Class 8 Science Chapter 4 – Electricity: Magnetic and Heating Effects from NCERT Curiosity with easy Notes and complete Book Solutions. Get simple explanations, important concepts, questions and answers, and revision material for effective learning, practice and exam preparation.
Chapter 4: Electricity: Magnetic and Heating Effects
Class 8 Science – Curiosity
Probe and Ponder
z If we don’t have an electric lamp while making an electric circuit with an electric cell, is there any other way through which we can find out if current is flowing in the circuit?
z Is it possible to make temporary magnets? How can these be made?
z We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?
z How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
Short Notes and Easy Concepts
1. Magnetic Effect of Electric Current
When electric current flows through a wire, it produces a magnetic field around the wire. This is called the magnetic effect of electric current.
2. Compass and Electric Current
A compass needle is a tiny magnet. When current flows through a nearby wire, the magnetic field of the wire makes the compass needle deflect.
When the current stops, the magnetic effect disappears and the compass needle returns to its original direction.
3. Oersted’s Discovery
Hans Christian Oersted discovered in 1820 that electricity and magnetism are connected. He observed that a current-carrying wire could deflect a nearby compass needle.
4. Electromagnet
A current-carrying coil that behaves like a magnet is called an electromagnet.
An iron core makes an electromagnet stronger.
5. Strength of an Electromagnet
The strength of an electromagnet can be increased by:
- Increasing the electric current.
- Increasing the number of turns of the coil.
- Using an iron core.
Changing the direction of current can reverse the poles of the electromagnet.
6. Lifting Electromagnets
Strong electromagnets are used in cranes to lift iron and steel objects. When the current is switched ON, the magnet lifts the objects. When it is switched OFF, the objects are released.
Heating Effect of Electric Current
7. Heating Effect
When electric current passes through a conductor, the conductor offers resistance to the current. Some electrical energy changes into heat energy.
8. Nichrome Wire
Nichrome has higher resistance than copper. Therefore, it produces more heat when current passes through it.
It is commonly used in heating appliances.
9. Heating Appliances
- Electric heater
- Electric stove
- Electric kettle
- Electric iron
- Water heating immersion rod
- Hair dryer
- Material of the wire
- Thickness of the wire
- Length of the wire
- Amount of current
- Time for which current flows
How Does a Battery Generate Electricity?
10. Voltaic Cell
A Voltaic cell has two electrodes made of different materials and an electrolyte.
A chemical reaction between the electrodes and electrolyte produces electricity.
11. Dead Cell
As the chemicals inside a cell get used up, the cell stops supplying electricity. Such a cell is called a dead cell.
12. Lemon Cell
A lemon can be used to make a simple cell. Copper and iron can act as electrodes, while lemon juice acts as the electrolyte.
13. Dry Cell
A dry cell contains a thick moist paste as its electrolyte. Its zinc container acts as the negative terminal and the carbon rod acts as the positive terminal.
A dry cell is generally a single-use cell.
14. Rechargeable Battery
Rechargeable batteries can be charged and used many times. They are used in mobile phones, laptops, cameras, inverters and electric vehicles.
Snapshots – Quick Revision
- Electric current produces a magnetic field around a conductor.
- A current-carrying coil that behaves like a magnet is called an electromagnet.
- An iron core makes an electromagnet stronger.
- The heating of a conductor due to electric current is called the heating effect of electric current.
- A cell or battery generates electricity because of chemical reactions inside it.
- Rechargeable batteries can be recharged and reused many times.
Keep the Curiosity Alive – Book Questions and Solutions
1. Fill in the blanks:
(i) The solution used in a Voltaic cell is called ________.
(ii) A current carrying coil behaves like a _______ .
(i) electrolyte
(ii) magnet
2. Choose the correct option:
(i) Dry cells are less portable compared to Voltaic cells. (True/False)
(ii) A coil becomes an electromagnet only when electric current flows through it. (True/False)
(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
(i) False
(ii) True
(iii) False
3. An electric current flows through a nichrome wire for a short time.
(i) The wire becomes warm.
(ii) A magnetic compass placed below the wire is deflected.
Choose the correct option:
(a) Only (i) is correct
(b) Only (ii) is correct
(c) Both (i) and (ii) are correct
(d) Both (i) and (ii) are not correct
4. Match the items in Column A with those in Column B.
| Column A | Column B |
|---|---|
| (i) Voltaic cell | (a) Best suited for electric heater |
| (ii) Electric iron | (b) Works on magnetic effect of electric current |
| (iii) Nichrome wire | (c) Works on heating effect of electric current |
| (iv) Electromagnet | (d) chemical reactions |
(i) – (d)
(ii) – (c)
(iii) – (a)
(iv) – (b)
5. Nichrome wire is commonly used in electrical heating devices because it
(i) is a good conductor of electricity.
(ii) generates more heat for a given current.
(iii) is cheaper than copper.
(iv) is an insulator of electricity.
6. Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Give reason(s) to support this statement considering societal impact.
7. Look at the Fig. 4.4a. If the compass placed near the coil deflects: (i) Draw an arrow on the diagram to show the path of the electric current. (ii) Explain why the compass needle moves when current flows. (iii) Predict what would happen to the deflection if you reverse the battery terminals.
(i) The arrow should show the direction of current from the positive terminal of the battery through the coil towards the negative terminal.
(ii) The current-carrying coil produces a magnetic field, which affects the compass needle.
(iii) The direction of current will reverse, so the compass needle will deflect in the opposite direction.
8. Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position (in introduction story). After some time, the iron nail no longer picks up the iron paper clips, but the wire wrapped around the iron nail is still warm. Why did the lifting electromagnet stop lifting the clips? Give possible reasons.
9. In Fig. 4.12, in which case the LED will glow when the switch is closed?
10. Neha keeps the coil exactly the same as in Activity 4.4 but slides the iron nail out, leaving only the coiled wire. Will the coil still deflect the compass? If yes, will the deflection be more or less than before?
11. We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.13.
| Figure | Material |
|---|---|
| (a) | Iron |
| (b) | Copper |
| (c) | Aluminium |
| (d) | Nichrome |
When current is passed through the coils, compass needles placed near the coils will show deflection.
(i) Only in circuit (a)
(ii) Only in circuits (a) and (b)
(iii) Only in circuits (a), (b), and (c)
(iv) In all four circuits
All the coils are conductors, so electric current can flow through them and produce a magnetic field.
Additional Easy Concepts from the Chapter
Why does an electromagnet stop working when current stops?
The magnetic field of an electromagnet is produced by electric current. When current stops, the magnetic field disappears.
Why does an iron core make an electromagnet stronger?
The iron core strengthens the magnetic effect of the current-carrying coil.
Why can the poles of an electromagnet be reversed?
Reversing the direction of electric current reverses the magnetic poles of the electromagnet.
Why does a nichrome wire become hot?
Nichrome offers resistance to electric current. Electrical energy changes into heat energy when current passes through it.
Why are heating elements made from special wires?
Materials such as nichrome have relatively high resistance and can produce considerable heat when current passes through them.
Important Differences
| Electromagnet | Permanent Magnet |
|---|---|
| Works mainly when current flows. | Has magnetism without an electric current. |
| Can be switched ON and OFF. | Cannot normally be switched ON and OFF. |
| Its strength can be changed. | Its strength is generally fixed. |
| Dry Cell | Rechargeable Battery |
|---|---|
| Generally used once. | Can be charged and reused many times. |
| Contains a paste-like electrolyte. | Uses materials designed for repeated charging and discharging. |
| Commonly used in simple portable devices. | Commonly used in phones, laptops, cameras, inverters and vehicles. |
Book Questions – Short Answer Practice
Why?
When?
How long?
Where?
Why not?
Discover, Design, and Debate – Activity Ideas
1. Number of Turns and Electromagnet Strength
Make coils with 25, 50, 75 and 100 turns. Connect each to the same cell and observe the compass deflection. More turns generally produce a stronger magnetic effect under the same conditions.
2. Nichrome Wire Investigation
Compare nichrome wires of different thicknesses and lengths. Observe how much they heat after the same amount of time. Perform the activity only under teacher supervision.
3. Fruit and Vegetable Cells
Try making simple cells using fruits or vegetables and different metal electrodes. Observe whether an LED glows.
Safety Points
- Do not touch a heated nichrome wire for a long time.
- Carry out electrical experiments under teacher supervision.
- Do not keep a wire connected to a cell for unnecessarily long periods.
- Use suitable wires, plugs and sockets for electrical appliances.
- Do not throw used batteries into regular garbage.
- Used batteries should be sent to appropriate e-waste or battery recycling facilities.
Quick Revision – One Page
| Term | Easy Meaning |
|---|---|
| Magnetic effect of current | Current flowing through a conductor produces a magnetic field. |
| Magnetic field | Region where magnetic effect can be felt. |
| Electromagnet | A current-carrying coil that behaves like a magnet. |
| Heating effect | Production of heat when current flows through a conductor. |
| Nichrome | A high-resistance material commonly used in heating elements. |
| Voltaic cell | A cell in which chemical reactions produce electricity. |
| Dry cell | A cell containing a thick moist paste as electrolyte. |
| Rechargeable battery | A battery that can be charged and reused many times. |
Electricity can produce both magnetic and heating effects. These effects are used in many useful devices around us.
Prepare with this NCERT Curiosity worksheet for Chapter 4. The worksheet includes important questions and revision exercises to help students understand key concepts, practise what they have learned, and prepare effectively for class tests and exams.
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