Class 12 Physics practical reading To determine resistance per unit length of a given wire by plotting a graph of potential difference versus current.

Aim

To determine resistance per unit length of a given wire by plotting a

graph of potential difference versus current.

Material Required

A wire of unknown resistance, battery, voltmeter (0-5 V),

ammeter (0 – 1.5A), rheostat, plug key, connecting wires and a

piece of sand paper.

Principle for Class 12 Physics practical reading To determine resistance

Ohm's law states that the electric current flowing through a conductor

is directly proportional to the potential difference

across its ends, provided the physical state of the conductor

remains unchanged.

If I be the current flowing through the conductor and V the potential

difference across its ends, then according to Ohm's law

V ∝ I

and hence

V = RI

where R is the constant of proportionality and is termed as the

electrical resistance of the conductor. If V is expressed in volts and I

in amperes, then R is expressed in ohms. The resistance R, depends

upon the material and dimensions of the conductor. For a wire of

uniform cross-section, the resistance depends on the length l and

the area of cross-section A. It also depends on the temperature of

the conductor. At a given temperature the resistance

R =  ρl/A

where ρ is the specific resistance or resistivity and is characteristic

of the material of wire.

Procedure

1. Clean the ends of the connecting wires with the help of sand paper

in order to remove any insulating coating on them.

2. Connect various components - resistance, rheostat, battery, key,

voltmeter and ammeter as shown in Fig.

Circuit diagram

3. Note whether pointers in milliammeter

and voltmeter coincide with the zero mark

on the measuring scale. If it is not so,

adjust the pointer to coincide with the zero

mark by adjusting the screw provided

near the base of the needle using a screw

driver.

4. Note the range and least count of the

given voltmeter and milliammeter.

5. Insert the key K and slide the rheostat

contact to one of its extreme ends, so that

current passing through the resistance

wire is minimum.

6. Note the milliammeter and voltmeter

readings.

7. Remove the key K and allow the wire to cool, if heated. Again

insert the key. Shift the rheostat contact slightly to increase the

applied voltage. Note the milliammeter and voltmeter reading.

8. Repeat step 7 for four different settings of the rheostat. Record

your observations in a tabular form.

Observations

Result

Resistance per unit length of the given wire is 0.055 ohm/cm.

By graph 0.054 ohm/cm

Precautions

1. The voltmeter should be connected in parallel and the ammeter

in series with the circuit. It should be ensured that current enters

at the positive terminal and leaves at the negative terminal.

2. The key should be inserted only while taking observations, as

excessive flow of current causes unnecessary heating of the wire.

3. Zero error in measuring instruments (voltmeter, ammeter, metre

scale) must be taken cognizance of and should be eliminated in

case of ammeter and voltmeter by adjusting the pointer with

the help of the screw provided at the base of the needle, using a

screw driver

4. Before plugging in the key ensure that the connections are tight. 

5. The wire should be free from kinks and should have uniform thickness.

Sources of error

1. Rheostat may have high resistance. 

 2. Thick connecting wires may not be available 

3. The connections may not be tight. 

 4. The resistance of wire is very low.