Class 8 Science · Chapter 4 NotesElectricity: Magnetic and Heating Effects

When electric current flows through a wire, it produces a magnetic field around it. This is called the magnetic effect of electric current. A simple way to detect this is to place a magnetic compass near a current-carrying wire: the compass needle deflects while the current flows and returns to its original direction when the current stops. The region around a magnet or a current-carrying wire where its magnetic effect can be felt is called a magnetic field. Hans Christian Oersted discovered this link between electricity and magnetism in 1820. The magnetic effect is used in many devices, including electromagnets, electric bells, motors, fans, and loudspeakers.

• Magnetic fieldthe region around a magnet or current-carrying wire where its magnetic effect can be felt.
• Magnetic effect of electric currenta current-carrying conductor produces a magnetic field around it.
• The magnetic field disappears when the current stops.

A current-carrying coil that behaves like a magnet is called an electromagnet. For practical use, most electromagnets have an iron core, which makes them much stronger. An electromagnet has two poles, North and South, just like a bar magnet. Its strength can be increased by increasing the current (for example, using more cells) or by increasing the number of turns in the coil. Reversing the direction of current reverses the poles. Lifting electromagnets are strong electromagnets hung from cranes. When current is switched on, they lift iron and steel objects; when current is switched off, the objects are released. They are widely used in factories and scrap yards to move, lift, and sort heavy metal items.

• Electromagneta current-carrying coil that behaves as a magnet.
• Strength increases with more current or more turns of the coil.
• Poles can be reversed by changing the direction of current.
• Lifting electromagnets are used in cranes and scrap yards.

When electric current flows through a conductor, it faces opposition called resistance. This resistance causes some electrical energy to be converted into heat energy, so the conductor gets warm. This is known as the heating effect of electric current. Different materials offer different resistance. Nichrome wire, for example, offers higher resistance than a copper wire of the same size and length, so it heats up more. The amount of heat produced depends on the magnitude of current, the material, thickness, and length of the wire, and the duration for which current flows. Many household appliances, such as electric room heaters, electric stoves, electric kettles, electric irons, water heating immersion rods, and hair dryers, work on this principle. They contain a heating element, which may glow red hot.

• Resistanceopposition to the flow of electric current in a conductor.
• Heating effectgeneration of heat in a conductor due to electric current.
• Nichrome has higher resistance than copper of the same size and length.
• Heat depends on current, material, thickness, length, and time.

The chapter does not give formulas for electric power or energy consumption. It explains that the heating effect is useful in appliances but can also cause problems, such as energy loss in wires during transmission. Overheating may damage plugs and sockets, melt plastic parts, or even lead to fires. In household circuits, safety devices are placed to minimise such incidents. It is important to use wires, plugs, and sockets rated for the specified electric current. Beyond homes, the heating effect is used in industry—for example, in specially designed high-temperature furnaces that use electric current to produce heat for melting and recycling scrap steel.

• Overheating can damage plugs and sockets or cause fires.
• Use wires, plugs, and sockets rated for the required current.
• High-temperature electric furnaces are used in steel manufacturing.

A cell or battery generates electric current because of chemical reactions taking place inside it. A Voltaic cell (also called a Galvanic cell) contains two metal rods called electrodes, made of different materials, partly dipped in a liquid called an electrolyte (usually a weak acid or salt solution). A chemical reaction between the electrodes and the electrolyte produces electricity. Over time, the chemicals get used up, and the cell becomes 'dead' and cannot supply electricity. A simple Voltaic cell can be made using lemons, copper wires, and iron nails, with lemon juice acting as the electrolyte. Some common metal pairs for Voltaic cells are zinc/copper, zinc/silver, aluminium/copper, iron/copper, magnesium/copper, and lead/copper.

• Electrodemetal rod in a cell where chemical reaction occurs.
• Electrolyteliquid or paste that conducts electricity in a cell.
• Voltaic cellcell that produces electricity from chemical reactions between electrodes and electrolyte.

Dry cells are widely used today. They are called 'dry' because the electrolyte is a thick moist paste, not a liquid. A dry cell has a zinc container that acts as the negative terminal and a carbon rod at the centre with a metal cap that acts as the positive terminal. The carbon rod is surrounded by the paste-like electrolyte. A dry cell is a single-use cell; once used up, it must be disposed of. Rechargeable batteries can be recharged and reused many times, which prevents wastage and saves money over time. They are used in watches, phones, laptops, tablets, cameras, inverters, and electric vehicles. However, rechargeable batteries also wear out after many charges. Lithium-ion batteries are the most common type of rechargeable battery today.

• Dry cellzinc container (negative terminal), carbon rod with metal cap (positive terminal), paste electrolyte.
• Rechargeable batteries can be recharged and reused multiple times.
• Lithium-ion batteries are the most common rechargeable batteries.

The magnetic and heating effects of electric current are used in many everyday devices. Electromagnets are used in electric bells, motors, fans, loudspeakers, and lifting cranes. Heating devices include electric heaters, stoves, kettles, irons, immersion rods, and hair dryers. The chapter also highlights safety: do not touch a heating wire for an extended period, do not connect wires to a cell for more than a few seconds (as the cell may weaken quickly), and use appropriate wires and plugs. Used batteries should not be thrown in regular garbage because they may contain harmful materials; they should be taken to e-waste recycling facilities.

• Electromagnetselectric bells, motors, fans, loudspeakers, cranes.
• Heating effectheaters, stoves, kettles, irons, immersion rods, hair dryers.
• Dispose of used batteries at e-waste recycling facilities.