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

Download Tamil Nadu 12th Standard Physics question papers, model tests, one-mark questions, important questions, and public exam papers in PDF format. Free study materials and answer keys for TN State Board students.

Questions:

Multiple Choice Question

25 x 1 = 25

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

(a)

\(\frac{1}{3}\)

(b)

(c)

(d)

\(\frac{1}{9}\)

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

(a)

half wave

(b)

full wave

(c)

voltage

(d)

bridge

__________ type of modulation costless.

(a)

Frequency

(b)

Time

(c)

Amplitude

(d)

Phase

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

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

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

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

The electric potential of an electron is given by \(V={ V }_{ 0 } \ In\left( \frac { r }{ { r }_{ 0 } } \right) \), where r₀ is a constant. If Bohr atom model is valid, then variation of radius of n^th orbit r_n with the principal quantum number n is _____.

(a)

\({ r }_{ n }∝ \frac { 1 }{ n } \)

(b)

\({ r }_{ n }∝ n\)

(c)

\({ r }_{ n }∝\frac { 1 }{ { n }^{ 2 } } \)

(d)

\({ r }_{ n }∝ { n }^{ 2 }\)

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)

4λ

(b)

5λ

(c)

\(\frac{5}{2}λ\)

(d)

3λ

10.

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

11.

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

12.

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

13.

A long straight 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

14.

Eddy currents was first observed by ________________.

(a)

Foucault

(b)

Newton

(c)

Faraday

(d)

Fleming

15.

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

16.

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

17.

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

(a)

copper

(b)

platinum

(c)

mercury

(d)

carbon

18.

The expression for electric potential 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)

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

19.

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)

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

20.

A simple pendulum with charged bob is oscillating with time period T and lets θ be the angular displacement. If the uniform magnetic field 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

21.

A wire of length l carrying a current I along the Y direction is kept in a 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)

\(\sqrt { \frac { 2 }{ { 3 } } } \beta Il\)

(b)

\(\sqrt { \frac { 1 }{ { 3 } } } \beta Il\)

(c)

\(\sqrt { 2 } \beta Il\)

(d)

\(\sqrt { \frac { 1 }{ 2 } } \beta Il\)

22.

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

23.

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

24.

An electric dipole is placed at an alignment angle of 30^o with an electric field of 2 x 10⁵ NC^-1. It experiences a torque equal to 8 N m. The charge on the dipole if the dipole length is 1 cm is

(a)

4 mC

(b)

8 mC

(c)

5 mC

(d)

7 mC

25.

What is the current drawn out from the battery?

(a)

1 A

(b)

2 A

(c)

3 A

(d)

4 A

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Answers Key:

Multiple Choice Question

25 x 1 = 25

(a)
\(\frac{1}{3}\)

(a)
half wave

(d)
both of them

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

\(\begin{array}{l} \alpha=\frac{I_c}{I_{\mathrm{c}}} ; \mathrm{I}_{\mathrm{c}}=\alpha \mathrm{I}_{\mathrm{t}}=9 \mathrm{~mA} \\ \mathrm{I}_{\mathrm{B}}=\mathrm{I}_{\mathrm{t}}-\mathrm{I}_{\mathrm{c}}=10-9=1 \mathrm{~mA} \end{array}\)

(b)
Frequency modulation

Electric potential in n^th orbit
\(\mathrm{V} =\mathrm{V}_0 \ln \left(\frac{\mathrm{r}_{\mathrm{n}}}{\mathrm{r}_0}\right) \)
\(=\mathrm{V}_0\left(\ln \mathrm{r}_{\mathrm{n}}-\ln \mathrm{r}_0\right) \)
\(=\mathrm{V}_0 \ln \mathrm{r}_{\mathrm{n}}-\mathrm{V}_0 \ln \mathrm{r}_0\)
\(\left|\mathrm{F}_\epsilon\right|=\mathrm{e} \frac{\mathrm{dv}}{\mathrm{dr}} =\mathrm{e} \frac{\mathrm{d}}{\mathrm{dr}}\left(\mathrm{V}_b / n \mathrm{r}_B-\mathrm{V}_0 / m \mathrm{r}_0\right) \)
\(=\mathrm{c}\left(\frac{\mathrm{V}_0}{\mathrm{r}_n}-0\right)=\frac{\mathrm{eV}}{\mathrm{r}_{\mathrm{n}}} \)
Centripetal force = coulomb force
\(\frac{m v^2}{r_n}=\mathrm{c} \frac{V_0}{r_n} \Rightarrow v=\sqrt{\frac{e V_0}{m}}=\text { constant }\)
Angular momentum,
\(\mathrm{mvr}_n=\frac{\mathrm{nh}}{2 \pi}\)
\(\mathrm{m}, \mathrm{v}, \mathrm{h}, 2 \pi\) are constants
hence, r_n ∝ n

\(\frac{\mathrm{hc}}{\lambda}=\phi+\mathrm{eV} \) .....(1)
\(\frac{\mathrm{hc}}{2 \lambda}=\phi+\frac{\mathrm{eV}}{4}\) .....(2)
multiply (2) eqn by 4
\(\frac{2 h c}{\lambda}=4 \phi+\mathrm{eV}\) .....(3)
subtract eqn (1) from (3), we get
\(\frac{ h c}{\lambda}=3 \phi \Rightarrow \phi = \frac{ h c}{3\lambda}\)
\(\frac{ h c}{\lambda_o}=\frac{ h c}{3\lambda}\)
⋋_o = 3⋋

10.

λ = 500 nm = 500 x 10^-9 m
x = 3 mm = 3 x 10^-3 m
a = 1 mm = 1 x 10^-3 m
\(d=\frac{xa}{1.22 \lambda}\)
\(d=\frac{3 \times1\times10^{-6}}{1.22 \times500\times10^{-9}}\)
\(=\frac{3 \times1\times10^{-6}}{6.10 \times 10^{-7}}\)
\(d=\frac{30}{6.1}=5 m\)

11.

(b)
0.4 ampere

12.

(a)
heating effect of current

13.

(d)
none of the above

14.

(a)
Foucault

15.

(b)
greater than the individual resistance

16.

(d)
zero

17.

(d)
carbon

18.

(d)
\(\int _{ R }^{ P }{ -\overset { \rightarrow }{ E } .\overset { \rightarrow }{ dr } } \)

19.

(a)
\(\frac { { q }^{ 2 } }{ 4\pi { \varepsilon }_{ 0 }r } \)

20.

21.

\(\vec { B } =\frac { \beta }{ \sqrt { 3 } } =(\hat { i } +\hat { j } +\hat { k } )T\)
Using an equation,
Lorentz force, \(\vec{F}=Il\hat{j}\times\vec B\)
We can get,
Lorentz force \(F=\sqrt { \frac { 2 }{ { 3 } } } \beta Il\)

22.

\(\mathrm{V}_{\mathrm{P}}=220 \mathrm{~V}, \mathrm{~V}_{\mathrm{s}}=11 \mathrm{~V} \)
\(\mathrm{I}_{\mathrm{P}}=6 \mathrm{~A}, \mathrm{I}_{\mathrm{s}}=100 \mathrm{~A} . \)
\(\text {Efficiency }=\frac{\mathrm{V}_{\mathrm{s}} \mathrm{I}_{\mathrm{s}}}{\mathrm{V}_{\mathrm{P}} \mathrm{I}_{\mathrm{P}}} \)
\(=\frac{11 \times 100}{220 \times 6}=\frac{1100}{220 \times 6}=\frac{5}{6}=0.83\)

23.

24.

(b)
8 mC

25.

\(\frac{1}{R_p}=\frac{1}{15}+\frac{1}{15}+\frac{1}{15}.\)
\(=\frac{1+1+1}{15}=\frac{3}{15}=\frac{1}{5}\)
∴ R_p = 5 Ω
\(\therefore I=\frac{V}{R_p}=\frac{5}{5}=1A\)