Showing posts with label 2nd Year. Show all posts
Showing posts with label 2nd Year. Show all posts
Sunday, 23 April 2017
Saturday, 22 April 2017
Friday, 21 April 2017
Thursday, 28 November 2013
Q.No.13.9: - What is Wheatstone bridge? How it be used to determine an unknown resistance?
Q.No.13.9: - What is Wheatstone bridge? How
it be used to determine an unknown resistance?
Ans: - Wheatstone bridge is used to determine unknown resistance of
a wire. The circuit is consists of four resistances R1, R2,
R3, R4 connected in such a way as to form a loop or mesh
ABCDA. A battery of emf E is connected between points A and C. A galvanometer
of resistance Rg is connected between the points B and D.
When the bridge is balanced, it
satisfied the following relation
R1/R2 = R3/R4
Or, R4
= R2.R3/R1
If the value of R1,
R2, and R3 are known then R4 can be
calculated. The galvanometer should not show any deflection by adjusting the
potential between terminals B and D. (Sorry for Diagram.
Pleas concert with Text Book for diagram.)Q.No.13.8: - Explain why the terminal potential of a battery decreases when the current drawn from it is increased?
Q.No.13.8: - Explain why the terminal
potential of a battery decreases when the current drawn from it is increased?
Ans: - The terminal potential difference of a batter is
IR = E
– I.r
Vt = E
– I.r
Where, E is the emf of the battery, r is the
internal resistance of battery, and I.r is the potential difference across
internal resistance.
When I increased
then I.r becomes larger and terminal potential becomes small. Thus, we can say that
when we draw more current from battery then its terminal potential difference
will decrease.
Here
is a rough example for this concept. The example of current flow is just like a
water flow. Let suppose the pressure of water is just like the potential in the
electrical system. The dam have a large capacity of water and suppose you are
drawing more and more water from the dam and as a result water in the dam
becomes low level and now it has low pressure on water to flow. Similarly when
we draw more current from the battery then the terminal potential of the
battery decreases just like dam pressure on water flow. I just tried to make this answer
clear this is not a fit example for it but in my view you can understand
something from it.
Q.No.13.7: - Describe a circuit which will give a continuously varying potential?
Q.No.13.7: - Describe a circuit which will
give a continuously varying potential?
Ans: - A potential divider or potentiometer is a circuit which can
give a continuously varying potential. Consider a resistance R in the form of a
wire on which a terminal C can slide. The resistance between A and C can be
varied from 0 to R, as C slides from A to B.
If we connect a battery of emf E
across a resistance R. The current flowing thought it is
I = E/R
If we represent
the resistance between A and C by r, the potential drop between these points
will be
V = rI
Now,
V = r
x E/R
Thus as C slides
from A to B, r varies from 0 to R, and the potential drop between A and C
changes from zero to E. This arrangement by which potential can be varied
continuously from 0 to E is known as a potential divider. (Sorry for Diagram. Pleas
concert with Text Book for diagram.)
Q.No.13.6: - Is the filament resistance lower or higher in a 500W, 220V light bulb than in a 100W, 220V bulb?
Q.No.13.6: - Is the filament resistance
lower or higher in a 500W, 220V light bulb than in a 100W, 220V bulb?
Ans: -
Power of light
bulb = P1 = 500W
Potential
difference = V1 = 220V
Now apply,
P1 = V12/R1
R1 = V12/P1
R1 = (220)2/500
R1 = 48400/500
R1 = 96.8
Ω
For second Bulb
Power P2 = 100W
Voltage V2 = 220V
Now,
R2 = V12/P2
R2 = (220)2/100
R2 = 48400/100
R2 = 484
Ω
It means that the
filament resistance of 100W, 220V light bulb is higher than that of other.
Q.No.13.5: - What are the difficulties in testing whether the filament of a lighted bulb obeys Ohm’s law?
Q.No.13.5: - What are the difficulties in
testing whether the filament of a lighted bulb obeys Ohm’s law?
Ans: - According to Ohm when we increase voltage then current will
also increases in direct proportion but resistance of the conductor must remain
same and hence temperature also must remain same or constant. But when we test
the filament of a lighted bulb that either it obeys Ohm’s law or not then with
the passage of time its temperature increases and resistance also increases.
These are the difficulties in testing whether filament of a lighted bulb obeys
Ohm’s law or not.
Q.No.13.4: - Why does the resistance of a conductor rise with temperature?
Q.No.13.4: - Why does the resistance of a
conductor rise with temperature?
Ans: - Electrical resistance is due to the collision of free
electrons with atoms of the conductors. Atoms in the conductors are in
vibratory motion when electrons collide with atoms they transfer some of their
energy to the atoms and atoms starts vibrating with larger amplitude as the
amplitude of vibration increases the probability of collision of electrons also
increases. As a result increases resistance. When temperature increases then
also amplitude of vibration increases and then probability of electrons
collision also increases and then increases resistance.
Q.No.13.3: - What are the resistances of the resistors given in the figure A and B? What is the tolerance of each? Explain what is meant by tolerance?
Q.No.13.3: - What are the resistances of
the resistors given in the figure A and B? What is the tolerance of each?
Explain what is meant by tolerance?
(Sorry for Diagram but I’m trying to
describe this figure here)
1st band is Brown 2nd
is Green 3rd is Red 4th is Gold. In B fig. 1st
band is Yellow 2nd is White 3rd is Orange
Ans: - For Figure A
First band
brown = 1
Second band Green = 5
Third band Red (have number two so)= 00
Now, R =
1500Ω
Fourth band Gold which shows tolerance = ±5%
So, R
= 1500Ω±5%
For Figure B
First Band Yellow = 4
Second band White= 9
Third band orange = 000 (no. of zeros)
R =
49000 Ω
Fourth band silver = ±10%
Now resistance is
R =
49000±10% Ω
Tolerance: -
Tolerance means the possible variation from
the marked value.
Q.No.13.2: - Do bends in a wire affect its electrical resistance?
Q.No.13.2: - Do bends in a wire affect its
electrical resistance?
Ans: - The resistance of wire is
R =
ρ.L/A
Where “ρ” is the
resistivity of the conductor, L is the length and A is the area of cross
section of the wire. So according to this equation we can say that bends in a
wire will never affect the electrical resistance because we bending the wire
its ρ, L and A remain same so its resistance remain same.
Q.No.13.1: - A potential difference is applied across the ends of a copper wire. What is the effect on the drift velocity of free electrons by (i) Increasing the potential difference. (ii) Decreasing the length and the temperature of the wire.
Q.No.13.1: - A potential difference is
applied across the ends of a copper wire. What is the effect on the drift
velocity of free electrons by
(i)
Increasing the potential difference.
(ii)
Decreasing the length and the temperature of the wire.
Ans: - (i) It is obvious that drift velocity of free
electrons in the copper wire will increase do to increasing potential
difference.
(ii) If the drift velocity of free electrons is increasing then it
means that resistance in the conductor is low at that time and when we decrease
the length of the conductor then basically we are trying to decrease the number
of atoms and decreasing resistance. As a result the drift velocity of free
electrons increases. We know that resistance depends upon temperature change
and if we decrease temperature of wire then it mean we are decreasing
resistance and as a result increasing drift velocity. So, we can say that by
increasing potential difference, decreasing length and temperature of conductor
we are increasing drift velocity of electrons.
Friday, 22 November 2013
Q.No.12.9: - Do electrons tend to go to region of high potential or of low potential.
Q.No.12.9: - Do electrons tend to go to
region of high potential or of low potential.
Ans: - By convention we use positive potential as a high and
negative potential as low potential. So according to this convention we can say
that electrons which have negative charge tend to go to the region of high
potential (positive) from low potential (negative).
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