Our objective is to determine the resistance per centimeter of a given wire by plotting a graph of potential difference versus electric current.
According to the Ohm's law, "The electric 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 is the current flowing through a conductor and V is the potential difference across its ends, then according to Ohm's Law,
where, R is the constant of proportionality. It is known as resistance of the conductor, or
R depends upon the 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.
To establish the current-voltage relationship, it is to be shown that the ratio V / I remains constant for a given resistance and therefore a graph between the potential difference(V) and the current (I) must be a straight line.
The value of unknown resistance can be calculated using the equation ,
Here R is a constant.
To find the length of the resistance wire, l. Then
This gives the resistance per unit cm.
- Students learn Ohm’s law.
- Students know the relation between voltage, current and resistance.
- Lead Accumulator or battery eliminator
- A resistance wire
- One way Key
- Metre Scale
- Connecting wires
- Draw the circuit diagram.
- Arrange the required materials on the table.
- Make the connections according to the circuit diagrams.
- Determine the least count of the voltmeter and ammeter, and also note the zero error, if any.
- Insert the key K, then slide the rheostat contact and see that the ammeter and voltmeter are working properly.
- Adjust the sliding contact of the rheostat such that a small current passes through the resistance coil or the resistance wire.
- Note down the value of the potential difference (V) from the voltmeter and current (I) from the ammeter.
- Shift the rheostat contact slightly so that both the ammeter and voltmeter show full divisions readings.
- Record the readings of the voltmeter and ammeter.
- In each case V/I is calculated, which gives the resistance R of the resistor and it is found as a constant.
- Take at least six sets of independent observations.
- Cut the resistance wire at the points where it leaves the terminals, stretch it and find its length by the meter scale.
- A graph is drawn with I along the X-axis and V along the Y-axis. The graph obtained is a straight line. This also verifies Ohm’s law.
- Note a particular point of current on X axis and extend a line joining the straight line and the point on Y axis. Mark these points.
- Note the values of OB and AB then calculate the value of R as AB/OB.
- Calculate the resistance per cm of the resistance wire can be found out by using the equation 2.
Simulator Procedure (as performed through the Online Labs)
- Use the slider to select the length of the resistance wire.
- To see the circuit diagram, click on the 'Show circuit diagram' check box seen inside the simulator window.
- Connections can be made as seen in the circuit diagram by clicking and dragging the mouse from one connecting terminal to the other connecting terminal of the devices to be connected.
- Drag the wire and place it on the voltmeter to have it connected.
- Once all connections are made, click and drag the key to insert it into the switch.
- Slowly move the rheostat contact to change the volt and current in the Voltmeter and Ammeter accordingly.
- The readings are shown both in Voltmeter and Ammeter and also as readings on the left side pane.
- Calculate the resistance based on the length of the wire selected and enter the resistance of the wire and resistance per cm in the respective text boxes.
- Click on the 'Check' button to see if the result is right or wrong.
- An external work sheet is shown below the simulator window that can be used to plot a graph based on the data provided in the worksheet.
- To see the graph, click on the graph icon in the worksheet.
- Click on the 'Reset' button to redo the experiment.
Length of the resistance wire l =......cm
Least count of ammeter = .......A.
Least count of voltmeter = .......V.
Zero correction for ammeter, (-e1) = .......A.
Zero correction for voltmeter, (-e2) = ......V.
- Table for Ammeter and Voltmeter Readings:
Serial no of obs
Ammeter Reading I (A)
Voltmeter Reading V (V)
Mean value of resistance, R = ....................ohms.
Length of the resistance wire, l=…………cm
Resistance per cm , =…………..Ωcm-1
Resistance per cm of the wire is ........................Ωcm-1.