Heating Effect of Electric Current & Electric Power | Science Class 10 PDF Download

Heating Effect of Electric Current

When an electric current flows through a bulb, the filament heats up so much that it glows and produces light. Similarly, when a current goes through the filament of an electric iron, the iron becomes very hot. This rise in temperature is caused by the heating effect of electric current, often called Joule's heating or Joule's law. If a resistor R is connected to a battery, the battery creates a voltage V across the resistor, causing a current i to flow through it.

So, V = iR  ......(i)  

The current flowing through the resistor is actually a movement of negative charges (electrons). In the battery, these negative charges move from the positive terminal to the negative terminal. The battery does work = QV to move a charge across the voltage V between its terminals. This energy is released as heat in the resistor. Therefore, for a steady current I, the heat H produced over time t is:

U = QV        .....(ii)

The charge that passes through the wire in time t is

Q =it.    ......(iii)

Using (i), (ii) and (iii), we find that the heat produced in the wire in time t is

U =QV = (it)

(iR) =i² Rt.

This equation shows that the heat produced is directly related to the square of the current if R and t are constant. So if the current is doubled, the heat produced increases by four times. Also, for a fixed current I and time t, the heat produced is directly related to the resistance R. If the same current flows through two resistors, more heat will be generated in the one with the higher resistance. The heat produced can also be written as.

or

For a given V and t, the heat produced is inversely proportional to R. So, if the same potential difference is applied across two resistances, more heat will be produced in the smaller resistance.

We have seen above that the increased energy of a charge gets converted to heat in the resistor. The increase in energy comes from the work done by the cell. This uses up the chemical energy of the cell. So, the energy appearing as heat in the resistor ultimately comes at the expense of the chemical energy of the cell.

Not always is the work done by a cell converted to heat. Immediately after a motor is connected to a cell, the speed of the shaft of the motor increases. A part of the work done by the cell goes into producing the increase in kinetic energy. And a part is used to overcome friction, etc. When the motor achieves a constant speed, its kinetic energy does not change. So the work done by the cell is only used to overcome friction, etc. This appears as heat. That is why the cover over a motor becomes warm on use.

Electric Power 

Power is how quickly work is done or how fast energy is produced or used. The electrical energy produced or consumed in a certain time is known as electric power. In an electric circuit, the power is calculated using these formulas:

Using iR = V

P = Vi

The energy consumed and power are related as

U = Pt.

Unit of Power 

The SI unit of energy is the joule, and that of time is the second. The SI unit of power is, therefore, joule/second. This unit is called the watt, whose symbol is W.

Practical Applications of Heating Effect of Electric Current 

The heating effect of electric current has many uses. Electric bulbs, room heaters, electric irons, immersion heaters, toasters, electric fuses, and a number of other appliances work on this principle. In all of these, a wire of suitable resistance, commonly called the heating element, is connected to the power supply. The current passing through the element produces heat in it, which is used for some specific purpose.

Electric bulb

An electric bulb has a simple structure. It consists of a sealed glass bulb that has a tungsten filament connected to two electrical contacts. The bulb is filled with an unreactive gas like argon or nitrogen. To produce white light, the filament has to be heated to about 3000°C by passing a current through it. Obviously, the material of the filament should such that it does not melt at this temperature. Tungsten is used for the filament because its melting point is about 3400°C. The sealed glass bulb serves two purposes. First, it protects the filament from oxidation and the effects of humidity. Secondly, the small enclosed volume makes it easier to maintain the required temperature, as without it the loss of heat would be more.

Fuse  

A fuse is a safety device that does not allow excessive current to flow through an electric circuit. It consists of a metallic wire of low melting point, fixed between the two terminals of a fuse plug. The fuse plug fits into a fuse socket connected in the circuit. Fuses are available in various shapes. The fuse plug is used in household wiring. It is made of porcelain.

A fuse is connected in series with an appliance (such as a TV) or a group of appliances (such as the lights and fans in a room). So, the current through the fuse is the same as the current through the appliance or the group of appliances. If this current exceeds a safe value, the heat produced in the fuse wire causes it to melt immediately. This breaks the circuit, preventing any damage. Figure shows examples of how a fuse is connected in circuits.

Good-quality fuse wires are made of tin, as it has a low melting point. Some fuse wires are made of an alloy of tin and copper. The thickness of the fuse wire depends on the circuit in which it is to be used. If a section of the circuit is meant to carry a maximum of 5A current, the fuse wire should also be able to carry currents up to 5A. Similarly, for wiring meant for 15A, the fuse wire should be thicker, and should be able to carry currents up to 15A.

Disadvantages of the Heating Effect of Current 

A current always produces some heat, whether we use the heat or not. If the heat produced cannot be utilized, it represents a wastage of energy. A considerable amount of energy is thus wasted in the transmission of electricity from the generating station to our homes. Sometimes, the heat produced in a device is so much that it can damage the device, unless proper cooling arrangements are made. To dissipate the heat produced in TV sets, monitors, etc., their cabinets have grills for air to pass. Certain components of a computer get so hot that they have fans to cool them. 

Rating of Electric Appliances 

Take an electric bulb and observe what is written on it. Besides the brand name and logo, you will see numbers related to power and voltage. For instance, it might say 60W, 220V. This indicates that 220V should be applied to the bulb, and it will consume 60W of power when that voltage is used. Similar information is found on all electric appliances. The combination of power and voltage values defines the rating of the appliance. From this rating, you can easily calculate its resistance with the formula P = V²/R. Remember, when the voltage is constant, higher power means lower resistance. For example, a 1000W heater has less resistance than a 100W bulb. You can also figure out the current drawn by an appliance using the formula I = P/V.

Kilowatt hour 

• Energy is calculated by multiplying power and time; the unit for electric energy is watt hour (Wh).
• One watt hour is the energy used when 1 watt of power operates for 1 hour.
• The standard unit of electric energy is kilowatt hour (kWh), often referred to as a 'unit'.
• 1 kWh = 1000 watts x 3600 seconds = 3.6 × 10⁶ watt seconds = 3.6 × 10⁶ joules (J).
• The electrical energy consumed in homes and factories is measured in kilowatt hours.
• The cost of electricity is set per kilowatt hour. One kilowatt hour of energy is known as one unit.