• Take three resistances R1, R2 and R3.
• Connect them in series combination to complete a circuit; as shown in figure.
• First of all, connect the ammeter between battery and R1 and take its reading.
• After that, connect the ammeter between R1 and R2 and take its reading.
• Then, connect the ammeter between R2 and R3 and take its reading.

Observation:
It is observed that ammeter shows same reading in all situation. This shows that same current flows through every part of the circuit containing three resistances in series.

 

 

 

• Take three resistors R1, R2, and R3 and connect them in parallel to make a circuit; as shown in figure.
• Use voltmeter to take reading of potential difference of three resistors in parallel combination.
• Now, remove the resistor R1 and take the reading of potential difference of remaining resistors’ combination.
• Then, remove the resistor R2 and take the reading of potential difference of remaining resistor.

Observation: Voltmeter reading was same in each case. This shows that the same potential difference exists across three resistors connected in a parallel arrangement. 
 

The Joule’s Law of Heating states that the heat produced in a resistor is:
 (a) Directly proportional to the square of current for a given resistor.
 (b) Directly proportional to resistance for a given current, and
 (c) Directly proportional to the time for which the current flows through the resistor.
 This can be expressed by following equation:
 
 H =I²Rt
 
 Here; I is electric current, R is resistance, t is time and H is heating effect.
 
Experiment to Demonstrate Joule’s Law of Heating:
• In this experiment, we will show the effect of current on heating.
• Take a water heating immersion rod and connect to a socket which is connected to regulator. It is important to recall that a regulator controls the amount of current flowing through a device.
• Keep the pointer of regulator on minimum and count the time taken by immersion rod to heat a certain amount of water.
• Increase the pointer of regulator to next level. Count the time taken by immersion rod to heat the same amount of water.
• Repeat above step for higher levels on regulator to count the time.

Observation: It is seen that with increased amount of electric current, less time is required to heat the same amount of water. This shows Joule’s Law of Heating. 
Application: Electric toaster, oven, electric kettle and electric heater etc. work on the basis of heating effect of current. 
 

Find out the following in the electric circuit given in Figure 12.9

(a) Effective resistance of two 8 Ω resistors in the combination
Ans. These resistors are in parallel, so effective resistance can be calculated as follows:

1/ R = 1/8 + 1/8
=1/4 Ω

R= 4Ω

 

(b) Current flowing through 4 Ω resistor
Ans. For this, we first need to find effective resistance in the circuit,
= 4Ω +4 Ω =8 Ω
v / I = R or

I = 1 A

(c) Potential difference across 4 Ω resistance
Ans. Potential difference is getting divided into two resistance of 4Ω each. Hence,
potential difference across 4Ω = = 8 V/ 2 = 4V

(d) Power dissipated in 4 Ω resistor
Ans. P= I2Rt = (IA)2 x 4Ω =4W

(e) Difference in ammeter readings, if any.
Ans. Since resistors are connected in series, so there will be no difference in ammeter reading.