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Heating and Magnetic Effects of Electric Current

During your childhood, have you ever accidentally touched a light switch and suddenly experienced a jolt of electricity? Well, even though the tingling goes away in a few minutes, we couldn’t get over the feeling of shock that we received. For days we avoid touching another light switch, eventually getting over the fear. But how and why this happened? And what exactly are the heating and magnetic effects of electric current? Let’s find out.

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Electric Current

(Source: Wikipedia)

Electric current is the flow of electrons in a circuit. To make it easier to understand, imagine water flowing through a pipe. The water flowing through pipes is similar to electrons flowing through wires. Let ‘I’ denote the current which is measured in Ampere which is equivalent to the flow of one coulomb per second (6.241 x 1018 electrons).

When we switch on a light bulb, it is due to the heating effect of electric current, and when we turn the ceiling fan on, it is due to the magnetic effect of current. Let us learn more about the heating and magnetic effects of electric current.

Heating Effects of Electric Current

The fundamental law of conservation of energy states that the total energy in an isolated system is always constant. It means that energy can neither be created nor destroyed – it can only be transferred from one form to the other.

To understand this, take a look at this example. When we line up a row of dominoes and tip over the first piece, it results in a chain reaction which causes them to fall. This happens because the mechanical energy of the first domino is transferred to the mechanical energy of the next domino and so on. And the energy remains mechanical, as it is passed on from one domino unto another.

How does it work?

An electric current is passing through a conductor which becomes hot after some time and produces heat. This is due to the conversion of some of the electrical energy that passes through the conductor, into heat energy. This effect of electric current is called the heating effect of current.

Mathematical Expression of Heat Produced

When a unit charge moves from one point to the other, some work is required to do so. The potential difference is the measure of work that is done in moving the charge across the circuit. Current in a circuit is equal to the amount of charge flowing in one second.

Therefore, work that is done in moving charge ‘Q’ through a potential difference ‘V’ in time ‘t’ is given by

Workdone = potential difference × current × time

W = VIt

Using ohm’s law, we know

V = IR

Therefore work can also be expressed as

W = (IR) It = I2Rt

Thus, we can say that the heat produced is directly proportional to resistance, to time and the square of the current.

Some applications of the Heating Effect

1. When you are late for work or for a date, you need to iron your shirt; you reach over for the iron. This is the most basic example of the heating effect.
2. In a microwave oven, electric energy is converted into heat which gives us some of the most delicious food and desserts to eat.
3. When girls find it hard to tame their hair, they turn to their hair curler or straightener. When you touch your hair, it feels warm to the touch. Well, it’s because it works on the same principle.

Magnetic Effects of Electric Current

Let us set up a simple electric circuit consisting of a wire, a battery, a switch and a bulb. When current passes through the circuit, the bulb lights up. Now try bringing a magnetic compass near the circuit and notice how the needle deflects when the circuit is complete.

These effects are called the magnetic effects of electric current and they occur because they experience a force. The first scientist who showed that electric current also produces magnetic effect was Hans Christian Oersted.

The direction of the force depends on the direction of the current that flows through the conductor. You can find the direction with the simple right-hand rule, which states that: the index finger points in the direction of velocity ‘v’, middle finger points to the direction of magnetic field ‘B’ and the thumb points in the direction of the cross product ‘F’. The magnetic field can be denoted by

$$\vec{F}=q \vec{v} \times \vec{B}$$

A magnetic field is formed around a conductor when current flows through it which means it acts like a magnet. We also know that in magnets, unlike poles attract and like poles repel each other.

We know that in magnets like poles repel and unlike poles attract each other, so depending on the direction of the magnetic field induced, the conductor will either get attracted to or get repelled by the permanent magnet.

Electric Bell

When you aren’t busy knocking on doors, you are definitely ringing the doorbell. But have you ever wondered how a bell actually works? Well, one application of electromagnets is an electric bell.

This brings us to the question: what is an electromagnet? The magnetic effects of electric current can be used to make an electromagnet.

Experiment

How: Take a wire and wrap it around an iron rod in many turns. As long as we apply current to the rod, it will act as a magnet. And this type of magnet is defined as an electromagnet. Now, increase the number of turns of wire and watch it become a more powerful magnet. It will then attract a piece of iron attached to the clapper which hits the bell, in turn, making it ring. So, now you understand the basic principle behind the working of your doorbell.

Other applications of the electromagnet are in the following things that we use.

1. Though they have been replaced by mobile phones, telephones still remain a part of our household. Electromagnets are used in the earpiece of a telephone where your sound is converted into electric current by the mouthpiece.
2. Even the radio in our cars that we listen to when we are stuck in traffic use electromagnets. Radio signals are a form of electromagnetic waves.

Solved Example for You

Q: An electric heater kept in a vacuum is heated continuously by passing electric current. It’s temperature will:

1. go on rising with time
2. stop after some time as it will lose heat to the surroundings by conduction
3. rise for some time and thereafter will start falling
4. become constant after some time because of loss of heat due to radiation

Ans: D. become constant after some time because of loss of heat due to radiation

Treating initially that there is no radiation and after applying a voltage to the heater, the temperature of the heater rises. As the temperature rises, the radiation also increases. As the input power (i2R) is constant, the output power should also be constant. When the heater reaches a certain temperature, the radiation power equals the input power and the temperature of the heater stops rising. Thus temperature becomes constant after some time.

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