors are parallel to each other an spaced 20cm apart. An object is kept in between them at lScm from A. out of the following at which point on image is not formed in mirror A (distance measured from mirror A)

(a)

15 cm

(b)

25 cm

(c)

45 cm

(d)

55 cm

18. A concave mirror of radius of curvature 60 cm is place at the bottom and height of 20 cm, The mirror faces upwards with its axis vertical. Solar light fall normally on the surface of water and image of sun is formed. If a $\\ \\ \\ { \mu }_{ \omega }=\cfrac { 4 }{ 3 }$ then will the observer in air, the distance of image from the surface of water is

(a)

30cm

(b)

10 cm

(c)

7.5 cm above

(d)

7.5 cm below

19. A light source of wavelength 520 nm emits 1.04 × 1015 photons per second while the second source of 460 nm produces 1.38 × 1015 photons per second. Then the ratio of power of second source to that of first source is

(a)

1.00

(b)

1.02

(c)

1.5

(d)

0.98

20. The photo current in an experiment on photoelectric effect increases if

(a)

the intensity of the source is increased

(b)

the exposure time is increased

(c)

the intensity of the source is decreased

(d)

the exposure time is decreased

21. The clocks in moving ships will appear to go _______ clocks on earth.

(a)

faster than

(b)

slower than

(c)

the same as

(d)

stops after leaving earth

22. The half-life period of a radioactive element A is same as the mean life time of another radioactive element B. Initially both have the same number of atoms. Then

(a)

A and B have the same decay rate initially

(b)

A and B decay at the same rate always

(c)

B will decay at faster rate than A

(d)

A will decay at faster rate than B.

23. According to to Bohr, the relation between principal quantum number (n) and radius of the orbit (r) is

(a)

r α $\frac1n$

(b)

r α $\frac{1}{n^2}$

(c)

r α n

(d)

r α n2

24. A N-P-N transistor' conducts when collector is _________ and emitter is ________ with respect to base

(a)

positive, negative

(b)

positive, positive

(c)

negative, negative

(d)

negative, positive

25. 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