Electricity: Magnetic and Heating EffectsClass 8 Science NCERT Solutions
18 Solutions
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Solution 1 of 18
Q1Discover, 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.
Solution
Objective: To investigate the effect of the number of turns in a coil on the strength of an electromagnet.
Expected Observations:
When each coil is connected to the same cell and a magnetic compass is placed at a fixed distance, the following will be observed:
- The coil with 25 turns will cause a small deflection of the compass needle.
- The coil with 50 turns will cause a larger deflection than the 25-turn coil.
- The coil with 75 turns will cause an even larger deflection.
- The coil with 100 turns will cause the largest deflection of the compass needle.
The angle of deflection of the compass needle increases as the number of turns in the coil increases.
Conclusion:
The strength of an electromagnet is directly proportional to the number of turns of wire in its coil. Increasing the number of turns concentrates the magnetic field produced by the current, resulting in a stronger magnet.
Q2Discover, design, and debate
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.
Solution
Brief Report of Activity
Part 1: Effect of Thickness
- Procedure: Two nichrome wires of equal length but different thicknesses (0.3 mm and 0.6 mm) were connected to a cell for 30 seconds each.
- Principle: Electrical resistance is inversely proportional to the cross-sectional area of the wire. The thinner wire (0.3 mm) has a smaller area and therefore a higher resistance than the thicker wire (0.6 mm).
- Expected Observation: The thinner (0.3 mm) wire will heat up more and feel significantly warmer than the thicker wire.
- Reason: For a constant voltage source like a cell, the power (heat generated per second) is given by . Since the thinner wire has higher resistance (), it might seem it would heat up less. However, the total heat generated also depends on the current and the material properties. The high resistance of the thin wire leads to a more intense heating effect in the wire itself.
Part 2: Effect of Length
- Procedure: Two nichrome wires of the same thickness but different lengths were connected to the cell for 30 seconds each.
- Principle: Electrical resistance is directly proportional to the length of the wire. The longer wire has higher resistance than the shorter wire.
- Expected Observation: The shorter wire will heat up more than the longer wire.
- Reason: The shorter wire has lower resistance. According to Ohm's law (), the circuit with the shorter wire will draw a larger current from the cell. The heat generated is given by . The current () is squared, so its effect is more dominant. The larger current in the shorter wire will cause it to generate heat more rapidly.
Conclusion:
The heating effect of an electric current in a wire depends on its dimensions. For a given material and voltage:
- A thinner wire gets hotter than a thicker wire of the same length.
- A shorter wire gets hotter than a longer wire of the same thickness.
Q3Discover, design, and debate
Try to make an electric cell using various fruits and vegetables. Also try with electrodes of different metals. Prepare a brief report.
Solution
Brief Report of Activity
Objective: To construct simple Voltaic cells using various fruits/vegetables as electrolytes and different pairs of metals as electrodes.
Procedure:
- Various fruits and vegetables (e.g., lemon, potato, tomato, apple) were used as the source of electrolyte.
- Different pairs of metal electrodes (e.g., a copper strip and a zinc-coated (galvanized) nail; a copper strip and an iron nail) were inserted into the fruit/vegetable, ensuring they did not touch each other.
- A small LED was connected to the electrodes to check for the generation of electricity.
Expected Observations:
- Effect of Electrolyte: Most acidic fruits and vegetables will work. A lemon, being highly acidic, is expected to produce the brightest glow in the LED, indicating a stronger current. A potato will also work, likely producing a dimmer glow than the lemon.
- Effect of Electrodes: The combination of copper and zinc electrodes is expected to be the most effective, producing a higher voltage and a brighter LED glow compared to the copper and iron pair. This is because zinc and copper are farther apart in the electrochemical series than iron and copper.
- Multiple Cells: A single fruit cell may not be powerful enough to light an LED brightly. Connecting several fruit cells in series (copper of one to the zinc/iron of the next) will increase the total voltage and make the LED glow much brighter.
Conclusion:
A simple electric cell (Voltaic cell) can be successfully constructed using common fruits/vegetables and metals. The performance of the cell depends on the acidity of the fruit/vegetable (electrolyte) and the specific combination of metals used for the electrodes. The principle is the conversion of chemical energy into electrical energy.
Q1Keep the curiosity alive
Fill in the blanks:
(i)
The solution used in a Voltaic cell is called _____ .
(ii)
A current carrying coil behaves like a _____.
Solution
Answers:
(i)
The solution used in a Voltaic cell is called electrolyte.
(ii)
A current carrying coil behaves like a magnet (or electromagnet).
Q2Keep the curiosity alive
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)
Solution
Answers:
(i)
False
Reason: Dry cells are called 'dry' because their electrolyte is a moist paste, not a liquid. This makes them sealed, spill-proof, and much more convenient and portable for everyday use compared to Voltaic cells, which contain liquid electrolyte in an open container.
(ii)
True
Reason: The magnetic field is produced by the flow of electric current through the coil. When the current stops, the magnetic field disappears, and the coil ceases to be an electromagnet.
(iii)
False
Reason: A battery of 2 cells provides a larger current than a single cell. The strength of an electromagnet increases with the amount of current flowing through its coil. Therefore, the electromagnet will be stronger and attract more iron paper clips with a 2-cell battery.
Q3Keep the curiosity alive
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
Solution
Answer: (c) Both (i) and (ii) are correct
Explanation:
An electric current flowing through a conductor has two main effects described in the chapter:
- Heating Effect: Due to the resistance of the wire, electrical energy is converted into heat, causing the wire to become warm. This is statement (i).
- Magnetic Effect: A current-carrying wire produces a magnetic field around it. This magnetic field will interact with a nearby magnetic compass, causing its needle to deflect. This is statement (ii). Since both effects occur simultaneously when current flows, both statements are correct.
Q4Keep the curiosity alive
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
Solution
Answers:
- (i) Voltaic cell → (d) Generates electricity by chemical reactions
- A Voltaic cell converts chemical energy into electrical energy through reactions between electrodes and an electrolyte.
- (ii) Electric iron → (c) Works on heating effect of electric current
- An electric iron uses a heating element to get hot and press clothes.
- (iii) Nichrome wire → (a) Best suited for electric heater
- Nichrome has high resistance, making it an ideal material for the heating element in electric heaters.
- (iv) Electromagnet → (b) Works on magnetic effect of electric current
- An electromagnet is a temporary magnet created by the flow of electric current through a coil.
Q5Keep the curiosity alive
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.
Solution
The question implies selecting the best reason. Although multiple factors can influence material choice, the primary scientific reason is related to its heating properties.
Best Reason: (ii) generates more heat for a given current.
Explanation:
- Nichrome wire has a significantly higher electrical resistance compared to excellent conductors like copper.
- The amount of heat generated in a wire is proportional to its resistance (Heat = ). Because of its high resistance, nichrome converts electrical energy into heat more effectively than a low-resistance wire like copper would for the same current.
- While it is a conductor (ruling out (iv)), it is not as good a conductor as copper (making (i) less relevant as the primary reason). Its effectiveness in generating heat is the key property for its use in heating devices.
Q6Keep the curiosity alive
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.
Solution
Electric heating devices are considered more convenient and have a positive societal impact compared to traditional methods for several reasons:
-
Health and Air Quality: Electric devices do not produce smoke, soot, or harmful gases (like carbon monoxide) at the point of use. This leads to better indoor air quality, reducing the risk of respiratory illnesses. Burning wood or charcoal releases pollutants that are harmful to health.
-
Environmental Conservation: Using electricity for heating reduces the demand for firewood, which helps in preventing deforestation and conserving forests. This is crucial for maintaining ecological balance and biodiversity.
-
Safety: Electric appliances, when used correctly, are generally safer than open flames. They reduce the risk of accidental fires in homes. Traditional methods carry a constant risk of fire and burns.
-
Convenience and Control: Electric devices are easy to operate with the simple flick of a switch. They provide instant heat and allow for precise temperature control, which is not possible with burning wood or charcoal.
-
Cleanliness: Electric heating is clean and does not produce ash or soot, making homes easier to maintain.
Q7Keep the curiosity alive
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.
Solution
This solution describes the setup and answers the questions without referring to the figure directly.
Setup Description: An electromagnet is constructed by connecting a coil of wire to a cell. A magnetic compass is placed near one end of the coil.
(i) Path of the electric current:
The electric current flows from the positive terminal of the cell, through the connecting wires and the coil, and then back to the negative terminal of the cell. The direction of current is conventionally taken as from positive to negative.
(ii) Explanation for compass needle movement:
When an electric current flows through the coil of wire, it generates a magnetic field in the region around the coil. This phenomenon is known as the magnetic effect of electric current. The coil effectively becomes an electromagnet. Since the compass needle is itself a tiny magnet, it is affected by this magnetic field. The force exerted by the electromagnet's magnetic field on the compass needle causes it to move and align itself with the direction of the field lines.
(iii) Prediction for reversing battery terminals:
If the battery terminals are reversed, the direction of the electric current flowing through the coil will also be reversed. Reversing the direction of the current reverses the polarity (the North and South poles) of the electromagnet. Consequently, the magnetic field produced by the coil will point in the opposite direction. This will cause the compass needle to deflect in the opposite direction compared to its original deflection.
Q8Keep the curiosity alive
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.
Solution
Reason: The lifting electromagnet stopped lifting the clips because the electric cell (battery) powering it became 'dead' or depleted.
Explanation:
- When the switch is left in the 'ON' position, a continuous electric current flows from the cell through the coil of the electromagnet.
- An electric cell stores a finite amount of chemical energy. This energy is converted into electrical energy to produce the current. Over time, the chemical reactions inside the cell slow down and eventually stop as the chemicals are consumed.
- As the cell weakens, the current it can supply decreases significantly. The strength of an electromagnet depends directly on the current flowing through its coil. When the current becomes too weak, the magnetic field produced is no longer strong enough to overcome the force of gravity on the iron paper clips, so it cannot pick them up.
- The wire is still warm because even a very weak current flowing through it will generate some heat due to the wire's resistance (the heating effect of current). This indicates that the cell is not completely exhausted but is too weak to produce a useful magnetic effect.
Q9Keep the curiosity alive
In Fig. 4.11, in which case the LED will glow when the switch is closed?
Solution
This solution describes the setup and provides the answer without referring to the figure directly.
Setup Description: Two circuits, labeled (a) and (b), are shown. Each contains a cell, a switch, and an LED (Light Emitting Diode). An LED has two terminals: a longer one (positive, or anode) and a shorter one (negative, or cathode). In circuit (a), the longer terminal of the LED is connected towards the positive terminal of the cell. In circuit (b), the longer terminal of the LED is connected towards the negative terminal of the cell.
Answer: The LED will glow only in case (a).
Reason:
An LED is a type of diode, which is a semiconductor device that allows electric current to flow through it in only one direction. This is known as forward bias. For an LED to light up, its positive terminal (the longer wire) must be connected to the positive terminal of the battery, and its negative terminal (the shorter wire) must be connected to the negative terminal of the battery.
- In circuit (a), the connections are correct for forward bias, so current will flow, and the LED will glow.
- In circuit (b), the connections are reversed (reverse bias). This blocks the flow of current, and therefore the LED will not glow.
Q10Keep the curiosity alive
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?
Solution
Answer:
Yes, the coil will still deflect the compass.
The deflection will be less than before.
Explanation:
A coil of wire with an electric current flowing through it is called a solenoid, and it acts as an electromagnet on its own, producing a magnetic field. This magnetic field is strong enough to deflect a nearby compass needle. Therefore, even without the iron nail, the coil will function as a magnet.
However, when an iron nail (a ferromagnetic material) is placed inside the coil, it becomes strongly magnetized by the coil's magnetic field. The magnetic field of the iron core adds to the magnetic field of the coil, making the overall electromagnet much stronger. Since the electromagnet is weaker without the iron nail, the magnetic force it exerts on the compass needle will be weaker, resulting in a smaller or less pronounced deflection.
Q11Keep the curiosity alive
We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.12. 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
Solution
Answer: (iv) In all four circuits
Explanation:
The magnetic effect of electric current is a fundamental property that occurs whenever current flows through an electrical conductor. Iron, copper, aluminium, and nichrome are all materials that conduct electricity, although with varying degrees of resistance.
When the switch is closed in each of the four circuits, an electric current will flow through the respective coil. In each case, this current will produce a magnetic field around the coil. This magnetic field will then cause the nearby compass needle to deflect.
Therefore, the deflection of the compass needle will be observed in all four circuits, as all four materials are conductors.
Q1Probe 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?
Solution
Yes, there are other ways to detect the flow of current in a circuit. Based on the effects of electric current, we can use the following methods:
-
Magnetic Effect: Place a magnetic compass near one of the wires in the circuit. If the compass needle deflects from its usual North-South alignment, it indicates that a magnetic field has been created around the wire, which confirms that current is flowing. When the circuit is opened (switched off), the needle will return to its original position.
-
Heating Effect: If the wire in the circuit is made of a material with some resistance (like a nichrome wire), you can carefully touch the wire after the circuit has been closed for a short time. If the wire feels warm, it indicates that electrical energy is being converted into heat energy, which confirms the flow of current. (Caution: This should be done carefully as the wire can get very hot).
Q2Probe and ponder
Is it possible to make temporary magnets? How can these be made?
Solution
Yes, it is possible to make temporary magnets. These are called electromagnets.
An electromagnet can be made by following these steps:
- Take a piece of soft iron, such as an iron nail. This will act as the core.
- Take a long piece of insulated copper wire.
- Tightly wrap the insulated wire around the iron nail to form a coil.
- Connect the two ends of the wire to the terminals of an electric cell or a battery, possibly through a switch.
When the electric current is switched on, it flows through the coil, and the iron nail becomes a magnet. It can attract magnetic materials like iron paper clips. This magnetism is temporary because as soon as the current is switched off, the iron nail loses most of its magnetism and stops acting like a magnet.
Q3Probe and ponder
We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?
Solution
Heat is generated in various electrical appliances due to the heating effect of electric current.
Here is how it works:
- When an electric current flows through a conductor (like a wire), the moving electrons collide with the atoms of the conductor.
- This flow of electrons is not perfectly smooth; the conductor offers some opposition or resistance to the flow of current.
- Due to this resistance, some of the electrical energy is converted into heat energy. This causes the conductor to heat up.
Electrical appliances designed for heating, such as electric heaters, irons, stoves, and water heaters, use a special coil of wire called a heating element. This element is typically made of a material like nichrome, which has a high resistance. When current passes through this high-resistance element, a large amount of heat is generated, which is then used for the appliance's intended purpose.
Q4Probe and ponder
How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
Solution
How to know if a cell is dead:
A cell or a battery is considered 'dead' when it can no longer provide a sufficient electric current to power a device. This happens because the chemical reactions inside the cell that generate electricity have slowed down or stopped as the chemicals get used up. We can know a cell is dead if a device that was working correctly (like a torch or a remote control) stops working when that cell is used to power it.
Rechargeability of cells:
No, not all cells and batteries can be recharged. There are two main types:
- Primary Cells (Non-rechargeable): These cells are designed for single use. The chemical reactions inside them are irreversible. Once the chemicals are depleted, the cell is 'dead' and must be disposed of. Common examples include the standard dry cells (like AA, AAA) used in toys and remote controls.
- Secondary Cells (Rechargeable): These batteries are designed to be reused multiple times. The chemical reactions are reversible. By passing an electric current through them in the opposite direction (charging), the original chemical composition can be restored. Examples include lithium-ion (Li-ion) batteries used in mobile phones, laptops, and electric vehicles, and lead-acid batteries used in cars and inverters.