NCERT Solutions: Chapter4 Electricity: Magnetic and Heating Effects (class 8 Science)
All 24 questions and solutions from Chapter 4 are fully pre-rendered below, covering Magnetic Field Detection, Electromagnets, Heating Effects of Nichrome Wire, Voltaic Cells, and Industrial Applications.
Yes, there are other ways to check this[cite: 9]. A simple method is to use a magnetic compass[cite: 9]. Place the magnetic compass near the wire and close the circuit[cite: 9]. If electric current flows through the circuit, the compass needle will deflect[cite: 9]. This deflection shows that there is a magnetic field around the wire produced by the electric current[cite: 9]. Other practical indicators are devices such as a buzzer or an electric bell[cite: 9]. These devices will produce a sound when current passes through them[cite: 9]. An ammeter can also be used, as it gives a direct reading of the current flowing in the circuit[cite: 9].
Yes, it is possible to make temporary magnets[cite: 9]. These temporary magnets are called electromagnets[cite: 9]. They can be made easily by wrapping an insulated conducting wire many times around an iron nail[cite: 9]. Then, connect the free ends of the wire to a battery[cite: 9]. It is observed that when electric current flows through the coil, the iron nail behaves like a magnet[cite: 9]. Therefore, the coil acts as an electromagnet[cite: 9]. When the current is switched off, the iron nail loses most of its magnetism and stops acting like a magnet[cite: 9].
Electrical appliances produce heat because of the heating effect of electric current[cite: 9]. When electric current passes through a conductor or a coil that has high electrical resistance, such as a nichrome wire, some of the electrical energy is converted into heat energy[cite: 9]. This conversion happens because of the resistance offered by the conductor[cite: 9]. This heat makes the wire or heating element very hot[cite: 9]. Electrical appliances such as electric heaters, toasters, and electric irons use this heating effect to produce useful heat for our daily needs[cite: 9].
A cell or battery is considered dead when it cannot provide enough electric current to operate a device[cite: 9]. For example, it cannot light a small bulb or run a toy motor[cite: 9]. We can check this by trying the cell in a device that we know works perfectly, or by using a device called a voltmeter[cite: 9].
Not all cells are rechargeable[cite: 9]. Most common dry cells, like the ones used in TV remotes, are single-use and cannot be recharged safely[cite: 9]. On the other hand, rechargeable batteries, such as those used in mobile phones and laptops, are specially designed to be charged again and again using a suitable charger[cite: 9].
Questions are as follows[cite: 9]:
- Why does reversing the battery terminals in a coil change the direction of the magnetic compass needle?[cite: 9]
- What happens if we use longer wires to make an electromagnet?[cite: 9]
- Which fruit or vegetable makes the strongest electric cell?[cite: 9]
- How does a rechargeable battery work differently from a single-use dry cell?[cite: 9]
- What materials are best for making heating elements in electric appliances?[cite: 9]
Yes, we can use electric current to make a magnet[cite: 9]. The magnetic effect of electric current is used for this purpose[cite: 9]. When electric current is passed through a conducting wire coiled around an iron nail or rod, the nail or rod becomes a magnet as long as the current flows[cite: 9]. This type of magnetism is temporary and disappears as soon as the electric current stops[cite: 9].
Yes, an electromagnet has two poles just like a normal bar magnet[cite: 9]. When electric current flows through the coil wrapped around an iron nail, one end of the nail behaves as a North Pole and the other end behaves as a South Pole[cite: 9]. This can be easily demonstrated by bringing the North Pole of a magnetic compass near each end of the nail while the current is flowing[cite: 9]. The compass needle will show deflection[cite: 9]. The basic rule that like poles repel and unlike poles attract proves that the electromagnet has two distinct poles[cite: 9].
Yes, electromagnets are widely used in factories and scrap yards to lift and move heavy magnetic objects[cite: 9]. A large electromagnet is hung from a crane, moved over the metal items, and switched on[cite: 9]. The strong magnetic field lifts the heavy metal objects easily[cite: 9]. The crane then moves to the desired location, and the electromagnet is switched off to release the items safely[cite: 9].
The wire ends become warm because of the heating effect of electric current[cite: 9]. All conductors offer some resistance to the flow of electric current[cite: 9]. When current passes through them, a part of the electrical energy is converted into heat energy[cite: 9]. This heat makes the ends of the wire feel warm[cite: 9]. During experiments, it is important to avoid touching wires that have been carrying electric current for a long time to prevent burns[cite: 9].
Yes, a simple Voltaic cell can be made using easily available materials such as lemon pieces, iron nails, and copper strips[cite: 9]. Insert five iron nails and five copper strips into the lemons, keeping one pair in each lemon piece[cite: 9]. Connect them in a series by joining the copper strip of one lemon to the iron nail of the next lemon[cite: 9]. Connect the free iron nail to the negative terminal of an LED and the free copper strip of the last lemon to the positive terminal[cite: 9]. If the connections are correct and the lemons are fresh, the LED will glow dimly[cite: 9]. This shows that the Voltaic cell is producing electricity[cite: 9].
The first iron nail is connected to the negative terminal of the LED and the last copper strip to the positive terminal of the LED[cite: 9]. The glowing of the LED indicates that the simple Voltaic cell is successfully producing a small electric current[cite: 9].
(ii) A current carrying coil behaves like a _______[cite: 9]
(i) The solution used in a Voltaic cell is called an electrolyte[cite: 9].
(ii) A current-carrying coil behaves like a magnet[cite: 9].
(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)[cite: 9]
(i) False[cite: 9]
Explanation: Dry cells are actually more portable because they are compact and contain a paste-like electrolyte instead of a liquid[cite: 9]. This makes them much easier to carry and use anywhere compared to a simple Voltaic cell, which requires liquid solutions and separate metal plates[cite: 9].
(ii) True[cite: 9]
Explanation: A coil behaves as an electromagnet only as long as electric current flows through it[cite: 9]. The magnetic effect exists only during the flow of current and disappears when the current stops[cite: 9].
(iii) False[cite: 9]
Explanation: Two cells combined together form a battery that provides a larger electric current than a single cell[cite: 9]. A stronger electric current produces a stronger magnetic field in the electromagnet[cite: 9]. As a result, it will attract more iron paper clips[cite: 9].
(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[cite: 9]
Correct Answer: (c) Both (i) and (ii) are correct[cite: 9]
Explanation: Both statements (i) and (ii) are correct[cite: 9]. When electric current flows through a nichrome wire, the wire gets warm due to the heating effect of electric current[cite: 9]. At the same time, the magnetic compass needle placed below the wire deflects due to the magnetic effect of electric current[cite: 9]. Therefore, both phenomena happen simultaneously[cite: 9].
Column A: (i) Voltaic cell, (ii) Electric iron, (iii) Nichrome wire, (iv) Electromagnet
Column B: (a) Best suited for electric heater, (b) Works on magnetic effect of electric current, (c) Works on heating effect of electric current, (d) Generates electricity by chemical reactions[cite: 9]
| Column A | Matched Column B | Working Principle |
|---|---|---|
| (i) Voltaic cell[cite: 9] | (d) Generates electricity by chemical reactions[cite: 9] | Chemical energy is converted into electrical energy using an electrolyte and electrodes[cite: 9]. |
| (ii) Electric iron[cite: 9] | (c) Works on heating effect of electric current[cite: 9] | Converts electrical energy into thermal heat to smooth out fabrics[cite: 9]. |
| (iii) Nichrome wire[cite: 9] | (a) Best suited for electric heater[cite: 9] | Offers high resistance and does not melt or oxidize easily at high temperatures[cite: 9]. |
| (iv) Electromagnet[cite: 9] | (b) Works on magnetic effect of electric current[cite: 9] | Acts as a temporary magnet only when an electric current flows through its coil[cite: 9]. |
(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.[cite: 9]
Correct Answer: (ii) generates more heat for a given current.[cite: 9]
Explanation: Nichrome wire is commonly used in electrical heating devices because it generates more heat for a given amount of current[cite: 9]. It has high electrical resistance and can easily withstand very high temperatures without melting[cite: 9]. Therefore, it produces a large amount of heat for the same current compared to low-resistance metals like copper[cite: 9].
Electric heating devices are often more convenient and beneficial for society due to the following reasons[cite: 9]:
- They require very little storage space and are much easier to keep inside the house compared to storing bulky firewood or charcoal[cite: 9].
- They do not produce any smoke[cite: 9]. This significantly reduces indoor air pollution and helps prevent health problems such as eye irritation and breathing difficulties[cite: 9].
- They do not emit soot or harmful gases inside the home[cite: 9]. This keeps the immediate environment cleaner and safer for everyone living there[cite: 9].
Overall, electric heating eliminates smoke-related health hazards and is very neat and convenient to use on a daily basis[cite: 9].
(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.[cite: 9]
- (i) Path of Electric Current: The electric current flows from the positive terminal of the cell to the end marked A of the coil[cite: 9]. It then travels through the coil to the end marked B, and finally returns to the negative terminal of the cell[cite: 9].
- (ii) Cause of Needle Deflection: The magnetic compass needle moves because the current-carrying coil behaves like a magnet[cite: 9]. It creates a magnetic field around itself[cite: 9]. The compass needle naturally aligns with this new magnetic field and therefore deflects from its original resting direction[cite: 9].
- (iii) Effect of Reversing Terminals: If we reverse the battery terminals, the direction of the electric current flowing through the coil is reversed[cite: 9]. This reverses the direction of the magnetic field of the coil[cite: 9]. As a result, the compass needle will deflect in the exact opposite direction compared to before[cite: 9].
The lifting electromagnet might have stopped lifting the clips due to the following possible reasons[cite: 9]:
- The battery may have become weak or completely exhausted[cite: 9]. A weak battery provides a much smaller electric current[cite: 9]. This small current may be enough to produce some heat in the wire, but it is not enough to create the strong magnetic field required to lift the iron clips[cite: 9].
- There could be a loose or poor connection in the circuit[cite: 9]. This allows only a small amount of current to flow, which slightly heats the wire but fails to strongly magnetise the iron nail[cite: 9].
- If the iron nail becomes too hot from continuous use over a long time, its magnetic strength can decrease[cite: 9]. As a result, it will attract fewer or no iron paper clips[cite: 9].
In all these cases, checking the condition of the battery and ensuring tight circuit connections will help identify and fix the problem[cite: 9].
The LED will glow when the switch is closed in case (a)[cite: 9].
Scientific Reason: In setup (a), the lemon juice contains citric acid which dissociates into free ions and acts as an electrolyte between the copper strip and iron nail[cite: 9]. This completes the circuit and allows an electric current to flow[cite: 9]. In setup (b), pure water is used[cite: 9]. Pure water, which does not contain any dissolved acids or salts, is a very poor electrolyte[cite: 9]. Therefore, it will not produce the required electric current to light up the LED[cite: 9].
Yes, the coiled wire alone will still produce a magnetic field when electric current flows through it[cite: 9]. Therefore, it will still deflect the magnetic compass needle[cite: 9].
However, the deflection of the needle will be much less than before[cite: 9]. This is because the iron nail placed inside the coil acts as a ferromagnetic core that helps to concentrate and strengthen the magnetic field[cite: 9]. Removing the iron nail makes the electromagnet weaker[cite: 9].
(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[cite: 9]
Correct Answer: (iv) In all four circuits[cite: 9]
Explanation: The compass needles will show deflection in all four circuits[cite: 9]. This is because an electric current flowing through any conducting coil always produces a magnetic field around it[cite: 9]. However, the amount of deflection will be different in each case[cite: 9]. Coils that have iron or magnetic materials will produce stronger magnetic fields and larger deflections[cite: 9]. On the other hand, coils with non-magnetic materials will produce smaller deflections[cite: 9].
Activity and Observations: Coils consisting of 25, 50, 75, and 100 turns are each connected to the same cell one after the other[cite: 9]. A magnetic compass is placed at the exact same position near each coil, and its deflection is carefully observed[cite: 9]. It is noticed that the compass needle deflects more and more as the number of turns in the coil increases[cite: 9].
| Number of Turns | Deflection of Compass Needle Observed |
|---|---|
| 25 turns[cite: 9] | Small deflection observed[cite: 9] |
| 50 turns[cite: 9] | More deflection than 25 turns[cite: 9] |
| 75 turns[cite: 9] | Even greater deflection[cite: 9] |
| 100 turns[cite: 9] | Maximum deflection[cite: 9] |
Conclusion: Increasing the number of turns in the coil increases the strength of the electromagnet[cite: 9]. This happens because more turns produce a stronger magnetic field for the same amount of electric current[cite: 9]. Therefore, the compass needle shows a greater deflection with a higher number of turns[cite: 9].
Part 1: Same Length, Different Thickness
Two nichrome wires of the exact same length but different thicknesses (0.3 mm and 0.6 mm) are connected one by one in a circuit[cite: 9]. Electric current is passed through them for 30 seconds[cite: 9].
| Wire Thickness | Observation After 30 Seconds |
|---|---|
| 0.3 mm (thin)[cite: 9] | Feels hotter to touch[cite: 9] |
| 0.6 mm (thick)[cite: 9] | Warms up less, not as hot as 0.3 mm[cite: 9] |
Result: The thinner wire (0.3 mm) becomes hotter[cite: 9]. A thinner wire has higher electrical resistance[cite: 9]. Because of this higher resistance, it produces more heat for the same amount of electric current[cite: 9].
Part 2: Same Thickness, Different Lengths
Two nichrome wires of the exact same thickness but different lengths (10 cm and 20 cm) are tested in the same way[cite: 9].
| Wire Length | Observation After 30 Seconds |
|---|---|
| 10 cm (short)[cite: 9] | Warms up[cite: 9] |
| 20 cm (long)[cite: 9] | Warms up, noticeably hotter than the shorter wire[cite: 9] |
Result: The longer wire gets hotter[cite: 9]. This happens because a longer wire offers greater electrical resistance than a shorter wire of the same thickness[cite: 9].
Activity: We can use easily available fruits like lemons, potatoes, or tomatoes for this activity[cite: 9]. Insert two different metal strips, such as copper and zinc or copper and iron, into each fruit or vegetable[cite: 9]. Connect several such fruit cells in a series and attach an LED or a small bulb to test if electricity is produced[cite: 9].
Observations:
- When a copper and zinc combination is used inside a lemon, the connected LED glows dimly[cite: 9].
- Connecting more fruit cells in a series makes the LED glow brighter because the total voltage adds up[cite: 9].
- Lemons usually work better than plain water because lemon juice is highly acidic[cite: 9]. It conducts ions well and acts as a very good electrolyte[cite: 9].
- Using metals that differ greatly in their chemical properties, such as zinc and copper, produces a stronger electric cell[cite: 9].
- Plain water without any dissolved salts or acids gives almost no electricity[cite: 9].