**Ohm's Law Experiment Viva Questions and Answers**

**(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: Resistivity = RA/ l. It is equal to the resistance of a wire of length one metre and cross sectional area 1 m^{2}

**(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). What is an ohmic conductor?**

A conductor that obeys the rule of Ohm's law is known as an ohmic conductor.

**(xiii). Define ampere, volt and ohm.**

ampere: ampere is the unit of current. The current is said to be one ampere if one coulomb of charge flows per second across any cross section of the conductor.

volt : volt is the unit of potential. It is the potential difference across a conductor of resistance one ohm, when a current of one ampere flows through it.

ohm : ohm is the unit of resistance. The resistance of a conductor is said to be one ohm if a potential difference of one volt is established across the conductor when unit current flows through it.

**(xiv). We have two wires of copper, one thicker than the other.**

a) Which of these will have more resistance?

b) Which of these will have more resistivity?

a) The thinner will have more resistance.

b) Both the wires will have same resistivity.

**(xv). Why is an ammeter always connected in series in a circuit?**

An ammeter has a very low resistance. It will not alter the current in the circuit only if it is connected in series.

**(xvi). Why is a voltmeter always connected in parallel in a circuit?**

A voltmeter is a high resistance instrument. When it is connected in parallel it draws negligible current from the main circuit and hence the current in the main circuit practically remains unaltered.

**(xvii). Why do we use thick connecting wires?**

Thick connecting wires offer negligible resistance.

**(xviii). What is the effect of temperature on the resistance of a conductor?**

The resistance of the conductor increases with increase in temperature.

**(xix). Define resistance.**

The resistance of a conductor is the obstruction shown by the conductor to the flow of electric current through it.

**(xx). Name some substances whose resistance decreases with increase in temperature.**

Si, Ge, C

**(xxi). On what all factors resistance of a wire depends?**

(i) Resistance of the conductor is directly proportional to the length.

(ii) The resistance of the conductor is inversely proportional to the area of cross section or the conductor.

(iii) It depends on the nature of material and the temperature across the conductor.

**(xxii). Define resistivity or specific resistance of a conductor.**

The resistance of a conductor of unit length and unit area of cross section is known as resistivity.

**(xxiii). Define electrical conductivity.**

The reciprocal of resistivity is called electrical conductivity.

**(xxiv) 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.

**(xxv) Apparatus of the Ohm's Law Experiment?**

Ans: Lead accumulator, a resistance wire (resistor), ammeter, voltmeter, rheostat, key etc.

**(xxvi) 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,

Resistivity = R x (π r^{2})/l

**(xxvii) 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 of the resistor is calculated from the equation,

Resistivity= R x π r^{2}/l