NCERT Class 8 Science Curiosity Chapter 04 Electricity Magnetic and Heating Effects PDF Download

Chapter 4: Electricity: Magnetic and Heating Effects

Probe and ponder

• If we don't have an electric lamp while making an electric circuit with an electric cell, is there any other way through which we can find out if current is flowing in the circuit?
• Is it possible to make temporary magnets? How can these be made?
• We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?
• How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
• Share your questions

It was the day of the science exhibition, and the school was buzzing with energy. Mohini and Aakarsh, along with their friends, went from one exhibit to another, eagerly exploring different models, asking questions and taking notes. One simple model really fascinated them. It was a working model of a lifting electromagnet which was displayed by their senior, Sumana. In it, instead of a hook like a typical crane, there was an iron nail wrapped with a wire, which was connected to a battery. When Sumana closed the circuit, the nail picked up iron paper clips like a magnet. When she opened the circuit, the clips fell off. Mohini and Aakarsh, were surprised. They remembered learning earlier (in the chapter 'Exploring Magnets', Curiosity, Grade 6) that magnetic materials were attracted by magnet and that iron was a magnetic material. But in Sumana's model, there was no magnet, only an electric circuit. They were so excited that they wanted to try it out themselves.

4.1 Does an Electric Current Have a Magnetic Effect?

Activity 4.1: Let us investigate

• Collect a magnetic compass, an electric cell, a cell holder, two drawing pins, a safety pin, two nails, two pieces of connecting wires (one longer and one shorter), and two small pieces of cardboard.
• Using two drawing pins, a safety pin, and a cardboard piece, make a switch (as you made it earlier in the chapter 'Electricity: Circuits and their Components' in Curiosity, Grade 7).
• Place the cell in the cell holder.
• Fix two nails to a piece of cardboard as shown in Fig. 4.1a. Fix the middle portion of the longer wire stretched between the nails, such that it is slightly above the surface of the cardboard. Attach one end of that wire to the cell holder and another end to the switch.
• Connect the second wire between the cell holder and the switch.
• Place the magnetic compass beneath the wire between the two nails (Fig. 4.1a).
• While watching the compass needle, move the switch to 'ON' position to allow electric current to flow through the wire (Fig. 4.1b). What do you observe?
• Now again while watching the compass needle, move the switch to 'OFF' position. What do you observe this time?
• Move the switch between 'ON' and 'OFF' positions a few more times. Carefully observe how the compass needle behaves the each time.

[Figure 4.1: An electric circuit and a magnetic compass, See in your textbook]

You may have noticed that when the current flows, the compass needle gets deflected from its original direction. When the current stops, the needle returns to its original direction.

As we have learnt earlier (in the chapter 'Exploring Magnets' in Curiosity, Grade 6), the compass needle is a tiny magnet which deflects when a magnet is brought near it and this magnetic effect can act through any non-magnetic materials kept in between. But why does the compass needle deflect when the current flows through the wire? The deflection indicates that the current carrying wire has a magnetic effect on the compass needle. When the current stops, this magnetic effect disappears and the compass needle returns to its original direction. The region around a magnet or a current carrying wire where its magnetic effect can be felt, such as by the deflection of a compass needle, is said to have a magnetic field.

When electric current flows through a conductor (like a wire), it produces a magnetic field around it. This phenomenon is known as the magnetic effect of electric current. The magnetic field disappears when the current stops flowing.

Teacher's Note

The compass needle always points north because Earth itself has a magnetic field. When you switch on the circuit, the magnetic field from the wire interferes with Earth's field, causing the needle to deflect. The deflection direction depends on which way the current flows through the wire. This is how you know the current is actually flowing - not by seeing light or heat, but by observing the compass move.

Be a scientist

You have just now made the same discovery which was made by the scientist Hans Christian Oersted (1777-1851) in 1820, that is, the discovery that electricity and magnetism are linked. He was a professor at a university in Denmark. It is said that once while giving a demonstration, he noticed that whenever an electrical circuit was closed or opened, the needle of a magnetic compass, lying nearby, deflected. He investigated this and when he was certain that an electric current indeed produced a magnetic field, he published his findings. This led to other scientists repeating his experiment to check if they got the same results, and further investigating the connection between electricity and magnetism.

Teacher's Note

Oersted's discovery was accidental - he wasn't even looking for a link between electricity and magnetism. But because he was alert and curious about what he saw, he investigated further and changed the course of physics forever. In your own experiments, stay alert to unexpected observations; they often lead to the most interesting discoveries.

Key Points

• When electric current flows through a wire, it creates a magnetic field around the wire. This is called the magnetic effect of electric current.
• You can detect this magnetic field using a compass needle, which will deflect from its normal position when the current flows.
• The magnetic field only exists while the current is flowing; it disappears immediately when the current stops.
• This connection between electricity and magnetism was discovered by Hans Christian Oersted in 1820.

This is a preview of the first 3 pages. To get the complete chapter, click below.