(i) State Ohm's law
Ans: At constant temperature the ratio of the potential
difference between the end of a conductor to the current through it is
constant. V/I = constant
(ii) Distinguish between emf and potential difference
Ans: emf is the potential difference between the terminals
of a cell when no current is drawn from it. It is equal to the amount of work
done by the source in moving unit charge once around a complete circuit.
Potential differences between two points on a conductor are
the work done in moving unit charge from one point to other.
(iii) What is meant by specific resistance or resistivity?
Ans: ρ = RA/ l. It is equal to the resistance of a wire
of length one metre and cross sectional area 1 m2
(iv) What is the unit of resistivity?
Ans: Ohm-meter. Ωm
(v) Why are connecting wires thick and covered with
cotton thread?
Ans: Thick copper wire has negligible resistance. They are
covered with cotton to avoid short circuiting.
(vi) What is the material of wire used for making a
rheostat?
Ans: Manganin or constantan
(vii) What is the effect of temperature on resistance?
Ans: Resistance of a wire increases with temperature
(viii) How will you convert a galvanometer into (a)
ammeter (b) voltmeter?
Ans:
(a) By connecting a low resistance in parallel.
(b) By connecting a high resistance in series
(ix) Is Ohm's law a universal law?
Ans: No. It is not a universal law. It fails on semi
conductor and for resistances at very low temperature
(x) What is super conductivity?
Ans: In some substances the resistance completely disappears
below a critical temperature. This phenomenon is called super conductivity
(xi) What happens if voltmeter is connected in series in
a circuit?
Ans: A voltmeter is an tool for finding the potential
difference between two points. Hence the voltmeter is connected between the two
points. More over the voltmeter has a very high resistance. Hence the current
in the circuit is very much reduced.
(xii) Aim of the Ohm's Law Experiment?
Ans: To study current—voltage relationship (i.e. to verify
Ohm's Law) using an ammeter and a voltmeter; and, hence to find the resistivity
of the given resistor.
(xiii) Apparatus of the Ohm's Law Experiment?
Ans: Lead accumulator, a resistance wire (resistor),
ammeter, voltmeter, rheostat, key etc.
(xiv) Theory of the Ohm's Law Experiment?
Ans: If V is the potential difference applied to a conductor
of resistance R and I is the current flowing through the conductor, according
to Ohm's law,
V/I = R, a constant
If r is the radius
and l is the length of the conductor, the resistivity of the conductor,
ρ = R x (π r2)/l
(xv) Procedure of the Ohm's Law Experiment?
Ans: The accumulator, resistance wire R, ammeter, rheostat
and key are connected in series. The voltmeter is connected in parallel to R.
The circuit is closed and the rheostat is adjusted to make
voltmeter to show a definite reading V. The reading I of the ammeter is
noted. V/I is calculated.
Adjusting the rheostat suitably, a number of sets of values
are noted for V and I. In each case V/I is calculated. It is found to be a
constant verifying Ohm's law.
A graph is drawn with V along the Y-axis and I along the
X-axis. The graph is a straight line. This also verifies Ohm's law.
The mean value of V/I is calculated. This gives the
resistance R of the resistor. R is also calculated from the graph by finding
its slope. The average value of R is found out. The radius r of the resistor is
determined using a screw gauge. Its length l is also measured. The
resistivity ρ
of the resistor is calculated from the equation,
ρ = R x π r2/l
ρ = R x π r2/l
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