Practical applications of Joule's heating effect

 

Joule’s law of electrical heating

 

Heat produced in a resistor is directly proportional to

(i) the square of the current I,

(ii) the resistance R

(iii) the time t for which the current flows.

H = i2Rt joules

Therefore, H = VIt = (V2/R)t

 

Practical applications of Joule's heating effect

One of the inevitable consequences of the conduction of electric current through a conductor is the Joule's heating effect. It is an undesirable effect in many cases because, in these cases a part of the useful electrical energy is wasted as heat energy. For examples in electric motor, transformer, transmission lines etc., a part of the electrical energy is dissipated as heat energy. But Joule heating has also many applications. Some familiar domestic applications of joule heating effect in daily life are mentioned below:

 

1. Electric heating appliances

 

Electrical iron, electrical toaster, electrical oven, electrical kettle, electrical heater are some important electric appliances based on heating effect of electric current. The heating elements in these appliances should have (i) high melting point and (ii) high resistivity. Usually nichrome wire is used as the heating element.

 

2. Filament lamp [Incandescent electric bulb)

 

The filament of electric bulb when heated to a high temperature by electric current becomes white hot and emits light. The bulbs are usually filled with chemically inactive gases such as nitrogen, argon etc. to prolong the life of the filament. These bulbs give nearly 1 candela light energy for every watt of electric power consumed.

The material of the filament has (i) a high resistivity and (ii) high melting point. Usually tungsten (M.P = 3380°C) is used for the filament.

 

3. Safety fuse

 

The electrical wires that carry electricity to lights, fans and other electrical appliances have some resistance, although usually it is quite small. If the current is large enough, the wires will heat up and produce heat energy at a rate of i2R. One possible hazard is that the current-carrying wires become so hot as to start a fire. When a wire carries current beyond the safe limit, it is said to be overloaded. To prevent overloading fuses or circuit breakers are installed in circuits. Overloading may be due to the use of too many devices drawing current in that circuit or due to a fault somewhere in the circuit such as short circuit.

 

The safety fuse is a wire of high resistance with low melting point and made of an alloy of tin and lead (63% tin and 37% lead). It is connected in series with the electric installation. If excess current flows through the circuit the fuse wire melts and breaks the circuit. The wire melts at a constant temperature θ, when the heat produced per second by the current equals heat lost per second by radiation: i.e., i2R = H x A, where A is the surface area of the wire and H the heat radiated per unit area per second.

 

It can be shown that θ is directly proportional to specific resistance ρ, inversely proportional to the cube of the radius (i.e., r3), directly proportional to the square of the current (i2) and independent of the length (l) of the wire. The radius r of the wire can thus be calculated so that it melts at a specified value of current. The fuse wire blows at a current slightly below the rated current value. For a fuse wire, the length used does not matter, but should not be too small.

 

The fuse wire is commonly enclosed in a cartridge of porcelain or glass or related material having metal ends. The fuses for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, 15 A, etc. In house wiring, we generally employ either 5 A or 15 A fuse, the former for lights and fans; and later for power circuits.

 

In addition to tin-lead alloy, aluminum, copper, iron, etc. are used for the fuse wire. A fuse wire has (i) high resistance (not resistivity) and (ii) low melting point.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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