Question and answer
on
Electric Current
The theory of this topic:
Class 12, Physics Tutorials, Module – 02, Chapter – 01, Electric Current
For 1 marks :
1 ) Name the constant physical quantity related to Ohm’s law.
Ans : Temperature of the conductor and other physical condition.
2 ) What will be the resistance of the conductor, if its cross-sectional area is halved while its length remains unchanged?
Ans : If the cross-sectional area of the conductor is halved, its resistance becomes twice.
3 ) Name any one factor on which internal resistance depends?
Ans : Nature of the electrolytes
4 ) What will be the expression for the internal resistance of the cell if it is discharging?
Ans : ir = E – V
5 ) What will be the expression for the internal resistance of the cell if it is charging?
Ans : ir = E + V
6 ) What is the positive temperature coefficient?
Ans : If the resistance increases with the increase in temperature by unity, then the temperature coefficient is called positive temperature coeffiecient.
7 ) What is the negative temperature coefficient?
Ans : If the resistance decreases with the increase in temperature by unity, then the temperature coefficient is called negative temperature coeffiecient.
8 ) What is the nature of the temperature coefficient of the good conductor?
Ans : Positive
9 ) What is the nature of the temperature coefficient of the semiconductor?
Ans : Negative
10 ) How electric current varies with potential difference across the ends of the conductor?
Ans : Directly varies
11 ) Draw the graph for the non-ohmic conductor.
Ans :
12 ) What is relaxation time?
Ans : The relaxation time is defined as the time during which two successive collision occured.
13 ) What is the drift velocity of the electron moving under the influence of the electric field E and relaxation time is τ?
Ans : vd = ( e E / m ) τ
wher ‘ m ‘ is the electronic mass, ‘ e ‘ is the electronic charge.
14 ) What will be the effect on the electric current flowing through the conductor if the drift velocity becomes twice?
Ans : The electric current also become twice.
15 ) What will be the equivalent resistance of the parallel combination of two equal resistors?
Ans : The equivalent resistance of the parallel combination of two equal resistors is half of the resistance of each resistor that is’
Rp = R / 2
16 ) What is the SI – unit of electron mobility?
Ans : ms-1N-1C
For 2 or 3 marks :
1 ) What are the factors influencing the internal resistance of the cell?
Ans : Internal resistance of the cell depends on the following factors :
- distance between two plates
- nature of electrolytes
- nature of nature of electrodes
- area of plates
- the concentration of the electrolytes
2 ) Establish the expression for the internal resistance of the cell?
Ans :
Following the circuit,
r = internal resistance
R = external resistance
E = E.M.F of the cell
V = voltage drop across R = iR
i = current flowing throgh the circuit = emf of the battery / total resistance of the ciruit
i = E / ( R + r )
⇒ iR + ir = E
⇒ V + ir = E
⇒ ir = E – V ——————————————— ( A )
⇒ r = ( E – V ) / i ———————————– ( 1 )
∵ i = V / R, putting this value of i in the above eqaution, we get
r = ( E – V ) / ( V / R )
⇒ r = ( E / V – V / V ) R
⇒ r = ( E / V – 1 ) R ————————————- ( 2 )
3 ) What is the relation between the electric current and drift velocity?
Ans :
let
l = length of the conductor
A = cross sectional area of the conductor
V = potential differnce across the two ends of the conductor.
n = number of the free electrons per unit volume
N = total number of free electrons = n * volume of the conductor = nAl
All free electrons move randomly before applying any electric field. When the electric field is applied to the conductor , all the electrons arrnaged and move in a specific direction and constitutues electric current flowing in opposite direction of the electorn. This electric current is nothing but the drift velocity.
Let I = electric current flowing through the conductor
I = total charge / t
I = Ne / t
I = nAle / t
I = neA ( l / t )
I = neAvd
4 ) What is the relation between the electric current flowing through the electric conductor and the applied electric field?
Ans :
Let v1 , v2 , v3 …………… are the final velocity of the electrons movign under the influence of applied electirc field E. τ is the relaxation time between two succesive colission of electrons when electric current flows ‘n’ ihe electron density.
vd = ( v1 + v2 + v3 + ………………… ) / n
vd = { ( u1 + aτ1 ) + ( u1 + aτ2 ) + ………………………. } / n
vd = { ( u1 + u2 + …………….. ) / n } + { a ( τ1 + τ2 + ……………….. ) / n } —————- ( 1 )
Before applying electirc field, the average velocity is zero as all electrons move randomly and effect produced by these electrons are canclled by each other.
therfore,
( u1 + u2 + …………….. ) / n = 0
and we Know that force experineced by each electron is given by
F = e E
ma = eE
a = eE / m
and ( τ1 + τ2 + ……………….. ) / n = τ = average relaxation time
where, a is the acceleration produced by each electron, m is the mass of the electorn, E is the applied electric field.
Now, putting the value of ‘a ‘ in equaion ( 1 ), we get
vd = 0 + eEτ / m
vd = ( e E / m ) τ
5 ) Define the term electron mobility?
Ans : The drift velocity of free electrons per unit electric field is called the electron mobility.
6 ) What is the relation between the electron mobility and relaxation time?
Ans : Electron mobility ( μ ) = drift velocity / electric field
μ = ( eEτ / m ) / E
μ = eτ / m
where e is the electronic charge, m is the electronic mass.
7 ) What is the relation between the electron mobility and electric current?
Ans : We know that,
I = neAvd
I = neA ( eEτ / m )
I = neAE ( eτ / m )
I = neAEμ
where n = electron density
e = electronic charge
A = cross-sectional area
E = Electric field
μ = Electron mobility
8 ) Define the current density?
Ans : The quantity of the electric current flowing per unit cross-sectional area of the conductor is called current density.
9 ) What is the relation between the current density and electron mobility?
Ans : We know that,
I = neAvd
and current density is given by
J = I / A
J = ( neAvd ) / A
J = nevd
J = ne ( eEτ / m ) [ vd = eEτ / m ]
J = neE ( eτ / m )
J = neEμ
10 ) What is the relation between the current density and the relaxation time?
Ans :
We know that,
I = neAvd
and current density is given by
J = I / A
J = ( neAvd ) / A
J = nevd
J = ne ( eEτ / m ) [ vd = eEτ / m ]
J = ne2Eτ / m
11 ) Show that the resistance of parallel combination of resistors is less than the individual resistance.
Ans :Let the equivalent resistance of the parallel combination of the n resistors is Rp
1 / Rp = 1 / R1 + 1 / R2 + 1 / R3 + ……………………… + 1 / Rn
For two different resistors,
1 / Rp = 1 / R1 + 1 / R2
Rp = R1R2 / ( R1 + R2 )
1 / Rp = 1 / R1 + 1 / R2
Rp / R1 = R2 / ( R1 + R2 ) < 1
Rp < R1
Hence, we can see that the equivalent resitance of the parallel combination of the resistors is always less than the individual one.
12 ) Show that the resistance of series combination of resistors is greater than the individual resistors.
Ans : Let the n resistors are connected in series combination. The equivalent resistance is given by
Rs = R1 + R2 + R3 + ……………………… + Rn
( R2 + R3 + ……………………… + Rn ) is a positive quantity
Hence,
Rs = R1 + Positive quantity
Rs > R1
Thus the resistance of the series combination of the resistors is greater than the individual.
13 ) What is voltmeter? How it is connected in an electronic circuit ?
Ans : A voltmeter is an electronic device used for measuring the voltage in an electric circuit. It is connected in parallel in a circuit as it has high internal resistance.
14 ) Define ammeter. How the resistance is connected with the ammeter in an electric circuit?
Ans : An ammeter is an electronic device used for measuring the electric current in the electronic circuit. It is always connected in series as it has very low internal resistance.
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