We all know about the heating effect of electric current , when it flows through a circuit due to collision between electrons and atoms of wire. But precisely how much heat is generated during electric current flow through a wire, on what conditions and parameters does it depend?
To solve this problem, Joule invented a formula which explains this phenomenon accurately. This is known as Joule’s law. This law is explained in detail below.
To solve this problem, Joule invented a formula which explains this phenomenon accurately. This is known as Joule’s law. This law is explained in detail below.
Joule’s Law of Heating
i) The amount of heat produced in electric current conducting wire, is proportional to the square of the amount of electric current that is flowing through the circuit, when the electric resistance of the wire and the time of electric current flow is constant.
i.e. H ∝ i2 (When R & t are constant)
ii) The amount of heat produced is proportional to the electric resistance of the wire when the electric current in the circuit and the time of electric current flow is constant.
i.e. H ∝ R (when i & t are constant)
iii) Heat generated due to the flow of electric current is proportional to the time of electric current flow, when the resistance and amount of electric current flow is constant.
i.e. H ∝ t (when i & R are constant)
When these three conditions are merged, the resulting formula is like this -
i.e. H ∝ i2 R t
Here ‘H’ is the heat generated in Joules, ‘i’ is the electric current flowing through the circuit in ampere and ‘t’ is in seconds. When any three of these are known the other one can be equated out.
Here, 'J' is a constant, known as Joule's mechanical equivalent of heat. Mechanical equivalent of heat may be defined as the number of work units which, when completely converted into heat, furnishes one unit of heat.
Obviously the value of J will depend on the choice of units for work and heat.
It has been found that
J = 4.2 joules/cal (1 joule = 107 ergs)
It should be noted that the above values are not very accurate but are good enough for general work.
Now according to Joule's law I2Rt = work done in joules electrically when I ampere of electric current are maintained through a resistor of R ohms for t second.
Therefore
H =i2 Rt joules/4.2joules/cal
= i2Rt/4.2 cal
By eliminating I and R in turn in the above expression with the help of ohm's law we get alternative forms as
H=0.24 V I t cal = 0.24 v2 t/R cal
Video on joules law.
i.e. H ∝ i2 (When R & t are constant)
ii) The amount of heat produced is proportional to the electric resistance of the wire when the electric current in the circuit and the time of electric current flow is constant.
i.e. H ∝ R (when i & t are constant)
iii) Heat generated due to the flow of electric current is proportional to the time of electric current flow, when the resistance and amount of electric current flow is constant.
i.e. H ∝ t (when i & R are constant)
When these three conditions are merged, the resulting formula is like this -
i.e. H ∝ i2 R t
Here ‘H’ is the heat generated in Joules, ‘i’ is the electric current flowing through the circuit in ampere and ‘t’ is in seconds. When any three of these are known the other one can be equated out.
Here, 'J' is a constant, known as Joule's mechanical equivalent of heat. Mechanical equivalent of heat may be defined as the number of work units which, when completely converted into heat, furnishes one unit of heat.
Obviously the value of J will depend on the choice of units for work and heat.
It has been found that
J = 4.2 joules/cal (1 joule = 107 ergs)
It should be noted that the above values are not very accurate but are good enough for general work.
Now according to Joule's law I2Rt = work done in joules electrically when I ampere of electric current are maintained through a resistor of R ohms for t second.
Therefore
H =i2 Rt joules/4.2joules/cal
= i2Rt/4.2 cal
By eliminating I and R in turn in the above expression with the help of ohm's law we get alternative forms as
H=0.24 V I t cal = 0.24 v2 t/R cal
Video on joules law.
No comments:
Post a Comment