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21. Two wires one of
copper and the other of maganin have equal lengths and equal
resistance. Which wire is thicker?
If two wires one of copper and the other of maganin have equal lengths l and
equal resistance R. Let resistivities & cross-sectional area copper & maganin
wires are
Experimentally, it is found that
22. Why are alloys constantan and manganin used to make standard
resistors?
23.
A wire carrying current stays electrically neutral. Why?
Electric current is the flow of free electrons in the conductor. The number of
proton in wire is equal to the number of electrons at any instant so, the net
charge is zero. A wire carrying current stays electrically neutral because many
electrons enter one end of wire & leave it from other end.
24.
The electron drift speed in metals is small and the charge of
the electron is also very small, but we can still obtain a large
amount of current in a metal. Why?
The
electron drift speed in metals is definitely a small, of the
order of millimeters per second. Had there been less number
of free electrons, the overall charge passing through a
point per unit time would be less. However metals have very
high number of free electrons. So even if they traveled with
less velocity, their large number carries more charges every
second and then according to the relation
 ,
the current will be high.
25. A wire is
stretched to double its length. What happens to its resistance?
The resistance R of the resistor of the length l whose resistivity is ρ is given by:
As the resistivity of the resistor is the characteristics of the material of the wire which does not altered and remains constant for the same material. If a wire is stretched to double its length, then its length increases more than its cross-sectional area decreases so from above relation, resistance increases and becomes four times the initial resistance.
26. In a car, one
terminal of the battery is 'said' to be connected to ground even
if it is not really connected to the ground. Explain.
A large body has large capacitance and due to its large capacitance, if some
charge is given to it then its potential does not change but remains constant.
Hence, such a body is considered as at zero potential or ground. In a car, one
terminal of a battery is connected to the metal body which is assumed to
grounded even if it is not grounded. Hence, one terminal of the battery is said
to be connected to ground even if it is not really connected to the ground in a
car.
27. Why is an ammeter always connected in series with the load?
An ammeter is used to measure current flowing through the circuit. The current
flowing through the circuit can be easily measured when the ammeter is connected
in series. As the resistance of an ammeter is negligible or zero ( in practice, the resistance
of ammeter is not zero but very low as compared to other resistance) so, that
when it is connected in the circuit the equivalent resistance of the circuit
remains same. So, the current in the circuit is unaltered. Hence, an ammeter is
always connected in series with the load.
28.
Why is a voltmeter always connected in parallel with the load
(resistance)?
A voltmeter is an instrument used to measure the voltage across the load so when it is connected in the circuit it should not disturb the current flowing through the circuit. As a voltmeter contain high resistance so when it is connected in parallel with a load, resistance is not disturbed which doesn't disturbs the current. But, if a voltmeter is connected in series with the load then the equivalent resistance of the circuit increases which alters the current so, there is error in the reading of voltage. Hence, a voltmeter is always connected in parallel with the load.
29. Why do electrons
acquire a steady average drift velocity?
When a battery is connected across the ends of a conductor, an electric field is
set up. This electric field accelerates the electrons in the conductor.
Electrons gain kinetic energy depending on the amount of the field. During their
movement, electrons collide with atomic lattice and loses their energy. So, they
cannot be accelerated continuously but they soon acquires drift velocity.
30. Why is there no current established in a conductor in the
absence of electric field although a large number of free
electrons are moving with high speed?
The flow
of current requires a flow of charges in a particular
direction, which of course requires guiding the electrons
somewhere.
There are
large number of free electrons moving in a metallic
conductor. But in the absence of any external force (i.e.
the electric field), they are not guided anywhere. So, for
every electron moving to the right, there are equal number
of electrons moving to the left. For every of them moving
upwards, equal number of them would be moving downwards and
so on. Therefore there will not be any overall movement in
any direction, which makes the charge flow zero and so
current also zero.
31. In which condition will the
terminal p.d of a cell be more than its emf?
In case of charging of cell having internal resistance r, the terminal potential
difference V of the cell is greater than its emf E i.e. V = E + Ir: where I is
the current. Here, the terminal p.d. exceeds the emf of a cell by the term Ir.
32. The thermal speeds of the free
electrons in a conductor are quite large. Then, why don’t they
escape from the surface of the conductor?
For the propagation of free electrons, the medium of good conductor is required.
However, the thermal speeds of the free electrons in a conductor are quite
large, they do not escape from the surface of the conductor because there are no
such medium through which free electrons can flow from the surface of the
conductor. There is only the air
medium which behaves as insulator through which free electrons can not
flow. However, the thermal speeds of the free electrons in a conductor are quite
large they do not escape from the surface of the conductor.
33. Explain how do you distinguish
whether two resistances in a circuit are connected in parallel
or in series?
Firstly, I would like to measure
whole resistance of a circuit. Then, I would separate two resistors and I would
measure the value of resistances of each resistor. Then I would calculate the
equivalent resistance of two resistors when they are connected in parallel as
well as in series. After that, I would compare the value of whole resistance of
a circuit with the equivalent resistance of two resistors when they are
connected in parallel as well as in series. If one of the value of equivalent
resistance of two resistors satisfies the value of resistance of whole circuit,
then I would easily know whether two resistors are connected in parallel or
series.
34. When electron drift in a metal
from a lower to higher potential, all the free electrons of the
metal move in the same direction. Is this statement true?
Explain.
When electron drift in a metal from a lower to higher potential, all the free
electrons of the metal move in the same direction. Yes, this statement is true
because the drift velocity is superposed over the random thermal velocities of
electrons.
35. How does the resistance of a material vary with the
temperature?
The resistance of a metal increases with increase in the temperature. Let R1 & R2 are the resistance of resistor at temperature t10C & t20C respectively then R2 = {1 + α(t2 - t1)} where 'α' is the constant called the 'temperature coefficient' of the material. For metal, 'α' is positive but for insulator and semiconductor, 'α' is negative.
36.
37. In a metal,
there are a large number of free electrons. Then, why does it
offer resistance?
When an electric field is applied across a metal, the free electrons are
accelerated. When accelerated, the electrons then move across the metal. During
their motion, they have to make a large number of inelastic collisions with the
positive ions of the metal. In each collision, electrons lose their energy and
their speeds and directions changes during the motion. In addition, they also
undergo
collisions
with the
electrons
present in
various
orbits of
atoms, which
are also
responsible
to cause
loss of
energy. Hence, a metal offers
resistance to the free electrons.
38. Why is copper used as connecting
wire?
Copper is
the metal that has
less resistance and it is cheap as well as it is easily available in its pure
form. So, it can make flow of the electric current by least resisting or
opposing it. Hence, copper is used as connecting wires.
39. How are lamps connected in a
house? How is the fuse connected?
Generally, lamps are connected in a house in parallel combination. If the lamps are connected in the series then if the switch is turned off then all lamps will not glow but if they are connected in parallel then all lamps can be made to glow at a time and also individual lamp can also be made to glow. Hence, lamps are connected in a house in parallel combination.
As the fuse is the safety device which is made of the alloys of metals (tin and lead) having low melting point. It suddenly breaks when the high voltage flow through it resulting in the break up of the circuit of our houses that saves our expensive electronics device as well as it saves from firing. So, it must be connected in series with the circuit so that whole circuit is break off while it melts.
40. A conductor is carrying a
current that is charge per unit time. Can you say that the
conductor was charged?
We cannot say that the conductor was charged. The flow of current in a conductor
is due to the drifting of free electrons in a direction. In a conductor, at any
instant, the total number of electrons is equal to the total number of protons
so the net charge is zero.
Hence, we cannot say that
the conductor was charged.
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