Heating Effects of Currents

Author: J.B. Hoag

Due to the collision of free electrons with atoms, the passage of a current down a metal wire heats it up. The amount of heat that is generated depends upon the square of the current and varies directly with the resistance of the wire and the time. Thus, in power units,

where P is the power lost in the wire in watts, E = volts, I = amperes, R = ohms.

The heat developed in a wire must not be confused with the temperature to which the wire will eventually rise. The latter depends on both the rate at which heat is produced in it by the current and the rate at which heat is lost to the surroundings. The loss of heat depends on many factors. It is obvious that, if the wire is wound into a compact unit and is placed in a closed space, the heat losses will be less than if the wire is extended and out in the open air.