Electricity: Magnetic and Heating Effects
Science Class 8 — Chapter Notes
Table of Contents
1. Introduction
2. Does an Electric Current Have a Magnetic Effect?
3. Does a Current-Carrying Wire Get Hot?
4. How Does a Battery Generate Electricity?
5. Points to Remember: Electricity-Magnetic and Heating Effects
1. Introduction
Have you noticed how a bulb lights up when you switch it on? But what if you do not have a bulb? How else could you tell that electricity is flowing? Electric current can do more than produce light. It can also create magnetic effects and produce heat.
Electric Circuit
This chapter explains how electric current produces magnetic effects and heating effects. It also introduces electromagnets, electric cells, dry cells and rechargeable batteries.
2. Does an Electric Current Have a Magnetic Effect?
Magnetic Effect of Electric Current: When electric current flows through a conductor, such as a wire, it produces a magnetic field around the conductor. This is called the magnetic effect of electric current.
Magnetic Compass and Magnetic Field
Electric Circuit and Magnetic Compass
Magnetic compass: The presence of a magnetic field can be detected using a magnetic compass. When current flows through a nearby conductor, the compass needle is deflected. When the current stops, the needle returns to its original direction.
Magnetic field: It is the region around a magnet or a current-carrying wire where its magnetic effect can be felt. The deflection of a compass needle is one way of detecting this effect.
Discovery of the Magnetic Effect by Oersted
Hans Christian Oersted (1777-1851) discovered in 1820 that electricity and magnetism are linked. He found that when an electric circuit was switched on or off near a magnetic compass, the needle deflected. This showed that electric current produces a magnetic field.
This discovery laid the foundation for understanding the connection between electricity and magnetism.
Electromagnet: Definition and Features
A current-carrying coil of wire wrapped around an iron core, such as an iron nail, behaves like a magnet. Such an arrangement is called an electromagnet.
Electromagnet: An electromagnet consists of a coil of wire wound around a soft iron core. It produces a magnetic field when electric current flows through the coil.
The magnetism of an electromagnet disappears when the current is switched off.
How to Make an Electromagnet
1. Wrap insulated wire around an iron nail to form a coil.
2. Connect both ends of the wire to a cell.
3. When current flows through the coil, the iron nail can attract iron objects.
4. Disconnecting the current causes the nail to stop acting as a magnet.
Strength and Polarity of Electromagnets
Electromagnets have two poles, North and South, just like bar magnets.
The poles of an electromagnet can be determined using a magnetic compass.
polarity of an electromagnet reverses if the direction of the current is reversed.
Compass Needle near end A and end B
The strength of an electromagnet increases with:
Greater current, which can be obtained by using more cells in a suitable circuit.
More turns of wire in the coil.
Inserting a soft iron core inside the coil.
Lifting Electromagnets
Lifting electromagnets are strong electromagnets used in cranes for lifting and moving heavy iron or steel objects. They are widely used in factories and scrap yards to move, lift and sort heavy metal items efficiently.
Lifting Electromagnets
Turning the current on activates the electromagnet and allows it to lift the metal.
Turning the current off releases the metal.
Earth's Magnetic Field
Earth acts as a giant magnet. The movement of liquid iron in its outer core produces electric currents and helps generate Earth's magnetic field.
Many migratory birds, fish and animals use Earth's magnetic field to navigate.
Earth's magnetic field also helps shield life on Earth from harmful particles coming from space.
3. Does a Current-Carrying Wire Get Hot?
Observation: When electric current flows through a conductor, such as a nichrome wire, the wire becomes warm or even hot.
Reason: A conductor resists the flow of electric current. This resistance causes some electrical energy to change into heat energy.
Heating Effect of Electric Current: The warming up of a conductor due to the passage of electric current is called the heating effect of electric current.
Heating Effects in a Wire
What is Resistance?
Resistance is the property of a material that opposes the flow of electric current. Different materials offer different amounts of resistance:
Nichrome wire has high resistance and gets heated more than a copper wire of the same size.
The amount of heat produced depends on:
The type of material.
The thickness and length of the wire. Longer and thinner wires generally produce more heat.
The strength of the electric current. More cells in a suitable circuit can provide more current and therefore more heat.
The duration for which the current flows.
Everyday Examples of the Heating Effect of Electric Current
Applications: Many appliances work on the heating effect of electric current because they contain a wire element made of nichrome or a similar high-resistance material.
Household Electrical Appliances
Examples: Electric room heaters, electric stoves, irons, immersion rods, kettles, hair dryers and incandescent lamp filaments.
In some devices, the heating element becomes red-hot and may even glow.
Safety Measures
Wires and appliances can overheat if too much current flows through them. This may cause damage, melting or even fires.
Use wires, plugs and sockets rated for the correct amount of current. Safety devices should be installed in household wiring to prevent overheating.
Industrial Uses
High-temperature electric furnaces are used in industries to melt and recycle steel.
These furnaces rely on the heating effect produced when a large electric current flows through special heating coils.
Advantages and Disadvantages of the Heating Effect of Electric Current
Advantage: The heating effect is useful for many daily and industrial tasks.
Disadvantage: It can sometimes lead to energy loss and overheating of wires, which may be wasteful or dangerous.
4. How Does a Battery Generate Electricity?
What is an Electric Cell?
An electric cell is a device that converts chemical energy into electrical energy through a chemical reaction.
Cells have two terminals, positive and negative, which can be connected in a circuit to supply electric current.
Voltaic Cell or Galvanic Cell
A voltaic cell, also called a galvanic cell, is one of the earliest types of electric cell.
It has two different metal plates, called electrodes, partly immersed in an electrolyte.
An electrolyte is a liquid or paste that can conduct electricity through the movement of ions.
The chemical reaction between the metals and the electrolyte releases energy in the form of electricity.
In the external circuit, conventional electric current flows from the positive terminal to the negative terminal.
After some time, the materials inside the cell are used up. The cell then stops working and is called a dead cell.
Origin of the Voltaic Cell
Alessandro Volta and Luigi Galvani studied how electricity could be produced using metals and liquids.
Volta demonstrated that electricity was generated by the combination of different metals and an electrolyte, rather than by living things.
Making a Simple Cell: Lemon Battery Activity
You can make a simple electric cell using a lemon, a copper strip or wire and an iron nail.
Electric Circuit using Lemons
The lemon juice acts as an electrolyte.
Copper and iron act as the electrodes.
Connecting several lemon cells in series can light up an LED.
If the LED does not glow at first, reverse its connections because polarity matters.
Dry Cell
The dry cell is the most commonly used type of cell in daily life. It is used in torches, clocks and television remote controls.
a) Dry Cell b) It's internal Structure
It is called "dry" because its electrolyte is not a free-flowing liquid but a moist paste. Its main parts are:
Zinc container: It acts as the negative terminal.
Carbon rod: It acts as the positive terminal and is placed at the centre, surrounded by the paste.
A dry cell is usually single-use and is not rechargeable. Once it is exhausted, it must be disposed of properly.
Rechargeable Batteries
Rechargeable batteries can be recharged and used many times. They are used in phones, laptops, cameras and vehicles.
Rechargeable Batteries
They reduce waste and are more cost-efficient over time.
However, they wear out after many cycles of charging and use.
Lithium-ion batteries are the most common rechargeable batteries used today. They are found in many modern devices.
Scientists are working on solid-state batteries that may be safer, longer-lasting and more efficient in the future.
5. Points to Remember: Electricity-Magnetic and Heating Effects
Magnetic Effect of Electric Current: When electric current flows through a conductor, such as a wire, it produces a magnetic field around it. This is called the magnetic effect of electric current.
Electromagnet: A coil of wire carrying electric current behaves like a magnet and is called an electromagnet. Most practical electromagnets have an iron core to increase their strength.
Polarity: Electromagnets, like regular magnets, have two poles, North and South. The poles can be reversed by changing the direction of the current.
Lifting Electromagnet: Powerful electromagnets are used in industries to lift and move heavy metal objects. They can be switched on and off by controlling the flow of current.
Heating Effect of Electric Current: When electric current flows through a conductor, it generates heat because of the resistance of the material. This is known as the heating effect of electric current.
Heating Element: Devices such as electric heaters, irons and kettles have a coil or rod made of a high-resistance material, such as nichrome, that acts as a heating element.
Electric Cell and Battery: A cell or a battery generates electric current through chemical reactions occurring inside it. Cells have positive and negative terminals.
Dry Cell: A common type of cell used in daily life. It contains a moist paste as its electrolyte and is generally meant for single use.
Rechargeable Batteries: These batteries can be recharged and used multiple times, reducing waste. Examples include lithium-ion batteries used in phones and laptops.
Electrolyte and Electrode: An electrolyte is a substance that conducts electricity through the movement of ions. Electrodes are the terminals through which current enters or leaves a cell.
Resistance: Resistance is the opposition to the flow of electric current, and it can lead to heating in wires or devices.
Electric current can create both magnetism and heat. Devices such as electromagnets use the magnetic effect, while many household appliances use the heating effect. Batteries and cells are sources of electric current based on chemical reactions, and rechargeable batteries allow repeated use, making them useful in modern life.
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