New NCERT Curiosity · Class 8 Science · Term I · Chapter 4: Electricity: Magnetic and Heating Effects
This chapter shows that an electric current can make a magnet and can also produce heat. It then explains how cells and batteries produce current through chemical reactions, from the voltaic cell and lemon cell to dry cells and rechargeable batteries.
From the Class 8 Science Short Notes, made as per the new NCERT book Curiosity · Pages 20–25 of the book.
Key points
- A current-carrying wire deflects a nearby compass needle; the effect disappears when the current stops.
- Oersted showed in 1820 that an electric current produces a magnetic field — electricity and magnetism are linked.
- A current-carrying coil behaves like a magnet; an iron core makes this electromagnet stronger.
- An electromagnet has a North and a South pole; reversing the current reverses its poles.
- More current (more cells) and more turns in the coil make an electromagnet stronger.
- Lifting electromagnets on cranes pick up and release iron and steel objects in factories and scrap yards.
- Resistance converts some electrical energy into heat, so a wire carrying current gets heated.
- Heat produced depends on the current, the material, thickness and length of the wire, and the time for which current flows.
- Heaters, kettles, irons and hair dryers use a heating element; overheating can melt plugs or cause fires.
- A cell produces current from chemical reactions between two different metal electrodes and an electrolyte.
- A dry cell uses a thick moist paste; rechargeable batteries like Li-ion can be used many times but slowly wear out.
Key terms
- Magnetic field: The region around a magnet or a current-carrying wire where its magnetic effect can be felt.
- Magnetic effect of electric current: A conductor carrying current produces a magnetic field around it, which disappears when the current stops.
- Electromagnet: A current-carrying coil that behaves as a magnet, usually with an iron core to make it stronger.
- Resistance: The opposition a conductor offers to the flow of current.
- Heating effect of electric current: A conductor gets heated when current passes through it because resistance converts some electrical energy into heat.
- Cell / battery: A device that generates electric current because of chemical reactions inside it.
- Electrodes and electrolyte: Electrodes are the two rods of different metals; the electrolyte is the liquid (usually a weak acid or salt solution) they are dipped in.
Diagrams to learn
- A current-carrying wire deflects a compass (Fig 4.1)
- An electromagnet — coil on an iron nail (Fig 4.2)
- Lemon cells joined to light an LED (Fig 4.3)
- Table comparing a bar magnet and an electromagnet
- Table comparing voltaic cell, dry cell and rechargeable battery
- It is wrong to think electricity and magnetism have no link — a current always produces a magnetic field.
- Touch a hot nichrome wire only for a moment — never hold it.
- An LED glows only when its longer leg goes to the positive terminal.
- A 'dead' battery is not harmless; never throw it in regular garbage — give it to an e-waste recycling facility.
A real page from the book
Get the full chapter in the book
This page is a summary. The Class 8 Science Short Notes has the complete chapter in colour, with every diagram and worked example.
Frequently asked questions
How can you make an electromagnet stronger?
Use a larger current, for example a battery of 2 or 4 cells instead of one, and wind more turns in the coil. An iron core also makes it stronger.
On what factors does the heat produced in a wire depend?
It depends on the amount of current, the material of the wire, its thickness, its length and the time for which current flows.
How does a lemon cell work?
A copper wire and an iron nail act as the two electrodes and lemon juice is the electrolyte. A chemical reaction between them produces current; several lemon cells joined together can light an LED.
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