Thermal effect of current it’s use, calculation of power and electrical energy
When an electric current flows through a conductor, electrical energy is expended n over coming the frictional resistance between eh electrons and the molecules of the wire. If potential difference of V volts is applied across a conductor and current of I amperes flows through it for time of t seconds, then energy expended will be equal to VIt watt-secs or joules.
If R is the resistance of the conductor through which a current of I amperes flows and V is the potential difference applied across its end then by Ohm’s law V- IR
and energy expended, W = VIt = IR X I X t = I2Rt joules
or W also = V X V t = V2 t joules
According to the law of conservation of energy this electrical energy expended must be converted in some other form of energy an I that other form is hear i.e. electrical energy expended is converted into heat energy and conversion of electrical energy into heat energy is called the thermal effect of electric current.
When electric current is supplied to a purely resistive conductor, the energy of electric current is dissipated entirely in the form of heat and as a result, resistor gets heated. The heating of resistor because of dissipation of electrical energy is commonly known as Heating Effect of Electric Current.
Applications of heating effect of electric current
Most household electrical appliances convert electrical energy into heat by this means. These include filament lamps, electric heater, electric iron, electric kettle, etc.
In lighting appliances
Filament lamps: it is made of a tungsten wire enclosed in a glass bulb from which air has been removed. This is because air would oxidize the filament. The filament is heated up to a high temperature and becomes white hot. Tungsten is used due its high melting point; 34000 The bulb is filled with an inactive gas e.g. argon or nitrogen at low pressure which reduces evaporation of the tungsten wire. However, one disadvantage of the inert gas is that it causes convection currents which cool the filament. This problem is minimized by coiling the wire so that it occupies a smaller area which reduces heat loss through convection.
Fluorescent lamps: these lamps are more efficient compared to filament lamps and last much longer. They have mercury vapour in the glass tube which emits ultraviolet radiation when switched on. This radiation causes the powder in the tube to glow (fluoresce) i.e. emits visible light. Different powders produce different colours. Note that fluorescent lamps are expensive to install but their running cost is much less.
In electrical heating
Electric cookers: electric cookers turn red hot and the heat energy produced is absorbed by the cooking pot through conduction.
Electric heaters: radiant heaters turn red at about 9000C and the radiation emitted is directed into the room by polished reflectors.
Electric kettles: the heating element is placed at the bottom of the kettle so that the liquid being heated covers it. The heat is then absorbed by water and distributed throughout the whole liquid by convection.
Electric irons: when current flows through the heating element, the heat energy developed is conducted to the heavy metal base raising its temperature. This energy is then used to press clothes. The temperature of the electric iron can be controlled using a thermostat (a bimetallic strip).
calculation of power and electrical energy
The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh.
E = P × t
- E is the energy transferred in kilowatt-hours, kWh
- P is the power in kilowatts, kW
- T is the time in hours, h
power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000.
For example, 2,000 W = 2,000 ÷ 1,000 = 2 kW. Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600.
For example, 7,200 s = 7,200 ÷ 3,600 = 2 h.