What is Electric Energy
Electric Energy is product of Electric Power and Time
Electric Energy = Electric power × Time
What are Units of Electrical Energy?
It is Measured in Watt second or Joule
Explanation
Electric Energy = Electric power × Time
Electric Power is measured in Watt and Time in Second
So Electrical Energy-Watt × Hours = Watt Second
Meaning of Heating Effect
When an electric current passes through a conductor (like a high resistance wire)
the conductor becomes hot after some time and produces heat
This is called heating effect of Electric Current
Example 1
A bulb becomes hot after its use for some time.This is because of heating effect of electric current
Example 2
When we switch on an electric iron,it becomes hot.This is also because of heating effect of electric current
What causes Heating Effect of Electric Current
Heating Effect happens due to conversion of electric energy into heat energy
Explanation
We know that battery or cell is a source of electrical energy
Due to chemical reaction in this battery or cell,potential difference is generated
This potential difference causes electrons to flow through circuit
This circuit has resistors which resist flow of current
Work is to be done to overcome this resistance
While doing this work,
This source energy in conductor is dissipated(converted) in resistor as heat energy
What is Formula for Heating Effect of Electric Current
H = I 2 RT
How is Heat Formula Derived
This is known as Joule's law of heating.
The law implies that
the heat produced in a resistor is
-
directly proportional to the square of current for a given resistance
It means if we double the current,the heat becomes 4 times
if we half the current,the heat becomes 1/4 times
Hence,more the current,more the heat. Less the current, less heat produced.
-
directly proportional to resistance for a given current
It means if we use a wire made of metal having more resistance (like nichrome wire),it will produce more heat
But if we use wire made of metal having less resistance (like copper),it will produce less heat
-
directly proportional to the time for which the current flows through the resistor
It means if we switch on an electric gadget for more time,it will get heated up more
But if we use an electric gadget for less time (switch it off after use),it will get less heated
Why do some electric appliances get heated up more (like electric iron) while others get heated up less(like TV)
It is because electric iron has a heating element like high resistance wire which help in converting most of electric energy into heat energy
Explanation
We know that as per Joule's Law of Heating
Heat is Directly Proportional to the resistance of the wire for a given current
It means if we use a wire made of metal having more resistance (like nichrome wire),it will resist the flow of current and hence it will produce more heat.
Since the resistance of the wires used in electric iron is more than that used Tv, more heat is produced in an electric iron.
Such kind of nichrome wire is used in electric iron which acts as heating element
This converts most of electric energy into heat energy
Different formulas for Heat
We know that,
Heat = Electrical power x time
Hence, the three formulae become
- H = (VI)t
- H = (I 2 R)t
- H = (V 2 /R)t
Questions
Example 12.11 - 100 J of heat is produced each second in a 4 Ω resistance. Find the potential difference across the resistor.
Question 8 -
In an electrical circuit two resistors of 2 Ω and 4 Ω respectively
are connected in series to a 6 V battery. The heat dissipated by
the 4 Ω resistor in 5 s will be
(a) 5 J
(b) 10 J
(c) 20 J
(d) 30 J
In an electrical circuit two resistors of 2 Ω and 4 Ω respectively
are connected in parallel to a 6 V battery. The heat dissipated by
the 4 Ω resistor in 5 s will be
(a) 45 J
(b) 20 J
(c) 60 J
(d) 35 J
