Case Based Questions: Electricity: Magnetic and Heating Effects | Science Class 8 PDF Download

Case Study 1

During their visit to a science exhibition, Mohini and Aakarsh were introduced to the magnetic effect of electric current. They observed that when an electric current flows through a conductor, such as a wire, it creates a magnetic field around it. They used a magnetic compass to detect this field, which showed the deflection of the needle when the current was switched on. The exhibition also demonstrated electromagnets, which are created by coiling wire around an iron core and passing electric current through it. Mohini was particularly interested in how electromagnets can lift heavy metal objects when current is flowing, and how the strength of these electromagnets depends on factors like the number of wire turns and the strength of the current.

1. What happens when an electric current flows through a conductor, and how is this related to magnetism? (2 marks)
Solution:
When electric current flows through a conductor, it creates a magnetic field around the conductor. This is the magnetic effect of electric current, which is demonstrated when a compass needle is deflected by the magnetic field.

2. What is an electromagnet, and how does it differ from a permanent magnet? (2 marks)
Solution:
An electromagnet is a coil of wire with an iron core that becomes magnetic when an electric current flows through it. Unlike permanent magnets, the magnetism in an electromagnet disappears when the current is switched off.

3. How can electromagnets be used in industries for practical purposes? (3 marks)
Solution:
Electromagnets are used in industries to lift and move heavy metal objects. Cranes in scrap yards use electromagnets to pick up metal parts. When the current is turned on, the magnet attracts metals, and when the current is turned off, the metals are released.

4. Explain how electromagnets are created and how their strength can be controlled. (3 marks)
Solution:
Electromagnets are created by wrapping insulated wire around a soft iron core and passing electric current through the wire. The strength of the electromagnet can be controlled by increasing the number of turns in the wire coil, using a stronger current, or inserting a larger iron core, all of which intensify the magnetic field.

Case Study - 2

While experimenting in the lab, Aakarsh noticed that the wire in the electric heater he was testing became warm. He learned that this is due to the heating effect of electric current, which occurs when current flows through a conductor and some of the electrical energy is converted into heat energy. The amount of heat produced depends on the resistance of the wire, the thickness and length of the wire, and the strength of the electric current. Aakarsh was amazed to find that many household appliances like electric stoves, irons, and kettles use this heating effect to perform their functions.

1. What is the heating effect of electric current, and why does it happen? (2 marks)
Solution:
The heating effect of electric current occurs when electric current passes through a conductor, and the resistance of the material causes some electrical energy to be converted into heat. This happens because the flow of electrons in the conductor meets resistance, which produces heat.​

2. What factors affect the amount of heat produced in a conductor? (2 marks)
Solution:
The amount of heat produced in a conductor depends on the resistance of the material, the thickness and length of the wire, and the strength of the electric current. Longer, thinner wires with higher resistance produce more heat.

3. Describe how the heating effect of electric current is used in appliances like electric heaters and irons. (3 marks)
Solution:
In electric heaters and irons, a wire made of a high-resistance material like nichrome is heated by electric current. As the current flows through the wire, its high resistance causes it to heat up. The heat generated is then transferred to the surrounding air or objects, allowing the heater to warm a room or the iron to press clothes.

4. What are some safety measures that should be taken to prevent overheating of electrical appliances? (2 marks)
Solution:
To prevent overheating, electrical appliances should be used according to their power ratings, and the wires and plugs should be correctly rated for the current. It is also important to ensure that appliances are not left unattended and that safety devices like fuses or circuit breakers are installed.

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