To determine resistivity of two or three wires by plotting a graph for potential difference versus current.

Aim - To determine resistivity of two or three wires by plotting a graph for potential difference versus current.

Materials required - Voltmeter, Ammeter, Battery, Rheostat, Meter Scale, Connecting Wires, Sand paper

Theory - We know that, By ohms law, the current flowing through a conductor is directly proportional to the potential difference across the ends, if Temperature, dimensions, pressure are same then,

I ∝ V

I R = V

=> Where R = Constant of proportionality = Resistance of conductor

R = V / I

R = ρ ( L / A )

=> Where ρ = Resistivity = specific resistance

R = ρ ( L / [πd2/4] )

here R is in ohms , V is in Volts and I is in amperes by SI system.

Procedure - To conduct the experiment, first arrange the apparatus as shown in the diagram and make neat and tight connections. Determine the least count and zero error of voltmeter and ammeter if any, then slide the rheostat contact to obtain a measurable current. Note the values of potential difference and current, record the readings of the voltmeter and ammeter. Take at least three sets of observations. Find the area of all wires and for battery eliminator take readings at different measures of voltages.

Diagram - The labelled circuit diagram for the given experiment "To determine resistivity of two or three wires by plotting a graph for potential difference versus current " is shown below. The diagram is to be made on the left blank side of the practical book

Observations -

Length of wires ( L1 ) = 1 metre

( L2 ) = 1 metre

Range of Ammeter = 0 to 500 milliampere (mA)

Range of Voltmeter = 0 to 5 Volts (V)

Circular Scale Divisions ( C.S.D. ) = 100 , Lateral Scale Divisions ( L.S.D. ) = 1

Least Count of Ammeter = 10 mA

Least Count of Voltmeter = 0.1 V

Least Count of Screw Gauge = 0.01 millimetre (mm)

Zero Errors in Ammeter and Voltmeter = 0 , none

Zero Error in Screw Gauge = ( - 0.65 mm )

Corrected average Length , area , diameter

L1 = 1m , A1 = 0.070 , D1 = 0.30

L2 = 1m , A2 = 0.016 , D2 = 0.13

Observation Table - Take your observations properly and record them in the following table.

TABLE 1 : Ammeter & Voltmeter readings

Sr No.	Observation No.	Ammeter Reading	Voltmeter Reading	Resistance
I	a.	100mA	0.4V	4 ohm
	b.	150mA	0.5V	4 ohm
	c.	200mA	0.7V	4 ohm
II	a.	50mA	0.2V	4 ohm
	b.	100mA	0.4V	4 ohm
	c.	150mA	0.6V	4 ohm

TABLE 2 : Calculation of Diameter With Screw Gauge

Sr No.	C-Scale Divisions (N)	Diameter (N x L.C)	Diameter - Z.Error	Corrected
1	95	0.95	0.95 - 0.65	0.30
2	94	0.94	0.94 - 0.65	0.29
3	96	0.96	0.96 - 0.65	0.31
4	78	0.78	0.78 - 0.65	0.13
5	77	0.77	0.77 - 0.65	0.12
6	78	0.78	0.78 - 0.65	0.13

TABLE 3 : Calculation of Resistivity

Sr No.	Resistance	Length	Area	Resistivity
(1)	4 ohm	1m	0.070	57.14
(2)	4 ohm	1m	0.065	61.53
(3)	4 ohm	1m	0.070	57.14
(1)	4 ohm	1m	0.016	237.69
(2)	4 ohm	1m	0.014	285.71
(3)	4 ohm	1m	0.016	237.69

Results - After proper calculations , the resistivity of the first wire came out to be 58.33 Ohm metre and the resistivity for the second wire came out to be 253 ohm metre .

Precautions - Ensure that the connections of the circuits are tight and intact while taking readings.

- Note readings quickly to avoid heating of wire and variation in readings.

- The wires must be of uniform thickness .

Sources of Error - The zero errors in the screw gauge must be remembered.

- Calculation errors and error due to parallax must be avoided.

- Use standard measurement devices .

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Link to download the pdf file - ( Link for PHYSICS CLASS 12 PRACTICAL - 1 )

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Viva Questions Relevant to the Practical

Q1 What is meant by resistance of a wire in a circuit ?

Ans . When the charge particles move across the wire there are particles of the wire or positive charge particles which act as obstacles in their path and cause an opposition to the flow of current. This ability to resist the flow of current by the wire is termed as Resistance . Its SI units are Ohms .

Q2 Which wire is best to use as connecting wire in home appliances?

Ans . Copper wire is the best choice for connecting wires in home appliances due to its high electrical conductivity, low resistance, durability and copper is readily available and cost-effective. The high electrical conductivity, durability, and affordability of copper make it the best choice for connecting wires in home appliances. Its use ensures optimal electrical performance and long-term reliability

Q3 Why did we connect the voltmeter as parallel combination to the wire ?

Ans . In parallel combination since the starting and ending points are all connected together at both ends, so there is no voltage difference between all the circuit elements connected to that points and hence because voltage is same for all , so the amount of current flowing to the resistance wire would not be altered due to the involvement of the Voltmeter. The voltmeter was connected in parallel to measure the wire's potential difference accurately without altering the current. Parallel connection ensures minimal disturbance to the circuit and allows direct voltage measurement across the wire.

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