Electricity: Magnetic and Heating Effects - Questions and Answers
Questions from Inside the Chapter
Page 1 - Opening Questions
Q1: 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?
Answer: Yes, we can use a magnetic compass. When electric current flows through a wire, it produces a magnetic effect that deflects the compass needle from its original position.
Q2: Is it possible to make temporary magnets? How can these be made?
Answer: Yes, temporary magnets called electromagnets can be made. They are made by wrapping an insulated wire around an iron nail and connecting the ends of the wire to a battery. When current flows through the coil, the nail becomes a magnet. When the current stops, the magnetic effect disappears.
Q3: How is heat generated in various electrical appliances?
Answer: When electric current flows through a conductor, it faces resistance to its flow. This resistance causes some electrical energy to be converted into heat energy. This is known as the heating effect of electric current.
Q4: How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
Answer: A cell or battery is dead when it stops working and cannot supply electricity. Not all cells and batteries can be recharged - dry cells are single-use and cannot be recharged, while rechargeable batteries can be recharged and reused multiple times.
Page 2 - Activity 4.1 Investigation
Q5: What happens when the switch is moved to 'ON' position in Activity 4.1?
Answer: When the switch is moved to 'ON' position, electric current flows through the wire and the compass needle gets deflected from its original direction.
Q6: What happens when the switch is moved to 'OFF' position?
Answer: When the switch is moved to 'OFF' position, current stops flowing and the compass needle returns to its original direction.
Q7: Why does the compass needle deflect when current flows through the wire?
Answer: The compass needle deflects because the current-carrying wire produces a magnetic field around it. The compass needle is a tiny magnet that gets deflected when brought near a magnetic field.
Page 3 - Oersted's Discovery
Q8: Who discovered the link between electricity and magnetism?
Answer: Hans Christian Oersted (1777-1851), a Danish professor, discovered in 1820 that electricity and magnetism are linked when he noticed that a compass needle deflected whenever an electrical circuit was closed or opened.
Page 4 - Activity 4.2
Q9: What happens when a coil of wire wrapped around an iron nail is connected to a cell?
Answer: When connected to a cell, electric current flows through the coil and the nail behaves like a magnet, attracting iron paper clips. When the current is stopped, the clips fall down.
Page 5 - Activity 4.3
Q10: What happens when an iron nail is inserted in the core of a current-carrying coil?
Answer: When an iron nail is inserted in the core of the coil, the coil becomes a stronger magnet, the deflection of the magnetic compass needle is much more, and it attracts more iron clips.
Q11: What is an electromagnet?
Answer: A current-carrying coil that behaves as a magnet is called an electromagnet. For practical applications, most electromagnets have an iron core to make them stronger.
Q12: Does an electromagnet have poles like a bar magnet?
Answer: Yes, just like a bar magnet, an electromagnet also has two poles - North and South.
Page 6 - Think Like a Scientist
Q13: What happens when you increase the number of cells in the circuit?
Answer: A battery with more cells gives a larger current, creating a stronger magnetic field. The deflection of the compass needle is more and the coil can attract more clips.
Q14: What happens when you increase the number of turns of the coil?
Answer: The increase in number of turns of the coil makes the coil a stronger magnet.
Q15: Can the poles of an electromagnet be reversed?
Answer: Yes, the poles of an electromagnet can be reversed by changing the direction of the current.
Q16: Why does Earth behave like a magnet?
Answer: Deep inside the Earth, the movement of liquid iron in the core creates electric currents, which generate a magnetic field.
Page 7 - Activity 4.5
Q17: Why do the wire ends get warm when current passes through them?
Answer: When electric current flows through a conductor, it faces resistance to its flow. This resistance causes some electrical energy to be converted into heat energy, making the wire warm.
Q18: What is the heating effect of electric current?
Answer: The heating effect of electric current is the phenomenon where an electric current passing through a conductor causes it to get heated due to resistance.
Q19: Why does nichrome wire heat up more than copper wire?
Answer: Nichrome wire offers higher resistance compared to a copper wire of the same size and length, so it generates more heat when current passes through it.
Page 8 - Think Like a Scientist
Q20: What happens when you use 2 cells instead of 1 cell in the heating experiment?
Answer: The amount of heat generated is more with 2 cells because the heat generated depends on the magnitude of the electric current.
Q21: What factors determine the heat generated in a wire?
Answer: The heat generated in a wire depends on the material, thickness, length of the wire, and the duration for which the current flows.
Page 10 - Voltaic Cell
Q22: What is a Voltaic cell?
Answer: A Voltaic cell contains two metal rods (electrodes) made of different materials and a liquid called an electrolyte. A chemical reaction between the rods and the electrolyte produces electricity.
Q23: Why does a Voltaic cell stop working over time?
Answer: Over time, the chemicals get used up through chemical reactions, and the cell stops working.
Q24: What is the electrolyte in a lemon cell?
Answer: In a lemon cell, the lemon juice acts as the electrolyte, which helps conduct electricity.
Page 11 - Lemon Cell Activity
Q25: How can you make a cell using lemons?
Answer: Insert a copper wire and an iron nail in each lemon, keeping them apart. Connect all lemons in series by joining copper wires and nails alternately. Connect an LED between the first copper wire and the last iron nail. If the LED glows, the cell is working.
Q26: What is the role of the LED in the lemon cell activity?
Answer: A glowing LED indicates that your cell is working. The LED glows only when its positive terminal (longer wire) is connected to the positive terminal of the battery.
Page 12 - Dry Cells and Rechargeable Batteries
Q27: What is a dry cell?
Answer: A dry cell is a widely used electric cell where the electrolyte is not a liquid but a thick moist paste. It consists of a zinc container (negative terminal) and a carbon rod (positive terminal).
Q28: What is a rechargeable battery?
Answer: Rechargeable batteries can be recharged and reused multiple times. This prevents wastage and saves money over time.
Q29: Can rechargeable batteries last forever?
Answer: No, rechargeable batteries do not last forever. After being charged and used many times, they slowly wear out.
Exercises Questions (Page 13-15)
Q1: Fill in the blanks
(i) The solution used in a Voltaic cell is called ________.
Answer: electrolyte
(ii) A current carrying coil behaves like a ________.
Answer: magnet/electromagnet
Q2: Choose the correct option
(i) Dry cells are less portable compared to Voltaic cells. (True/False)
Answer: False. Dry cells are actually more portable and convenient for everyday use.
(ii) A coil becomes an electromagnet only when electric current flows through it. (True/False)
Answer: True
(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
Answer: False. A battery with more cells creates a stronger magnetic field and attracts more clips.
Q3: 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
Answer: (c) Both (i) and (ii) are correct
Q4: 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) Generates electricity by chemical reactions |
Answers:
(i) → (d) Generates electricity by chemical reactions
(ii) → (c) Works on heating effect of electric current
(iii) → (a) Best suited for electric heater
(iv) → (b) Works on magnetic effect of electric current
Q5: 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.
Answer: (ii) generates more heat for a given current.
Q6: 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.
Answer: Electric heating devices are more convenient because:
They are cleaner and produce no smoke or harmful gases
They are safer with proper safety devices and don't cause indoor air pollution
They are more efficient and can be controlled precisely
They don't require storage of fuel like firewood or charcoal
They reduce deforestation and associated environmental damage
They are easier to use and maintain
Q7: Look at 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.
Answer: Current flows from the positive terminal of the cell through the coil to the negative terminal.
(ii) Explain why the compass needle moves when current flows.
Answer: When current flows through the coil, it produces a magnetic field around it. The compass needle, being a tiny magnet, gets deflected due to this magnetic field.
(iii) Predict what would happen to the deflection if you reverse the battery terminals.
Answer: The compass needle would deflect in the opposite direction because the direction of the magnetic field reverses when the direction of current is reversed.
Q8: Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position (in the 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.
Answer: The lifting electromagnet stopped lifting the clips because:
The cell/battery got used up and became dead, so no current flowed through the circuit
The cell may have weakened due to continuous use and couldn't provide enough current to generate a strong magnetic field
The wire heating indicates that current was flowing and energy was being converted to heat, which may have drained the battery faster
Q9: In Fig. 4.12, in which case the LED will glow when the switch is closed?
Answer: The LED will glow only when its positive terminal (longer wire) is connected to the positive terminal of the battery and the negative terminal (shorter wire) is connected to the negative terminal of the battery. (Note: Without the specific figure, the correct connection is the one where LED is properly oriented with respect to battery polarity.)
Q10: 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?
Answer: Yes, the coil will still deflect the compass because a current-carrying coil itself behaves as a magnet (electromagnet). However, the deflection will be less than before because the iron nail acts as a core that makes the electromagnet stronger.
Q11: We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.13.
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
Answer: (iv) In all four circuits
Explanation: When current passes through any conductor (whether iron, copper, aluminium, or nichrome), it produces a magnetic field around it. This magnetic effect of electric current occurs in all conductors, regardless of the material. The compass needle will deflect in all four cases.
Discover, Design, and Debate - Activity Questions
Activity 1: Effect of Number of Turns
Q: 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 cases. Report your observations and draw conclusion.
Answer: The deflection of the compass needle increases as the number of turns increases. This shows that the strength of an electromagnet increases with the number of turns of the coil. More turns create a stronger magnetic field because the effects of each turn add up.
Activity 2: Heating Effect with Different Thickness and Length
Q: Take two thin nichrome wires of equal length and different thickness (0.3mm and 0.6mm). Connect them one by one in a circuit and allow current to flow for 30s. Which wire heats up more?
Answer: The thinner wire (0.3mm) heats up more because thinner wires offer higher resistance to current flow, converting more electrical energy into heat.
Q: Repeat with two nichrome wires of same diameter but different lengths. Which wire heats up more?
Answer: The longer wire heats up more because longer wires have higher resistance, generating more heat when current flows through them.
Activity 3: Electric Cell Using Fruits and Vegetables
Q: Try to make an electric cell using various fruits and vegetables with electrodes of different metals. Prepare a brief report.
Answer: Different fruits and vegetables can act as electrolytes. Using copper and zinc electrodes, a cell can be created. The acidity and moisture content affect the voltage produced. Citrus fruits like lemons and oranges work well. Different metal pairs produce different voltages based on their chemical properties.
0 Comments