To determine resistance per cm of a given wire by plotting a graph of potential difference versus current.

Aim

To determine resistance per cm of a given wire by plotting a graph of potential difference versus current.

Apparatus

A metallic conductor (coil or a resistance wire), a battery, one way key, a voltmeter and an ammeter of appropriate range, connecting wires and a piece of sand paper, a scale.

Formulae Used:

The resistance (R) of the given wire (resistance coil) is obtained by Ohm’s Law V/I=R

Where, V : Potential difference between the ends of the given resistance coil. (Conductor)

I: Current flowing through it.

If ι is the length of resistance wire, then resistance per cm of the wire = R/ι

Observation:

Range:

1. Range of given voltmeter = 3v
2. Range of given ammeter = 500 mA

Least count:

1. Least count of voltmeter = 0.05v
2. Least count of ammeter = 10 mA

Least count:

1. Zero error in ammeter, e₁=0
2. Zero error in voltmeter,e₂=0

Ammeter and Voltmeter Readings:

Mean R = 1.56

Length of resistance wire: 28 cm

Graph between potential difference & current:

Scale: X – axis : 1 cm = 0.1 V of potential difference

Y – axis: 1 cm = 0.1 A of current

The graph comes out to be a straight line.

Result

It is found that the ratio V/I is constant, hence current voltage relationship is established i.e. V ∝ I or Ohm’s Law is verified.

Unknown resistance per cm of given wire = 5.57 x 10²Ωcm^-1

Precautions:

• Voltmeter and ammeter should be of proper range.
• The connections should be neat, clean & tight.

Source of Error:

• Rheostat may have high resistance.
• The instrument screws may be loose.