12th physics NCERT chapter 3 CURRENT ELECTRICITY -short Question Answer
Q.1-An student connects voltmeter in series and ammeter in parallel in a circuit. What will be its result?
Bihar Board 2009
Answer - If by mistake voltmeter is connected in series in a circuit then resistance of the circuit becomes very high due to which very low current or No current flows through the circuit.
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On the other hand, if ammeter is connected in parallel then very high current flows through the circuit due to which needle of ammeter may break or coil may burn.
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| 12th physics chapter 3 |
Q.2- Potentiometer and voltmeter, both are used to measure potential difference. Why are the two devices used for a single work?
Answer - Voltmeter does not measure the exact value of potential difference because it takes few current from the circuit that is why it measures potential difference less than the exact value.
On the other hand potentiometer does not take any current from the circuit thus it measures exact value of potential difference. It works like a voltmeter of infinite resistance.
Q.3-Write down the limitations of ohm’s law.
Q.3-Write down the limitations of ohm’s law.
Answer- Followings are the limitations of Ohm's law.
(1)Ohm’s law is not obeyed by non-conductors and semi conductors.
(2)Ohm’s law is not obeyed by diodes, transistors and thyristor.
(3)Ohm’s law is not obeyed by vacuum tube.
(4)Ohm’s law is not obeyed by electrolyte.
(5)Ohm’s law is not obeyed in electric arc.
Q.4- why is internal resistance of a secondary cell is low ?
Bihar Board 2009
Answer : The plates of secondary cell (A rechargeable cell is called secondary
cell) are large and there is no polarization in secondary cell so internal
resistance of secondary cell is low.
Q.5-Write down the expression of e.m.f. for the parallel combination of cells.
Answer-
\[{{E}_{equivalent}}=\frac{{{E}_{1}}{{r}_{2}}+{{E}_{2}}{{r}_{1}}}{{{r}_{1}}+{{r}_{2}}}\]
Q.6- Write the expression for the resistivity of the conductor and explain each term of the expression.
Bihar Board 2011
Answer- Resistivity =\[\rho =\frac{R\times
A}{l}\]
Where R= resistance of the conductor
A= cross-section area of the conductor
l= length of the conductor
Q.7- Define the term resistivity and state its SI unit. Draw a graph showing variation of resistivity with change in temperature for a typical
semiconductor .
Bihar Board 2009
Answer- \[\rho =\frac{R\times
A}{l}\]
If l=1m
A = 1${{m}^{2}}$
then ρ=R
the numerical value of resistance of a wire of unit length and unit cross-section area
is called resistivity .
Resistivity of a semiconductor- resistivity of a semiconductor decreases non-uniformly with increase in temperature.
.
Q.8-What is difference between the flow of electricity through solids and liquids.
Answer- Flow of electricity is caused in solids by free electrons but in liquids by ions.
Q.9-Aqueous solution of sugar is not conductor. why?
Answer- In aqueous solution of sugar molecules of sugar do not disintegrate into ions that is why aqueous solution of sugar is not conductor.
Q.10-Although pure water and dry salt is non-conductor of electricity but their aqueous solution is conductor. Why?
Answer-There are not ions in pure water and not free electrons in dry salt so they are non-conductors of electricity. But in their aqueous solution salt disintegrates into ions that is why aqueous solution of salt is a conductor.
Q.11-Drift velocity is uniform why?
Answer- When we apply electric field across a wire. Electric force acts on free electron due to which it accelerated in the opposite direction of field. But these accelerated free electrons frequently collide with stationary atom and ions that is why the free electrons do not get net increase in their velocities. Hence drift velocity of free electrons in a conductor is uniform.
Q.12- Using theory of drift velocity, express Ohm's law.
Bihar Board 2018
Answer- 
Let PQ is a wire of length l and cross-section area A.
Potential difference of the wire = V
Electric field applied across the wire= E=V/l
since we know that ${{V}_{d}}=\frac{eE}{m}\tau
=\frac{e\tau }{m}\times \frac{V}{l}....(i)$
Again ,we know $I=nAe{{V}_{d}}=nAe\times
\frac{e\tau }{m}\times \frac{V}{l}$
$\frac{V}{I}=\frac{ml}{n{{e}^{2}}A\tau
}$= Constant
$\frac{ml}{n{{e}^{2}}A\tau
}$ is a constant quantity ,which is called resistance of the conductor.
$\frac{V}{I}=R$
$V\alpha I$
This is Ohm's law.
Q.13-Which resistance circuit is used in meter bridge. Write down the condition of balanced position.
Answer-Balanced Wheatstone bridge is used in meter bridge.
for balanced Wheatstone bridge.
$\frac{{{R}_{1}}}{{{R}_{2}}}=\frac{{{R}_{3}}}{{{R}_{4}}}$
This is required condition.
Q.14-An electric bulb is marked 220v, 100w. if its resistance is measured by a multimeter, it turns out to be 60Ω. Why?
Answer- P = 100w
V = 220v
P = V²/R
R = V²/P = 220 × 220/100
R = 484 ohm
This is resistance when bulb glows .Resistance of tungsten coil decreases with decrease in temperature that is why Multimeter measures its resistance 60Ω at room temperature.
Q.15- What do you mean by colour code of carbon resistance ?
Bihar Board 2020
Answer- see in note of (Nawendu physics classes–Muzaffarpur.)
Q.16- Write down two characteristics of fuse wire.
Bihar Board 2014
Answer- There are following two properties of fuse wire.
(i) fuse wire must have very low melting point so that during flow of heavy current it melts quickly.
(ii) fuse wire must have high conductivity (or low resistivity) so that it allows maximum current to flow through it.
Q.17-State kirchhoff's two laws for electrical network.
Bihar Board 2016
Answer- Write from the note of Nawendu physics classes–Muzaffarpur.
Q.18- State the principle of potentiometer. Draw a diagram used to compare the e.m.f of two primary cells.
Bihar Board 2009
Answer- Principle of potentiometer –
If constant current is following through a wire of uniform cross section area
then potential difference between any two points of the wire is directly proportional
to distance (length) between the points.
V∝ l
V = Kl
Where K = potential gradient
Circuit diagram to compare e.m.f of two cells-
If balance point for ${{E}_{1}}$ is found at ${{l}_{1}}$distance from A and that for ${{E}_{2}}$ is found at ${{l}_{2}}$ distance from A
then $\frac{{{E}_{1}}}{{{E}_{2}}}=\frac{{{l}_{1}}}{{{l}_{2}}}$
Q.19- Explain that Kirchhoff's second law is law of conservation of energy.
Bihar Board 2017
Answer- Kirchhoff's seond law states-
" For a closed loop of a circuit, algebraic sum of potential change is zero."
for given example
IR-E=0
E=IR
E=V
Energy (E.M.F.) given to unit charge by the cell is equal to work done (potential difference) on the unit charge to flow through the resister.
Hence, Kirchhoff's second law is (obeys) law of conservation of energy.
Q.20- What is Seebeck effect?
Bihar Board 2010,2011
Answer- When both ends of a thermoelectric couple (e.g.. Copper-Iron, Copper-Constantan, Constantan-Chromel ,Constantan- Iron ) is connected and one junction is heated and the other junction is cooled. The electric current begins to flow through the circuit. It is called Seebeck effect.
Q.21-What is Peltier effect?
Answer- When both ends of a thermoelectric couple is connected and electric current is allowed to flow through it then as a result one junction becomes hot and the other junction becomes cold. It is called Peltier effect.
Q.22- Write any two features on which thermo e.m.f produced in thermo
electric couple depends.
Bihar Board 2011
Answer- Thermo e.m.f depends upon following factors.
(i) Thermo e.m.f depends upon the temperature different between
both ends of a thermocouple.
(ii) Thermo e.m.f depends upon electro-negativity and electro-positivity of the thermocouple metals.
Q.23- State and explain Thomson effect.
Bihar Board 2011
Answer- When electric current flows through an unequally heated conductor , there is absorption or evolution of heat in the conductor.
This is known as Thomson effect.
Thomson effect is of two types.
(i) Positive Thomson effect- Heat is evolved when current is passed from hotter end to the colder end and heat is absorbed when current is passed from colder end to hotter end.
The metals which show positive Thomson effect are Sn, Cu, Ag, Cd, Zn etc.
In positive Thomson effect , it is found that hot end is at high potential and the cold end is at low potential.
(ii) Negative Thomson effect- Heat is evolved when current is passed from colder end to the hotter end and heat is absorbed when current is passed from hotter end to colder end. The metals which show negative Thomson effect are Fe, Co, Bi, Pt, Hg etc.
In negative Thomson effect , it is found that hot end is at low potential and the cold end is at high potential.
Q.24-What is difference between joule effect and peltier effect?
Q.24-What is difference between joule effect and peltier effect?
Answer - (i) joule effect produces in whole wire but peltier effect produces at junctions.
(ii) Direction of current does not make any difference in joule effect. But in peltier effect when we reverse the direction of current hot junction becomes cold and cold junction becomes hot.
(iii) joule effect occurs in any conductor but peltier effect occurs in thermoelectric couples.
(iv) Heat generated in joule effect is directly proportional to square of electric current but heat generated in Peltier effect is directly proportional to current.
Q.25- Describe the effect of temperature on resistivity of metal, resistivity of nichrome and resistivity of semiconductor.
Q.25- Describe the effect of temperature on resistivity of metal, resistivity of nichrome and resistivity of semiconductor.
Answer - Metal-resistivity of a metal (conductor) increases non-uniformly with increase in temperature.
Nichrome – Resistivity of nichrome increases (Alloy) linearly with increase in temperature.
Semiconductor – resistivity of a semiconductor decreases non-uniformly with increase in temperature.
Q.26- Write down two uses of potentiometer.
Bihar Board 2022
Answer – Two uses of potentiometer are as follows
(i) It is used to determine the internal resistance of a cell.
(ii) It is used to compare the e.m.f. of two cells.
Q.27- Explain parallel grouping of resistors.
Bihar Board 2022
Answer – Write from the note of Nawendu physics classes–Muzaffarpur.
Q.28- Write color code of carbon resistance for orange and yellow colors .
Bihar Board 2022
Answer – For orange color-
Numerical value= 3
Decimal multiple=\[{{10}^{3}}\]
For yellow color-
Numerical value= 4
Decimal multiple=\[{{10}^{4}}\]
Q.29 Differentiate between electromotive force (e.m.f.) and terminal potential difference of a cell .
Bihar Board 2023
Answer – electromotive force (e.m.f.)-
(i) Electromotive force is the potential difference between two electrodes of a cell in the open circuit . i.e. Electromotive force is the maximum potential difference between two electrodes of a cell .
(ii) e.m.f. is measured by a potentiometer. terminal potential difference-
(i) terminal voltage of a cell is the potential differnce between two electrodes of a cell in closed circuit
(ii) terminal potential difference is measured by a voltmeter. Q.30. What is the relationship between drift velocity of free electrons and electric current density ?
Bihar Board 2023
Answer – \[j=\frac{I}{A}=\frac{nAe{{V}_{d}}}{A}\]
$j=ne{{V}_{d}}$
i.e. drift velocity of free electrons is directly proportional to electric current density.
chapter 1 ELECTRIC CHARGES AND FIELDS short Q-A
chapter 2 ELECTROSTATIC POTENTIAL AND CAPACITANCE short Q-A
chapter 4 MOVING CHARGES AND MAGNETISM short Q-A
chapter 5 MAGNETISM AND MATTER short Q-A
chapter 6 ELECTROMAGNETIC INDUCTION short Q-A
chapter 7 ALTERNATING CURRENT short Q-A
chapter 8 ELECTROMAGNETIC WAVES short Q-A
chapter 2 ELECTROSTATIC POTENTIAL AND CAPACITANCE short Q-A
chapter 4 MOVING CHARGES AND MAGNETISM short Q-A
chapter 5 MAGNETISM AND MATTER short Q-A
chapter 6 ELECTROMAGNETIC INDUCTION short Q-A
chapter 7 ALTERNATING CURRENT short Q-A
chapter 8 ELECTROMAGNETIC WAVES short Q-A
Very useful sir🙏
ReplyDeleteSir very usefull for all of students
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