Creating a comprehensive document for Class 12 Physics Unit on Current Electricity involves curating a range of questions that cover various aspects of the topic thoroughly. Here’s an outline of what the document would include:
Document Title: Class 12 Physics Unit: Current Electricity Questio...
OPTIMUM PHYSICS(XII), Question Bank, CURRENT ELECTRICITY II-1
INDRA’S OPTIMUM PHYSICS
U-II CURRENT ELECTRICITY (7 marks) Kh. Indrajit, Senior Lecturer
Comprehensive Concept School
Basic Tips: E
Internal resistance, r 1 R
q V
Electric current, I
t Unidentical cells in series, εeq = ε1+ε2+……..+εn and req = r1+r2+……..+rn
dq Identical cells in series, εeq = nε and req = nr
Instantaneous current, I (t ) n
dt Current in the resistance R, I
I neAvd nr R
Ohms’s law V∝ I or V =IR r 2 r1 1 1 1
Two unidentical cells in parallel, eq 1 2 and
l r1 r2 req r1 r2
Resistance, R
A eq 1 2 eq 1 2 n
So, For ‘n’ number of cells .......
R2 R1 req r1 r2 req r1 r2 rn
Temperature co-efficient,
R1 t2 t1 For identical cells in parallel, εeq = ε and req = r/n
W V 2
Electric power, P VI I 2 R Current in a resistance R, I
t R r/nR
Heat energy, H=I2 Rt P R
Wheatstone’s bridge principle: at balance condition,
Relation between emf and terminal potential difference, E= V+Ir Q S
Questions and Answers
Q. Define 1A of current. q ne
I
1C t t
Ans: 1A
1s 1.6 10 -19
or 2 n
1A of current may be defined as the flow of 1C of charge through any 1
cross section in one second. or n 12.5 1018 electrons.
Q. How many electrons constitute a current of two ampere? Q. How many electrons pass through a lamp in one minute, if the current is
Ans: 500mA?
, OPTIMUM PHYSICS(XII), Question Bank, CURRENT ELECTRICITY II-2
Ans: Given, Q. Show that the electric current flowing through a conductor is directly
I 500 mA 500 10 3 A, t 60 s proportional to the drift velocity of electrons. (COHSEM 2011, 2017)
Ans: Let us consider a conductor of length ‘l’ and area cross section ‘A’.
ne
Now I Let n= no. of free electrons per unit volume.
t
Total no. of free electrons = n×Al
It 500 103 60
or n 19
1.875 1020 Total charge on the conductor, Q = (nAl)e
e 1.6 10 Q neAl l
Q. Define the term ‘drift velocity’. (COHSEM 2016)
Now, I neAvd where v d
t t t
Ans: It is define as the average velocity with which free electrons get drifted As n, A and e are constants
towards the positive end of the conductor under the influence of the I vd
electric field.
Q. What do you mean by resistance of a conductor? Give its S.I unit.
Q. How does the drift velocity of electron depend upon temperature of the (COHSEM 2015)
conductor? (CBSE 2002)
Ans: It is the opposition offered by the conductor to the flow of free electrons.
Ans: Drift velocity of a metal decreases with increase in temperature due to S.I unit-ohm (Ω)
increase in collision frequency of free electrons with ions or atoms of the Q. On what factors does the resistance of a conductor depend?
conductor. Ans: i) length of the conductor (𝑅 ∝ 𝑙)
Q. Inspite of large number of electrons in the conductor no current flows
1
through it in absence of any electric field across it, why? ii) area of cross section of the conductor ( R )
A
Ans: In a conductor, the free electrons move in all directions randomly. In the
iii) temperature (R∝ T)
absence of electric field, the average thermal velocity of free electrons is
Q How does resistance of a conductor increase with the rise in temperature?
zero. Therefore no current flows through the conductor in the absence of
(COHSEM 2013, 2022)
electric field. Ans: If the temperature is increased the frequency of collision of electrons with
Q. What do you mean by relaxation time or mean free time? ions increases. As a result relaxation time of electrons decreases.
Ans: The time interval between two successive collisions between electrons
1
and ions in a conductor is called relaxation time. But, R . Therefore resistance of the conductor increases as
Q. If the temperature of a good conductor increases, how does the relaxation
temperature increases.
time of electrons in the conductor change? (CBSE 2000)
Q. The figure shows graphs between potential difference (V) and current (I)
Ans: The relaxation time decreases as the frequency of electron collision with
of a metallic wire at three different temperatures T1 ,T2 , T3. Which of
ions increases.
them will have the least value of temperature? (COHSEM 2018)
Q. The drift velocity of electrons is very small, however the current is
established almost instantaneously when the circuit is closed, why?
Ans: When the circuit is closed, an electric field is set up inside the conductor
due to external battery. So, the free electrons experience a force at the
same time and start moving instantly in one direction. Therefore, current
starts flowing instantly.
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