Joule’s law for production of heat is relation between current and heat, the flow of current causes heat flowing through conductor due to the conductor resistance. The resistivity of conductor is not same for all conductor it varies conductor to conductor metal to metal. Due to flow of current heat production depends upon three factors viz. current, time and resistance of the conductor. The terms of heat are H, i, t, R Where H = produce heat, i = current flow through the conductor, t = time of current flow, R = resistance of the conductor.
Joule’s First Law – Current Law
If the resistance R and time t is constant the heat production is square proportional to current flow through the conductor.
Mathematically, H∝ I2
Where R and t is constant
Joule’s Second Law – Law of Resistance
If the time and current flow is constant then the production heat is directly proportional to the resistance.
That means if you increase the resistance the heat will be increase proportionally suppose the resistance is 100 ohm and heat produced is 100J and you increase the resistance to 200 ohm then heat will be also 200J.
Mathematically, H ∝ R
Joule’s Third Law – Law of Time
The production of heat is proportional to time of current flow when the resistance and current flow is constant.
Mathematically, H ∝ R
If you flow current through the resistance for 5 min and 10 min you will get more heat production for 10 min increase of time means increase of heat.
The flowing of current through conductor produce heat. We are using this heat due to flow of current in our daily life as in electric iron, electric bulb, electric fuse, electric heater and more. Heating effect of electric current are below
(i) Electric iron: In an iron the upper part is grooved. In this groove a coil is placed. Mica as an insulator is placed between the metal part and the coil, so that there is no electrical connection between them. Mica is a bad conductor of electricity but it is a good conductor of heat. Due to the flow of current through the coil it becomes heated and the heat transferred to the metal part through mica. Finally the metal part becomes heated. Iron is used with the heating effect of an electric current.
(ii) Electric bulb: we see a thick metallic wire in the bulb. It is made of tangsten metal. In a glass vessel or bulb the tangsten wire is kept sealed. The glass bulb is filled with neutral gas or vacuum. Neutral gases are using now a days commonly. The tangsten wire is known as filament when the current flows through the tangsten wire or filament it becomes heated and emit of light. Due to the flow of current heating effect of an electric current used as the source of light.
(iii) Electric heater: In an electric heater one type of coil is used. A high resistance material like nichrome or same type of material is used as coil. The coil is wound in grooves on ceramic format or china clay. Flowing electric current through the coil it becomes heated. Due to high resistance the coil becomes red color forms.
(iv) Electric fuse: In different types of electrical instruments fuses are used in our houses. In any electric circuit, in electric line, if there is a sudden high rise of current, then those instruments can be burnt down or there can be fire. The reason for this sudden high rise of current is the short circuit between two terminals of the electrical line. A low melting point conducting wire is connected in series to avoid this type of accident.