Introduction

In this science experiment, we will see the dependence of potential difference (V) on current (I) passing through the resistor and determine its resistance (R). And will try to plot a graph between V and I based upon the concept of Ohm’s Law.

Circuit

1. A circuit is a current-carrying closed conducting loop.
2. The arrangement of devices with the help of their symbol in such a way that they represent a close loop is called a circuit diagram. 

Electric Current

1. The charge flow through a conductor per unit time is called an electric current.
Electric Current = Charge flow/ Time.
                                                       I = Q/T.

2. The SI unit of electric current is Ampere.
3. The flow of one-coulomb charge in one second is called one-ampere current.
4.
1 microampere = (1 X 10^-6) ampere,
1 milliampere = (1 X 10^-3) ampere,
1 microampere = (1 X 10^-9) ampere.
5. Ammeter is the instrument for the measurement of electric current.

Potential

1. The work done in bringing one unit charge from infinity to a point inside an electric field is called the potential of that point.
2. The work done in taking charge from one point to another is called the potential difference between these two points.
3. The potential difference (V) between two points is equal to the work done (W) in taking charge from one point to another per unit charge (Q).

Potential Difference = Work Done/ Charge,

V = W/Q

4. The SI unit of potential difference is Joule per second or Volt.
5. 1 joule/coulomb = 1 volt.
6. The device which is used to measure the potential difference is Voltmeter.

Resistance

1. The property of conducting wire to resist the flow of current in a conductor is called resistance.
2. According to ohm’s law, Potential difference = Electric Current/ Resistance.

                                                                            V = IR 

                                                                           R = V/I

3. Therefore, the ratio of potential difference and electric current is called resistance.
4. The SI unit of resistance is ohm or volt/ampere.
5. The resistance of a conductor depends upon the area, length and material of that conductor.
    R L/A, where L= length and A = Area.
    R = ρL/A, where ρ = resistivity of the conductor.

Ammeter

1. It is an instrument used to measure the strength of the electric current in a conductor.
2. This instrument is represented as

3. One side represents the positive terminal, and the other side is the negative terminal.
4. The positive terminal of the ammeter is always connected with the positive terminal of the cell, and the negative terminal of the ammeter is connected with the cell’s negative terminal.

Voltmeter

1. This device is used to measure the potential difference in a circuit.
2. It is always connected in a way such that the positive terminal is connected to a higher potential, and the negative terminal is connected to a lower potential.

Sliding Rheostat

This device is used to control current in a circuit by changing its resistance.

Aim

To determine the dependence of potential difference (V) across the resistor on current (I) passing through it and determine its resistance (R). And will try to plot a graph between V and I.

Material Required

1. Conducting wire (nichrome or manganin),
2. Ammeter,
3. Voltmeter,
4. Battery eliminator,
5. Rheostat,
6. One-way plug key,
7. Connecting wires

Theory

According to Ohm’s law, if the physical quantity, such as temperature, pressure, stress, strain, etc., is constant, the current flowing through the circuit is directly proportional to its potential difference.

 V ∝  I , V = IR

Where R will be the resistance of the circuit.

We get a straight line if a graph is plotted between the current (I) and the potential difference (V).

Assembly of devices

Procedure

Step 1. Set up the diagram as shown in the figure.

Step 2. Note the least count of the ammeter.

Step 3. Note the least count of the voltmeter.

Step 4. Find their zero error, if any, and note it.

Step 5. Now, plug the key on the battery eliminator.

Step 6. Slide the variable of rheostat until the ammeter and voltmeter show a reading.

Step 7. Note their readings.

Step 8. Now, take out the key from the battery eliminator for a moment.

Step 9. Repeat steps 5, 6, 7, 8 and 9 for 3-4 times for a different value of current by sliding the variable of rheostat

Step 10. Write all the observations in the table and find V/I for every set of observations.

Step 11.  Draw a graph by taking V on X-axis and I on Y-axis.

Observation Table

Table 1; For ammeter and voltmeter

Table 2; For reading ammeter and voltmeter. 

Mean value of R = (R1 + R2 + R3 + R4)/3.

Calculations

1. Find the value of R for each set of observations, then find its mean value R = ……….. Ohm.
2. Plot the graph between X-axis and Y-axis as shown in the figure.
3. Find the slope of line AB = BC/AC = (I^2 – I^1)/(V^2 – V^2).
4. Resistance of wire = 1/ ( slope of line AB) = ………ohm.

Result

1. Straight line graph shows that the potential difference across the conductor is directly proportional to the current flowing through it.
2. The resistance of the conducting wire is equal to the mean value of R. It also verifies ohm’s law.
3. Resistance of the conducting wire = ………. Ohm.

Precautions

1. Zero error should be measured correctly.
2. Note the value of least count correctly.
3. There should be no defects in devices.
4. Area of the wire should be more as it offers the minimum resistance.
5. Rheostat must be of the low range.
6. The ends of the wire should be connected to the terminals of the voltmeter.
7. Positive terminal of the cell should be connected to the positive terminal of the voltmeter or ammeter.
8. Don’t allow the current to flow longer in the wire because it can heat up.

Sources of Error

1. While observing the devices, reading errors may occur.
2. Wire may heat up due to the flow of current for a longer period of time.
3. Area of wire may not be uniform.
4. The screws may not be adequately tightened.
5. The terminals of the cell may be connected to the wrong terminal of devices.

Conclusion

In this experiment, we have learnt to determine the dependence of potential difference (V) across the resistor on current (I) passing through it and determine its resistance (R). And will try to plot a graph between V and I.

Viva Questions And Answers

Q.1 Describe Ohm’s law.

ANS. According to Ohm’s law, if the physical quantity, such as temperature, pressure, stress, strain, etc., are constant, the current flowing through the circuit is directly proportional to its potential difference.

Q.2 What is the formula for ohm’s law?

ANS. V = IR    where,    V = potential difference

I = Current

R = Resistance.

Q.3 Name the SI unit of potential difference?

ANS. Voltage

Q.4 Which law of electric current is verified in this experiment?

ANS. Ohm’s law.

Q.5 What is the SI unit of current?

ANS. Ampere.

Q.6 What is the SI unit of resistance?

ANS. Ohm

Q.7 Why should we not allow the current to flow for a longer period of time?

ANS. Because according to the formula (H = I^28R), the wire will heat up and lead up to maximum resistance.

Q.8 Resistance of wire depends on what conditions?

ANS. 1. Area,

2. Length,
3. Nature of conductor.

Q.9 What is the composition of nichrome wire?

ANS. Nickel- 68%

Chromium – 15%

Iron- 15.5%

Manganese – 1.5%

Q.10 Why is it advised to take out the key when observations are not being taken?

ANS. To avoid the excessive heating of the wire.

Vikram Sarabhai

An Indian physicist and astronomer.