#### 12th Standard Physics Magnetism and magnetic effects of electric current English Medium Free Online Test One Mark questions with Answer Key 2020-2021

12th Standard

Reg.No. :
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Physics

Time : 00:20:00 Hrs
Total Marks : 20

20 x 1 = 20
1. 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)

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

2. 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)

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

3. A non-conducting charged ring of charge q, mass m and radius r is rotated with constant angular speed ω. Find the ratio of its magnetic moment with angular momentum is

(a)

$\\ \frac { q }{ m }$

(b)

$\\ \frac { 2q }{ m }$

(c)

$\\ \frac { q }{ 2m }$

(d)

$\\ \frac { q }{ 4m }$

4. 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)

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

5. In a thermocouple, the temperature of cold junction is 30°C and the neutral temperature is 310°C. The temperature of inversion is

(a)

580°C

(b)

590°C

(c)

600°C

(d)

690°C

6. In a tangent galvanometer a current 1A, produces deflection of 30° the current required to produce a deflection of 60° is

(a)

3A

(b)

2A

(c)

$\sqrt { 2 }$

(d)

$\frac { 1 }{ \sqrt { 3 } }$A

7. Ampere's circuital law is another form of

(a)

Tangent law

(b)

Biot-Savart law

(c)

Ampere's Swimming rule

(d)

End rule

8. The magnitude and direction of magnetic Lorentz force

(a)

$\vec { F } =\vec { v } \times \vec { B }$

(b)

$\vec { F } =q/(\vec { v } \times \vec { B } )$

(c)

$\vec { F } =q(\vec { v } \times \vec { B } )$

(d)

$\vec { F } =v(\vec { v } \times \vec { B } )$

9. The value of Bohr magneton is

(a)

9.27 x 10-34 Am2

(b)

9.72 x 10-34 Am2​​​​​​​

(c)

9.27 x 10-24 Am2

(d)

9.72 10-24 Am2

10. The unit of pole strength (magnetic charge) is _______

(a)

A/m

(b)

Am2

(c)

A/m2

(d)

Am

11. A wire of length 1 m carrying a current of 2 A is placed inside a field of magnetic induction 20 T such that it makes an angle of 30° with the direction of the field. The force experienced by the wire is

(a)

40N

(b)

10$\sqrt { 3 }$ N

(c)

20N

(d)

20$\sqrt { 3 }$N

12. To convert a galvanometer into an ammeter we connect which one of the following to the galvanometer?

(a)

a low resistance in series

(b)

a high resistance in series

(c)

a low resistance in parallel

(d)

a high resistance in parallel

13. The resistance of the filament of a 110 W, 220 V electric bulb is,

(a)

440 Ω

(b)

220 Ω

(c)

484 Ω

(d)

848 Ω

14. Which one of the following has positive Thomson effect

(a)

Zn

(b)

Bi

(c)

Ni

(d)

Hg

15. In a thermocouple, temperature of the cold junction is 20°C, the inversion of temperature is 600°C, then neutral temperature is

(a)

310°C

(b)

320°C

(c)

300°C

(d)

315°C

16. A deutron and an alpha particle move in a magnetic field along the same circular path, then the ratio of their velocities is _______

(a)

1:2

(b)

2:1

(c)

4:1

(d)

1:1

17. The tangent galvanometer has 50 turns of diameter 10 cm. If B = 13.14 x 10-5 wb/m2, the reduction factor is ______

(a)

0.4 A

(b)

0.05 A

(c)

0.4 x 10-3 A

(d)

0.4 x 10-6 A

18. Force on unit length of two parallel wires carrying equal current kept apart by 3m in air is 2 x10-7 N, then the current in each wire is ______

(a)

1A

(b)

3A

(c)

$\sqrt { 3 }$A

(d)

$\sqrt { 2 }$A

19. Torque acting on a coil is maximum when the coil is ______

(a)

parallel to the magnetic field

(b)

perpendicular to the magnetic field

(c)

at an angle of 45° to the magnetic field

(d)

at any position in a uniform magnetic field

20. The unit of Galvanometer constant is ________

(a)

Am-1

(b)

(c)

AC-1

(d)

As-1