Ever touched an electrical gadget after it has been working for a while? Take an example of an electric fan. It does turn hot after it has been working for some time. So, what causes this rise in temperature, and where does the heat come from? In this lesson, we’ll talk about the heating effect of electric current and the Joule’s Law of heating. We know that the battery is the source of electrical energy for any circuit. All the electrical energy provided by the battery is not converted to useful work. As we have observed, that when the current flows through a conductor or a gadget, this heating takes place in that conductor. So, some of the electrical energy is also dissipated in the form of heat. When the current flows through a purely resistive circuit, that is a circuit which comprises of only resistors and the battery, all the energy provided by the battery is converted in the form of heat. That is, all the electrical energy provided by the battery is dissipated in the form of heat through the resistors. This is called the heating effect of electric current. Let us consider a circuit operating under a potential difference V and a current I flowing through a resistor R. Also, let Q with the charge flowing in a time interval t. Now, the work done to move a charge Q from one point to other other under a potential difference of V is equal to V times Q. Therefore, our source or the battery has to do a work equal to V times Q in the time interval of t. Also, we define power to be the work done per unit time, that is V times Q divided by t. But, from our first lesson we remember that Q by t that is the charge flowing per unit time is equal to the current. Therefore, power is equal to V times I, where V is the potential difference and I is the current flowing in the circuit. For a steady current I flowing in the circuit, the heat produced in a resistor is equal to VI multiplied by the time t. Now, by using Ohm’s Law and replacing V by IR, we get the expression of heat as H is equal to I squared R t, and this is called the Joule’s Law of Heating, which implies that the heat produced in the resistor is directly proportional to the square of the current flowing in the circuit. It is directly proportional to the resistance used in the circuit and it is also directly proportional to the time for which the current flows in the circuit. We may feel that the heating effect of electric current is very undesirable as it converts our useful electrical energy into heat. But, it has many practical applications also. Consider our everyday household items, like the electric heater, electric kettle, laundry, iron, all are based on the Joule’s Law of heating. Even the functioning of an electric bulb is based on the Joule’s Law of heating. The heat energy provided by the current raises the temperature of the filament of the bulb to 3380 degrees Celsius. When it is this hot, it starts to emit light, and in order to protect the filament from melting, it needs to be kept in an insulated atmosphere. For this, a bulb is generally filled with inactive Nitrogen or Argon gas, so that the life of the filament is prolonged. A major part of energy given to the bulb goes in the form of heat, and only a small part is radiated in the form of light. Another common example of Joule’s heating is of an electric fuse. The function of an electric fuse is to protect our electronic devices from unduly high currents flowing in the circuit. A fuse wire is connected in series with our electronic device. When the current flowing in the circuit exceeds the maximum permissible value, it increases the temperature of the fuse wire due to which the fuse wire melts and break the circuit. A fuse wire has a low melting point and high value of resistance. And it generally comes in porcelain casings with metal ends. So, till now we have discussed the heating effect of electric current and the Joule’s Heating Law. Further, in this lesson we’ll talk about electric power. Electric power is defined as the rate of dissipation or consumption of electric energy. The SI unit of power is Watt. We have already derived the relation for power that is P is equal to V times I. It can also be written as V squared by R or I squared into R, by applying the Ohm’s law. Therefore, 1 Watt is the power consumed by electrical device which carries the current of 1 Ampere operated under the potential difference of 1 V. Therefore, 1 Watt is equal to 1 Volt Ampere. But in actual practice, 1 Watt is a very small unit of power. In our everyday life, we generally use a bigger unit that is 1 kiloWatt, and 1 kiloWatt is equal to 1000 Watts. Since, electrical energy is a product of power and time, 1 Watt hour is the energy consumed when a device of power 1 Watt is used for 1 hour. Similarly, 1 kiloWatt hour is the energy consumed when a device of 1 kiloWatt is used for 1 hour. 1 kiloWatt hour is the commercial unit of energy and it is also called one unit. 1 kilowatt hour or 1 unit is equal to 1,000 into 60 into 60 is equal to 3.6 into 10 raised to power 6 Joules. The electricity bills we pay in our homes is equal to the number of units of electricity we use in our homes multiplied by the rate of 1 unit of electricity in our locality. So, let us quickly recap what we learned in this lesson. We discussed about the heating effect of electric current and its applications. We derived the mathematical expression for the Joule’s Law of heating. We discussed electric power and we also explained the commercial unit of electrical energy.