Experts have designed these Class 8 Science Notes Chapter 4 Electricity: Magnetic and Heating Effects Class 8 Notes for effective learning.
Class 8 Science Chapter 4 Electricity: Magnetic and Heating Effects Notes
Class 8 Electricity: Magnetic and Heating Effects Notes
Class 8 Science Chapter 4 Notes – Electricity: Magnetic and Heating Effects Notes Class 8
→ Magnetic Effect of Electric Current: When an 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.
→ Compass Needle: It is a small magnet that aligns itself along the Earth’s magnetic field.
- When current flows through a nearby wire, the compass needle gets deflected due to the magnetic field produced by the current.
- When the current stops, the magnetic field disappears. The compass needle returns to its original direction gets aligned with Earth’s magnetic field.
→ Magnetic Field: The region around a magnet or a current-carrying wire where the magnetic effect can be felt is called the magnetic field.
→ Discovery of the Link Between Electricity and Magnetism: Hans Christian Oersted in 1820 while demonstrating an experiment, observed that a compass needle deflects when an electric current flows through a nearby wire. This led him to conclude that there is a connection between electricity and magnetism.
→ The magnetic effect of electric current has many practical applications, such as in devices like electromagnets, electric bells, motors, fans, loudspeakers, etc.
→ Electromagnet: A current-carrying coil that behaves as a magnet.
→ An electromagnet, like a regular magnet, has two poles – North and South.
→ Factors Affecting the Strength of an Electromagnet:
- Electric Current: Increasing the current increases the magnetic field.
- Number of Turns in the Coil: More turns result in a stronger magnetic field.
→ Reversing Poles of Electromagnet: The poles of an electromagnet can be reversed by changing the direction of the current.
→ Earth-A Giant Magnet: The movement of liquid iron in Earth’s core creates electric currents which generate a magnetic field around Earth causing it to behave like a giant magnet.
→ Earth’s Magnetic Field: Guide and Shield
- Many migratory birds, fish, and animals use this field to navigate across continents and oceans.
- Earth’s magnetic field acts as a protective shield, blocking harmful space particles and protecting life on Eart
→ Lifting Electromagnets: These are strong electromagnets which work on the principle of the magnetic effect of electric current. These electromagnets are used in factories and scrap yard to move, lift, and sort heavy items.
→ A moving magnet can also produce an electric current, just as an electric current produces a magnetic field. This close connection between electricity and magnetism forming the basis of many devices like electric motors and power generators.
→ Heating Effect of Electric Current: When an electric current flows through a conductor, it faces resistance to its flow. Due to this resistance, some electrical energy is converted into heat energy, causing the wire to get warm or heated. This warming of an electric wire is known as the heating effect of current.
- Nichrome wire offers higher resistance than copper, so it heats up more.
→ The heat generated in a wire depends on the material, thickness, and length of the wire, and the duration for which the current flows.
→ Many household appliances like room heaters, irons, and hair dryers work on the heating effect of electric current. These devices have a heating element (a rod or coil of wire) that gets hot when current passes through it, sometimes glowing red when visible.
→ Disadvantages of the Heating Effect of Electric Current:
- Energy loss in wires during long-distance electricity transmission.
- Overheating in appliances may damage plugs, sockets, and melt plastic parts.
- Risk of electrical fires due to excessive heating.
- To prevent unnecessary heating, it is important to use proper wires, plugs and sockets which are rated for the specified electric current of the connections.
→ Industrial Applications of the Heating Effect of Electric Current: The heating effect of electric current is also widely used in industries. For example, steel manufacturing industries use specially designed high-temperature furnaces where heat produced by electric current is used to melt and recycle scrap steel, converting it into usable steel.
→ Voltaic Cell/Galvanic Cell: A Voltaic Cell (also called Galvanic Cell) produces electricity using a chemical reaction. It consists of:
- Electrodes: Two metal plates made of different materials.
- Electrolyte: A liquid which is usually a weak acid or salt solution.
→ Working of Voltaic cell: The chemical reaction between electrodes and electrolyte generates an electric current. Current flows from the positive terminal to the negative terminal when the circuit is connected. Over time, the chemicals get used up, and the cell becomes dead, unable to supply more electricity.
→ Origin of Voltaic or Galvanic Cells: Voltaic or Galvanic cells are named after Italian scientists Alessandro Volta and Luigi Galvani. In the late 1700s, Galvani saw that a dead frog’s leg move when touched with copper and iron, thinking the electricity came from the frog. Volta showed it came from the metals and a liquid, proving this with saltwater-soaked paper. This led to the invention of the first battery.
→ Common Metal Pairs for Voltaic Cells: Common metal pairs used in Voltaic cells are:
- Zinc/Copper
- Zinc/Silver
- Aluminium/Copper
- Iron/Copper
- Magnesium/Copper
- Lead/Copper
→ Some metals (like copper) act as positive electrodes. Some metals (like zinc) act as negative electrodes. This depends on the chemical properties of the metals.
→ We can also make-our own Voltaic cell using lemon juice as electrolyte.
→ Dry Cells: Dry Cells are widely used today. They are called ‘dry’ because the electrolyte is a thick, moist paste, not a liquid.
→ Structure of a Dry Cell:
- Zinc container, acts as the negative terminal.
- Carbon rod at the centre, covered with a metal cap, acts as the positive terminal.
- Electrolyte, a thick paste surrounding the carbon rod.
→ Dry cells are single-use. Once used up, they are disposed of.
→ Rechargeable Batteries: They can be recharged and reused multiple times. This helps reduce waste and saves money over time. Today, they are increasingly being used in many applications.
→ There are different kinds of rechargeable batteries that are used for different applications.
– Rechargeable batteries also do not last forever. After being charged and used many times, they slowly wear out.
→ Lithium-ion Batteries: These are the most widely used rechargeable batteries today, found in almost all battery-powered devices. They depend on rare metals like lithium and cobalt, which are mined in limited regions. To deal with this, countries are trying to secure supplies, recycle old batteries, and develop new technologies.
→ Scientists are developing solid-state batteries, which use solid materials instead of liquid electrolytes. These future batteries will be safer, charge faster, and last longer.