Electricity: Magnetic and Heating Effects — Class 8 Science
Curiosity · Chapter 4
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- Space weighted by chapter size
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- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Magnetic effect of electric current
- The production of a magnetic field around a conductor when an electric current flows through it.
- Magnetic field
- The region around a magnet or current-carrying wire where its magnetic effect can be felt.
- Electromagnet
- A current-carrying coil that behaves as a magnet, usually made stronger with an iron core.
- Heating effect of electric current
- The generation of heat in a conductor due to its resistance when current flows through it.
- Resistance
- The opposition a conductor offers to the flow of electric current, causing heating.
- Electrolyte
- The liquid or moist paste in a cell that takes part in the chemical reaction producing electric current.
Extra questions & answers
What happens to a compass needle placed near a current-carrying wire?
The needle deflects when current flows and returns to its original direction when the current stops, showing the wire has a magnetic effect.
Who discovered the link between electricity and magnetism?
Hans Christian Oersted, who noticed a compass needle deflect whenever a nearby electric circuit was switched on or off.
What is an electromagnet?
A current-carrying coil that behaves like a magnet, usually with an iron core to make it stronger; it loses its magnetism when the current stops.
Name two ways to increase the strength of an electromagnet.
Increase the current by using more cells, or increase the number of turns in the coil.
How can the poles of an electromagnet be reversed?
By reversing the direction of the current flowing through the coil.
Why is nichrome wire used in electric heaters?
Nichrome has high resistance, so it generates more heat for a given current compared with a copper wire of the same size.
On which factors does the heat generated in a wire depend?
The magnitude of the current, and the material, thickness, length of the wire, and the time for which current flows.
How does a Voltaic cell produce electricity?
A chemical reaction between two different metal electrodes and an electrolyte produces an electric current.
Why is a dry cell called "dry"?
Because its electrolyte is a thick moist paste rather than a liquid.
Why should used batteries be disposed of as e-waste?
They contain acids and metals that can cause fires or harm the environment, and valuable materials in them can be recycled and reused.
Probe and ponder
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?
Yes. A magnetic compass placed near or under the wire can detect current. When current flows the wire produces a magnetic field and the compass needle deflects from its north-south direction. When current stops the needle returns. So deflection of the compass shows that current is flowing even without a lamp.
Is it possible to make temporary magnets? How can these be made?
Yes. A temporary magnet called an electromagnet can be made by winding insulated copper wire as a coil around an iron nail and passing electric current through the coil. The current-carrying coil with the iron core behaves like a magnet and attracts iron clips. When the current is switched OFF the magnetism disappears, so it is temporary.
We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?
In electrical appliances heat is generated by the heating effect of electric current. When current passes through a conductor it faces resistance. This resistance converts some electrical energy into heat energy and the conductor becomes warm or hot. Appliances like heaters, irons, kettles and stoves use a heating element made of high-resistance wire like nichrome to produce heat.
How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
A cell or battery is dead when it can no longer make a device work - for example the lamp does not glow, the electromagnet does not pick up clips, or the wire does not become warm and a voltmeter shows very low voltage. This happens because the chemicals inside have been used up. No, not all cells can be recharged. Dry cells with zinc container and carbon rod are single-use and must be disposed. Rechargeable batteries like those in phones, laptops and vehicles can be recharged many times, but even they wear out after many charge cycles.
Keep the curiosity alive
1. Fill in the blanks: (i) The solution used in a Voltaic cell is called ________.
The solution used in a Voltaic cell is called electrolyte. It is usually a weak acid or salt solution that conducts electricity and allows chemical reactions at the electrodes.
1. Fill in the blanks: (ii) A current carrying coil behaves like a _______ .
A current carrying coil behaves like a magnet. It is called an electromagnet and it shows two poles, attracts iron objects and deflects a compass needle while current flows.
2. Choose the correct option: (i) Dry cells are less portable compared to Voltaic cells. (True/False)
False. Dry cells are more portable and convenient than Voltaic cells. Voltaic cells contain liquid electrolyte in a glass container and are not easy to carry, while dry cells contain a thick moist paste and are compact and leak-proof.
2. Choose the correct option: (ii) A coil becomes an electromagnet only when electric current flows through it. (True/False)
True. The coil shows magnetic effect only while current is flowing. When the circuit is open or the cell is disconnected the magnetic field disappears and it no longer attracts clips or deflects a compass.
2. Choose the correct option: (iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
False. A battery of 2 cells gives larger current and produces a stronger magnetic field than a single cell. So the electromagnet with 2 cells attracts more clips and causes larger compass deflection for the same coil.
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
(c) Both (i) and (ii) are correct. Any conductor carrying current shows both effects at the same time: it gets heated due to the heating effect and it produces a magnetic field around it that deflects a compass, even if one effect is more noticeable than the other.
4. Match the items in Column A with those in Column B. (i) Voltaic cell
(d) chemical reactions - A Voltaic cell generates electricity due to chemical reactions between the two different metal electrodes and the electrolyte.
4. Match the items in Column A with those in Column B. (ii) Electric iron
(c) Works on heating effect of electric current - The heating element inside the iron becomes hot when current flows and is used to press clothes.
4. Match the items in Column A with those in Column B. (iii) Nichrome wire
(a) Best suited for electric heater - Nichrome offers high resistance and produces a lot of heat for a given current and does not melt easily, so it is used as heating element.
4. Match the items in Column A with those in Column B. (iv) Electromagnet
(b) Works on magnetic effect of electric current - A current-carrying coil with iron core acts as a magnet due to the magnetic field produced by the current.
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.
(ii) generates more heat for a given current. Nichrome has much higher resistance than copper of the same size, so more electrical energy is converted to heat. It also can withstand high temperature without melting or oxidising quickly.
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.
Electric heaters are cleaner and more convenient because they do not produce smoke, ash or soot inside the house, so they reduce indoor air pollution and health problems. They are easy to switch ON and OFF, temperature can be controlled quickly, and there is no need to collect, store and carry firewood. Societally this reduces deforestation, saves time especially for women and children, and lowers fire risk. However they need regular electricity supply.
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.
Current flows out from the positive terminal of the cell, through the connecting wire into one end of the coil, circulates around the coil and returns through the other connecting wire to the negative terminal of the cell, completing the circuit. On the diagram the arrow should go clockwise around the circuit following this loop.
7. Look at the Fig. 4.4a. If the compass placed near the coil deflects: (ii) Explain why the compass needle moves when current flows.
The compass needle is a small magnet. A current-carrying coil produces a magnetic field around it. This field exerts a force on the magnetic needle and makes it deflect from its original north-south direction. The region around the coil where this effect is felt is called the magnetic field.
7. Look at the Fig. 4.4a. If the compass placed near the coil deflects: (iii) Predict what would happen to the deflection if you reverse the battery terminals.
Reversing the battery reverses the direction of current and therefore reverses the polarity of the electromagnet. The north and south poles of the coil swap, so the compass needle will deflect in the opposite direction by about the same amount, if the current magnitude stays the same.
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.
The wire being warm shows current is still flowing and heating effect is present, but magnetism has become too weak to lift clips. Possible reasons are: the cell or battery has become dead or weak so current is now too small to create a strong magnetic field; the chemical energy is used up and voltage has dropped; the coil may have become partly loose or overheated increasing resistance; or the iron core has lost its strength while hot. A weak current still produces heat but not enough magnetic field to attract many clips.
9. In Fig. 4.12, in which case the LED will glow when the switch is closed? Fig. 4.12 (a) Iron nail, Lemon juice, Copper strip (b) Iron nail, Pure water, Copper strip
The LED will glow in case (a) with lemon juice. Lemon juice is an electrolyte containing acids and ions, so it allows chemical reactions between the iron nail and copper strip and completes the circuit. Pure water in case (b) is a very poor conductor and has almost no ions, so no useful current is produced and the LED will not glow.
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?
Yes, the hollow coil will still behave as an electromagnet and deflect the compass while current flows, because any current-carrying coil produces a magnetic field. However the deflection will be less than before. The iron nail acts as a core that strengthens the magnetic field, so without it the magnetism is weaker and the compass needle moves less and fewer clips are attracted.
11. We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.13 (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
(iv) In all four circuits. The magnetic effect of electric current is produced whenever current flows through a conductor, regardless of the metal. All four wires are conductors, so each current-carrying coil creates a magnetic field and deflects the compass, though the amount of deflection may differ because different resistances allow different amounts of current to flow.
Discover, design, and debate
Make coils of turns 25, 50, 75, and 100. Connect them to the same cell one by one. Note the deflection in a magnetic compass placed in the same position in all the cases. Report your observations. Draw conclusion of the effect of number of turns of the coil on the strength of the electromagnet.
Observation: With the same cell and same compass position, the compass deflection increases as the number of turns increases from 25 to 100, and more paper clips are picked up. Conclusion: For the same current, the strength of the electromagnet increases with the number of turns of the coil. More turns produce a stronger magnetic field.
Take two thin nichrome wires of equal length and different thickness (approximately one of these wire thickness to be double of the other, say 0.3 mm and 0.6 mm). Connect them one by one in a circuit which has a switch and a cell, and allow the current to flow for 30 s in each case. Momentarily touch these wires. Which wire heats up more? Now repeat the same activity with two nichrome wires of same diameter but of different lengths. Prepare a brief report of your activity.
Report: The thinner wire (0.3 mm) becomes noticeably warmer than the thicker wire (0.6 mm) when same length and same current duration are used, because thinner wire has higher resistance and converts more electrical energy to heat. When diameter is same but length differs, the longer wire has higher resistance and heats more overall, but heat per unit length may be similar. So for a given material and current, heating increases with length and decreases with thickness, and also increases with time and current.
Try to make an electric cell using various fruits and vegetables. Also try with electrodes of different metals. Prepare a brief report.
Report: Lemon, orange, potato and tomato can act as electrolytes. When a copper strip and an iron nail are inserted apart and connected to an LED, the LED glows faintly, showing a cell is formed. Using two different metals is essential; copper with iron or zinc gives higher voltage than iron with aluminium. Using two similar metals gives no glow. Joining several fruit cells in series increases voltage and makes the LED brighter. Vegetables with more juice and acidity generally give better results.