to determine resistance per cm of a given wire by plotting a graph of potential difference versus current
Answers
Answer:
Aim
To determine resistance per cm of a given wire by plotting a graph of potential difference versus current.
Apparatus
A resistance wire, a voltmeter (0-6) V and an ammeter (0-6) A of appropriate range, a battery (battery eliminator), a rheostat, a metre scale, one way key, connecting wires and a piece of sand paper.
Theory
According to the Ohm’s law the current flowing through a conductor is directly proportional to the potential difference across its ends provided the physical conditions (temperature, dimensions, pressure) of the conductor remains the same.
If I be the current flowing through a conductor and V be the potential difference across its ends, then according to, Ohm’s Law,
I is proportional to Vor takingV proportional to Iwe getV = IRwhere R is proportionally constant known as resistance with unit Ohm
R depends upon the nature of material, temperature and dimensions of the conductor.
In S.I. units, the potential difference V is measured in volt and the current I in ampere, the resistance R is measured in ohm.
(1) To establish the current-voltage relationship, it is to be shown that the ratio V/I remains constant for a given resistance,
therefore a graph between the potential difference (V) and the current (I) must be a straight line.
The Resistance in VI curve is given as V/I =R
Circuit diagram

Procedure
Arrange the apparatus in the same manner as given in the arrangement diagram.
Clean the ends of the connecting wires with sand paper to remove the insulations, if any.
Make neat, clean and tight connections according to the circuit diagram. While making connections ensure that +ve marked terminals of voltmeter and ammeter are joined towards the +ve terminal of the battery.
Determine the least count of voltmeter and ammeter, and also note the zero error, if any.
Insert the key K, slide the rheostat contact and see that ammeter and voltmeter are
working properly.Adjust the sliding contact of the rheostat such that a measurable current passes through the resistance coil or the resistance wire.
Note down the value of potential difference V” from voltmeter and current I from ammeter.
Shift the rheostat contact slightly so that both ammeter and voltmeter show full divisions readings and not in fraction.
Record the readings of the voltmeter and ammeter.
Note. In case of battery eliminator, follow these steps:
Turn the knob at 5 V in battery eliminator and put the constant point in rheostat at fixed position. Now record the reading in voltmeter and ammeter.
Without disturbing the rheostat, turn the knob of battery to different voltage such that 4, 6, 8, 10 and 12 Volts and record corresponding readings in voltmeter and ammeter.
Take at least five sets of independent observations.
Cut the resistance wire at the points where it leaves the terminals, stretch it and find its length by the metre scale.
Record your observations.
Observations
Length
Length of the resistance wire l = ……….
Range
Range of the given ammeter = ……….
Range of the given voltmeter = ……….
Least count
Least count of ammeter = ……….
Least count of voltmeter = ……….
Zero correction
Zero correction for ammeter, c1 = -e1 = ……….
Zero correction for voltmeter, c2 = -e2 = ……….
Table for Ammeter and Voltmeter Readings

Calculations
1. Find ratio of V and I for each set of observations.
2. Plot a graph between potential difference V (column 36) and current I (column 26), taking V along X-axis and I along Y-axis. The graph comes to be a straight line.

Result
Resistance per cm of the wire is …….. Ω cm-1.
The graph between V and I is a straight line.
Regards
The process to determine resistance per cm of a given wire by plotting a graph of potential difference versus current is given below.
- If you plot the potential difference along the X-axis and the current along the Y-axis, the reciprocal of the slope gives the material's resistance, R.
- If you interchange the quantities on the axes, the resistance will be given by its slope and not it's reciprocal.
- The resistance per cm is given by R/l if the length of the resistance wire is 'l' cm.
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