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Home Physics Class 12 Physics To determine resistance per cm of a given wire by plotting a graph of potential difference versus current.
Class 12 Physics Lab Experiments

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:

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:

S.No. Ammeter Reading I (A) Ammeter Reading I (A) Voltmeter Reading, V(v) Voltmeter Reading, V(v) V/I=R
Observed Value Observed Value
1 50 500 mA 16 16 x 0.05=0.8 1.6Ω
2 35 350 mA 11 0.55 1.57Ω
3 32 320 mA 10 0.50 1.56Ω
4 19 190 mA 6 0.30 1.58Ω
5 10 100 mA 3 0.15 1.5Ω

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.

Graph between potential difference & current

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.