## Current Electricity Important Questions for CBSE Class 12 Physics Resistance and Ohm’s Law

1.The directed rate of flow of electric charge through any cross-section of a conductor is known as electric current.

where, n = number of charged particles constitute the current
NOTE Current is a scalar quantity
I is in the direction of flow of positive charge and opposite to the direction of flow of negative charge.
SI unit of current is ampere and is represented by A.
Ampere is defined as,

2.The current density at a point in a conductor is the ratio of the current at that point in the conductor to the area of cross-section of the conductor at that point provided the area is held normal to the direction of flow of current.

NOTE Current density is a vector quantity

3.Flow of Electric Charge in Metallic Conductors Among the solids, all metals are good conductors of electricity. The cause of conductance is free electrons.
In Case of a Solid Conductor (i.e. Cu, Fe, Ag, etc) atoms are tightly bound to each other. There are large number of free electrons in them.
In Case of a Liquid Conductor Like electrolytic solution, there are positive and negative charged ions which can move on applying electric field.

4.Drift Velocity It is defined as the average velocity with which the free electrons move towards the positive end of a conductor under the influence of an external electric field applied

### 1 Mark Questions

1.Plot a graph showing variation of current versus voltage for the material GaAs. [Delhi 2014]
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2.Show variation of resistivity of copper as a function of temperature in graph. [Delhi 2014; All India 2014]
Ans.Graph of resistivity of copper as a function of temperature is given below (resistivity of metals increases with increase in temperature)

3.Define the term drift velocity of charge carriers in a conductor and write its relationship with the current flowing through it. [Delhi 2014]
Ans.The term drift velocity of charge carriers in a conductor is defined as the average velocity acquired by the free electrons along the length of a metallic conductor under a potential difference applied across the conductor.Its relationship is expressed as

where, I is current flowing through the conductor,n is concentration of free electrons
e is electron i.e.charge
A is cross-sectional area

4.Define the term electrical conductivity of a metallic wire. Write its SI unit.  [Delhi 2014]
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5.Show variation of resistivity of Si with temperature in graph.[Delhi 2014]
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6.Define the term mobility of charge carriers in a conductor. Write its SI unit.  [Delhi 2014]
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7.Write a relation between current and drift velocity of electrons in a conductor. Use this relation to explain how the resistance of a conductor changes with the rise in temperature ? [Compartment  2013]
Ans.Relation between current and drift velocity of electrons in a conductor is given by l = Anevd
where l = current, A = area of conductor, n = number density of electrons and vd = drift velocity.
with the increase in temperature of a metallic conductor, resistance increases and hence, drift velocity decreases

8.When electrons drift in a metal from lower to higher potential, does it mean that all the free electrons of the metal are moving in the same direction?  [Delhi 2012]
Ans.No, the drift speed of electrons is superposed over the random velocities of the electrons

9.Show on a graph, the variation of resistivity with temperature for a typical semiconductor. [Delhi 2012]
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10.Two wires of equal length, one of copper and the other of manganin have the same resistance. Which wire is thicker? [All India 2012]
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11.Define resistivity of a conductor. Write its SI unit.  [All India 2011]
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13.Two identical slabs, of a given metal, are joined together, in two different ways, as shown in figures
(a) and (b). What is the ratio of the resistances of these two combinations? [Delhi 2010 c]

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14.Two conducting wires X and Y of same diameter but different materials are joined in series across a battery. If the number density of electrons in X is twice than that in Y, then find the ratio of drift velocity of electrons in the two wires.[All India 2010]
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15.The three coloured bands, on a carbon resistor are red, green and yellow, respectively. Write the value of its resistance. [All India 2009c]
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16.Write an expression for the resistivity of a metallic conductor showing its variation over a limited range of temperatures. [Delhi 2008 C]
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17.Define ionic mobility. Write its SI Unit.[Foreign 2008]
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### 2 Marks Questions

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22.Draw a plot showing the variation of resistivity of a (i) conductor and (ii) semiconductor, with the increase in temperature. How does one explain this behaviour in terms of number density of charge carriers and the relaxation time? [Delhi 2014 C]
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23.Derive an expression for the current density of a conductor in terms of the drift speed of electrons. [Foreign 2014]
Ans.Let potential difference V is applied across a conductor of length / and hence, an electric field E produced inside the conductor.

24.A conductor of length / is connected to a DC source of potential V. If the length of the conductor is tripled by gradually stretching it, keeping V constant, how will
(i)drift speed of electrons and
(ii)resistance of the conductor be affected? Justify your answer.[HOTS; Foreign 2012]
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25.Plot a graph showing temperature dependence of resistivity for a typical semiconductor. How is this behaviour explained? [Delhi 2011]
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27.The sequence of coloured bands in two carbon resistors Rt and R2 is
(i)brown, green, blue and
(ii)orange, black, green.
Find the ratio of their resistances.[Delhi 2010 C]
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29.Derive an expression for drift velocity of free electrons in a conductor in terms of relaxation time. [Delhi 2009]
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30.Two metallic wires of the same material have the same length but cross-sectional area is in the ratio 1: 2. They are connected
(i)in series and
(ii)in parallel.
Compare the drift velocities of electrons in the two wires in both the cases.  [All India 2008]
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31.Derive an expression for the resistivity of a good conductor, in terms of the relaxation time of electrons.       [All India 2008]
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### 3 Marks Questions

Ans.(i)The term drift velocity of charge carriers in a conductor is defined as the average velocity acquired by the free electrons along the length of a metallic conductor under a potential difference applied across the conductor.Its relationship is expressed as

where, I is current flowing through the conductor,n is concentration of free electrons
e is electron i.e.charge
A is cross-sectional area

33.Define relaxation time of the free electrons drifting in a conductor. How it is related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.[All India 2012]
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37.Three resistors R1, R2 and Rare connected in parallel, across a source of emf E and negligible internal resistance. Obtain a formula for the equivalent expressions for the current through each of the three resistors. [All India 2009 c]
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38.Prove that the current density of a metallic conductor is directly proportional to the drift speed of electrons.[Delhi 2008]
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