Physics · Ch 2 — Current Electricity
Application of Joule's Heating Effect
Application of Joule's Heating Effect
Joule's heating effect has a wide range of familiar, everyday and industrial applications, all exploiting the deliberate conversion of electrical energy into heat:
1. Electric heaters. Devices like the electric iron, electric room heater and electric toaster (Figure 2.30) all use heating elements made of nichrome, an alloy of nickel and chromium. Nichrome is chosen specifically because it has a high specific resistance (so a compact element can dissipate a lot of power) and can be heated to very high temperatures repeatedly without oxidising or burning out.
2. Electric fuses. A fuse (Figure 2.31) is a short length of wire, made from a low-melting-point material, connected in series in a circuit specifically to protect the rest of the circuit from the heat generated by excessive current -- if the current exceeds a certain safe value, the fuse wire itself melts and breaks the circuit before the excess current can damage anything else. An alloy of lead and tin is typically used for fuse ratings below 15 A, while copper fuse wires are used above 15 A. The one disadvantage of a traditional fuse is that once it has burnt out, it must be physically replaced; modern homes instead often use circuit breakers (or "trippers", Figure 2.32), which trip open automatically on an excessive current and can simply be reset and switched back on once the underlying fault has been repaired.
3. Electric furnaces. Industrial furnaces (Figure 2.33) are used to manufacture a wide range of technologically important materials, such as steel, silicon carbide, quartz and gallium arsenide. To reach temperatures up to about 1500°C, a molybdenum-nichrome wire wound on a silica tube is used; carbon arc furnaces can push temperatures even higher, up to about 3000°C. …
What this figure shows. Three household heating appliances shown side by side: (a) an electric iron box with its flat heated sole-plate, (b) a coil-type electric room heater with its glowing nichrome element, and (c) an electric toaster with slots for bread, all sharing the same underlying nichrome resistive-heating principle. …
What this figure shows. A fuse assembly is shown in cutaway, with a base holding a fuse carrier that in turn holds the thin fuse wire itself, with metal contacts at either end connecting it into the circuit; the fuse wire is drawn noticeably thinner than the surrounding wiring, since it is specifically this thin section that is designed to melt and break the circu …
What this figure shows. A photograph-style illustration of modern household circuit-breaker switches (trippers) mounted in a distribution panel, contrasted implicitly with the older fuse-wire technology of Figure 2.31, since a breaker's switch mechanism trips open automatically on excess current but can simply be reset and closed again once the fault is fixed, unlike a fus …
What this figure shows. An industrial electric furnace shown as a large lined chamber capable of reaching very high temperatures, used to manufacture technologically important materials such as steel, silicon carbide, quartz and gallium arsenide, with its heating element made of molybdenum-nichrome wire wound on a silica tube (or, for the h …
What this figure shows. Three further everyday and industrial applications of Joule heating shown together: a glowing incandescent light bulb with its tungsten filament visible inside the glass envelope, an electric arc producing an intense bright discharge, and an electric welding torch fusing metal using the heat generated by a very large curr …
Worked out. An electric heater of resistance , connected to a 220 V supply, is immersed in 1 kg of water; the time needed to raise the water's temperature from to is required (specific heat capacity of water ). The current through the heater is A, so the heat produced per second is J (i.e. the heater's power rating is effectively 4.84 kW). The heat energy actually needed is J kJ. Setting the produced heat equal to the required heat, the time is $t=Q/ …