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Laboratory Manual For Class XII

Section A

EXPERIMENT-1


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, an ammeter of appropriate range, a battery eliminator, a rheostat, one way key connecting wire and a piece of sand paper.

THEORY: If I be the current flowing through a conductor and V be the potential difference across its ends, then according to Ohm’s Law  

                                                V α I

                                    Or        V=IR

where R is the constant of proportionality. It is known as resistance of the conductor.

                                    Or        V/I=R

R depends upon the material and dimensions of the conductor. In S.I unit, the potential difference V is measured in volts and the current I in ampere, the resistance R is measured in ohms.

            1. To establish the current–voltage relationship, it is to be shown that the ratio V/R remains constant for a given resistance, therefore a graph between the potential difference (V) and the current (I) must be a straight line

            2. The constant ratio V/R of the given wire gives the unknown value of resistance.

Circuit Diagram: Figure (i) above


PROCEDURE:

  1. Make neat, clean and tight connection according to the circuit diagram. While making the connection ensure that +ve marked terminals of the voltmeter and ammeter are joined towards the +ve terminals of the battery.
  2. Determine the least count of the voltmeter and ammeter and also note the zero error, if any.
  3. Adjust rheostat so as to pass minimum current.
  4. Insert the key K, slide the rheostat contact and see that ammeter and voltmeter are working properly.
  5. Adjust the sliding contact, a small current passes through the resistance coil or the resistance wire.
  6. Note down the value of the potential difference V from voltmeter and current I from ammeter.
  7. Record the reading of the voltmeter and ammeter.
  8. Repeat step 5 & 6, take at least six sets of independent observation
  9. Record your observation as given below.

OBSERVATIONS:

1. Range of the given ammeter            = …A

2. Range of the given voltmeter          = …V

3. Least count of ammeter                   = …A

4. Least count of voltmeter                  = …V

5. Zero error in ammeter e1                  = …A

6. Zero error in voltmeter e2                 = …V

7. Zero correction in ammeter (-e1)     = …A

8. Zero correction in voltmeter(-e2)     =….V

Table for Ammeter and Voltmeter Readings



No. of Obs

Ammeter Reading I (A)

Voltmeter Reading V (V)

V/I=R  Ω

Observed

corrected

Observed

Corrected

1

2

3

4

5

6






Calculations:

  1. Find the ratio of V and I for each set of observation
  2. Plot a graph between potential V and current I taking V along X-axis and I along Y-axis. The graph comes to be a straight line as shown in figure (ii) above

            From the graph, the resistance can be calculated.

            In Δ ABC,  tan θ = AB/CB = ΔI / ΔV

                   cot θ = V / I

                        R = V / I

                        R = cot θ

                        R = …Ω

            3.   Constant ratio V / I gives resistance of the wire.

4.       Resistance per cm of the given wire is  = … Ω-cm-1

RESULT

Resistance per cm of the given wire is  = …Ω-cm-1

PRECAUTIONS

1.                  The connections should be neat, clean and tight.

2.                  Thick copper wires should be used for the connections after removing the insulations near their ends by sand paper.

3.                 Voltmeter and ammeter should be of proper range. They should be connected properly (voltmeter in parallel and ammeter in series).

4.                  A low resistance rheostat should be used.

5.                   The unknown resistance should not be too low.

6.                  The key should be inserted only while taking observations to avoid heating of resistance.

SOURCES OF ERROR

1.                  The instrument screws may be loose.

2.         Thick connecting wires may not be available.

3.         Rheostat may have high resistance.

4.         The unknown resistance may be low