# Ohm's Law Experiment with Readings (Class 12)

Ohm's Law Experiment for Class 12 with Readings

Aim

To establish a relationship between voltage and current (Ohm's law) for a given resistance using an ammeter and a voltmeter and hence to find

(i) the resistance of a wire

(ii) resistivity of the material of the wire.

Apparatus

Battery (E), Ammeter (A), Voltmeter (V), A resistance wire (R), Rheostat (Rh). Key (K), Screw gauge, Connecting wires etc.

Principle

Ohm's law states that at constant temperature the current [I] flowing through a conductor is directly proportional to the potential difference [V] between its ends.

ie, I V or V/I = R, the resistance of the conductor.

The resistivity (ρ) of the material of the resistance wire is calculated using the equation

ρ = Rπr2/L

where, r - Radius of the wire

L - Length of the wire

Procedure

The battery (E), resistance wire (R), rheostat (Rh), ammeter (A) and key (K) are connected as shown in the figure. The voltmeter (V) is connected in parallel to the resistance wire (R). The key (K) is closed and the rheostat is adjusted to read a current I (say 0.2 A). The corresponding voltmeter reading (V) is noted. The ratio V/I = R is calculated. The experiment is repeated for different values of currents (0.4 A, 0.6 A, 1 A etc) by adjusting the rheostat. In each case the ratio V/I is calculated and the mean value of R is found.

A Graph is plotted taking current (I) along the X - axis and voltage (V) along the Y - axis. It is straight line. The diameter of the wire and hence radius of the wire (r) is measured using a screw gauge. Its length (L) is also measured using a meter scale. The resistivity (specific resistance) of the material of the wire is calculated using the formula, ρ = Rπr2/L

(i) To verify the Ohm’s law

 Trial Ammeter reading I(A) Voltmeter reading V(Volt) V/R = R(Ω) 1 2 3 4 5 6 .2 .24 .28 .32 .36 .4 .2 .24 .28 .32 .36 .4 1 1 1 1 1 1

Mean Resistance, R = 1Ω

Volt – Current graph

To find R from the graph

OB = 0.16 A

AB = 0.16 V

R = AB/OB = 1Ω

Mean value of R = 1Ω

2. To find the resistivity of the resistor

Radius of the conductor using screw gauge

Pitch = 1 mm

LC = .01 mm

Zero correction = +10 div

 Trial PSR HSR Corrected HSR Fraction HSR x LC (mm) Correct diameter 1 2 3 4 0 0 0 0 42 43 44 41 52 53 54 51 .52 .53 .54 .51 0.52 0.53 0.54 0.51

Mean diameter = 0.53 mm

Radius of the resistor = r = 0.265 mm = 0.265 x 10-3 m

Length of the resistor = l = 32.5 cm = 32.5 x 10-2 m

R = 1Ω

Resistivity of the resistor = ρ = Rπr2/L = 6.78 x 10-7 Ωm

Results

(i) V/I is found to be a constant. Hence Ohm's law is verified.

(ii). The resistivity of the material of the given wire = 6.78 x 10-7 ohm.m

MODEL VIVA VOCE QUESTIONS AND ANSWERS

1. State Ohm's Law.

Ohm's law states that current flowing through a conductor is directly proportional to the pd across it at constant temperature.

2. What is an ohmic conductor?

A conductor that obeys the rule of Ohm's law is known as an ohmic conductor.

3. Define ampere, volt and ohm.

ampere: ampere is the unit of current. The current is said to be one ampere if one coulomb of charge flows per second across any cross section of the conductor.

volt : volt is the unit of potential. It is the potential difference across a conductor of resistance one ohm, when a current of one ampere flows through it.

ohm : ohm is the unit of resistance. The resistance of a conductor is said to be one ohm if a potential difference of one volt is established across the conductor when unit current flows through it.

4. We have two wires of copper, one thicker than the other.

a) Which of these will have more resistance?

b) Which of these will have more resistivity?

a) The thinner will have more resistance.

b) Both the wires will have same resistivity.

5. Why is an ammeter always connected in series in a circuit?

An ammeter has a very low resistance. It will not alter the current in the circuit only if it is connected in series.

6. Why is a voltmeter always connected in parallel in a circuit?

A voltmeter is a high resistance instrument. When it is connected in parallel it draws negligible current from the main circuit and hence the current in the main circuit practically remains unaltered.

7. Why do we use thick connecting wires?

Thick connecting wires offer negligible resistance.

8. What is the effect of temperature on the resistance of a conductor?

The resistance of the conductor increases with increase in temperature.

9. Define resistance.

The resistance of a conductor is the obstruction shown by the conductor to the flow of electric current through it.

10. Name some substances whose resistance decreases with increase in temperature.

Si, Ge, C

11. On what all factors resistance of a wire depends?

(i) Resistance of the conductor is directly proportional to the length.

(ii) The resistance of the conductor is inversely proportional to the area of cross section or the conductor.

(iii) It depends on the nature of material and the temperature across the conductor.

12. Define resistivity or specific resistance of a conductor.

The resistance of a conductor of unit length and unit area of cross section is known as resistivity.

13. Define electrical conductivity.

The reciprocal of resistivity is called electrical conductivity.

##### 1 on: "Ohm's Law Experiment with Readings (Class 12)"
1. bro thank you very much