NCERT Solutions: Electricity: Magnetic and Heating Effects

Probe and Ponder (Page 46)

Q1: If we don't have an electric lamp while making an electric circuit with an electric cell, is there any other way to find out if current is flowing in the circuit?
Ans: Yes. A simple way is to use a magnetic compass. Place the compass near the wire and close the circuit; if current flows, the needle will deflect, showing that there is a magnetic field around the wire produced by the current. Other practical indicators are devices such as a buzzer or an electric bell, which will sound when current passes, or an ammeter (if available) which gives a direct reading of current in the circuit.

Q2: Is it possible to make temporary magnets? How can these be made?
Ans: Yes. Temporary magnets, called electromagnets, can be made easily. Wrap an insulated conducting wire many times around an iron nail and connect the ends of the wire to a battery. When current flows through the coil, the nail becomes a magnet and can attract small magnetic objects. When the current is switched off, the nail loses most of its magnetism and stops acting like a magnet.

Q3: We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?
Ans: Electrical appliances produce heat because of the heating effect of electric current. When current passes through a conductor or a coil that has electrical resistance (for example, a nichrome wire), some electrical energy is converted into heat energy. This makes the wire or element hot. Appliances such as electric heaters, toasters, and irons use this effect to produce useful heat.

Q4: How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
Ans: A cell or battery is considered dead when it cannot provide enough current to operate a device (for example, it cannot light a bulb or run a motor). Simple checks are: try the cell in a known good device, or use a voltmeter. Not all cells are rechargeable. Most common dry cells (used in remotes) are single-use and cannot be recharged safely. Rechargeable batteries (for example, those used in mobile phones and laptops) are designed to be charged again using a suitable charger.

Q5: Share your questions __________
Ans: Questions are as follows:

• Why does reversing the battery terminals in a coil change the direction of the compass needle?
• What happens if we use longer wires to make an electromagnet?
• Which fruit or vegetable makes the strongest electric cell?
• How does a rechargeable battery work differently from a single-use battery?
• What materials are best for making heating elements in electric appliances?

InText (Page 49 - 56)

Q1: Can we use electric current to make a magnet? (Page No. 49)
Ans: Yes. The magnetic effect of electric current is used to make a magnet. When electric current is passed through a conducting wire coiled around an iron nail or rod, the nail or rod becomes a magnet while the current flows. This magnetism is temporary and disappears when the current stops.

Q2: Does an electromagnet also have two poles like a bar magnet? (Page No. 50)
Ans: Yes. An electromagnet has two poles like a bar magnet. When current flows through the coil around an iron nail, one end of the nail behaves as a north pole and the other end behaves as a south pole. This can be demonstrated by bringing the north pole of a compass near each end of the nail while the current flows and observing the needle's deflection. The fact that like poles repel and unlike poles attract shows the electromagnet has two poles.

Q3: Are electromagnets also used in real life, for lifting objects? (Page No. 52)
Ans: Yes. Electromagnets are widely used in factories and scrap yards to lift and move heavy magnetic objects. A large electromagnet is hung from a crane, moved over the metal items and switched ON. The strong magnetic field lifts the metal items. The crane then moves to the desired place and the electromagnet is switched OFF to release the items.

Crane

Q4: While doing the activity for the electromagnet, did you also notice that the wire ends got warm? Why would that happen? (Page No 52)

Ans: The wire ends become warm because of the heating effect of electric current. Conductors offer some resistance to current flow, and when current passes through them, part of the electrical energy is converted into heat. The heat makes the wire ends warm. In experiments, it is important to avoid touching wires that have been carrying current for some time.

Q5: Can we also make our Voltaic cell using easily available materials? (Page No. 56)
Ans: Yes. A simple Voltaic cell can be made from lemon pieces, iron nails and copper strips. Insert five iron nails and five copper strips, one pair in each lemon, and connect them in series by joining the copper of one lemon to the iron of the next. Connect the free iron nail to the negative side of an LED and the free copper strip of the last lemon to the positive side. If connections are correct and the lemons are fresh, the LED will glow dimly, showing the cell produces electricity.

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. The LED glowing indicates the Voltaic cell is producing a small current.

Keep the curiosity alive (Pages 58-61) 

Q1: Fill in the blanks: 
(i) The solution used in a Voltaic cell is called ________. 
(ii) A current carrying coil behaves like a _______ . 
Ans: 
(i) The solution used in a Voltaic cell is called electrolyte.
(ii) A current-carrying coil behaves like a magnet.
Q2: 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)
Ans:
(i) False 

Dry cells are more portable because they are compact and contain a paste electrolyte. They are easier to carry and use than a simple Voltaic cell made from liquids and separate plates.

(ii) True

A coil behaves as an electromagnet only while electric current flows through it; the magnetic effect exists during the current flow.

(iii) False 

Two cells in combination usually give a larger current or voltage than a single cell. A stronger current produces a stronger magnetic field in the electromagnet, which attracts more iron paper 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
Ans: (c) Both (i) and (ii) are correct 

When current flows through a nichrome wire:
(i) The wire gets warm - due to the heating effect of current.
(ii) The compass needle deflects - due to the magnetic effect of current. Thus both statements are true.

Q4: Match the items in Column A with those in Column B

Ans:

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.

Ans: (ii) generates more heat for a given current 

Nichrome is used in heating devices because it has high electrical resistance and can withstand high temperatures without melting. Therefore it produces more heat for the same current compared to low-resistance metals like copper.

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. 
Ans: Electric heating devices are often more convenient and better for society because:
(i) They require less storage space and are easier to keep in the house compared to storing firewood or charcoal.
(ii) They do not produce smoke, so they reduce indoor air pollution and related health problems such as eye irritation and breathing difficulties.
(iii) They do not emit soot and harmful gases directly in the home, which helps to keep the immediate environment cleaner and safer for people. Overall, electric heating reduces smoke-related health hazards and is neater and more convenient to use.

Q7: 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.

Ans: (i) The current flows from the positive terminal of the cell to the end marked A of the coil, then through the coil to end marked B, and then to the negative terminal of the cell as shown by the red arrows in the Figure.

(ii) The compass needle moves because the coil with current behaves like a magnet; it creates a magnetic field around it. The compass needle aligns with this new magnetic field and therefore deflects from its original direction.

(iii) If you reverse the battery terminals, the direction of current through the coil reverses. This reverses the magnetic field of the coil, so the compass needle will deflect in the opposite direction compared to before.

Q8: 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.
Ans: Possible reasons include:
(i) The battery may have become weak or nearly exhausted. A weak battery gives a much smaller current, which may be enough to produce some heat in the wire but not enough to produce a strong magnetic field required to lift the clips.
(ii) There may be a loose or poor connection in the circuit so that only a small current flows; the small current may heat the wire slightly but not magnetise the nail strongly.
(iii) If the iron nail became too hot from continuous use, its magnetic behaviour could be reduced, so it would attract fewer clips. In all cases, checking the battery condition and the circuit connections helps to identify and fix the problem.

Q9: In Fig. 4.11, in which case the LED will glow when the switch is closed?

Ans: The LED will glow when the switch is closed in case of (a). Here the lemon juice acts as an electrolyte between the copper and iron plates and allows a current to flow. In case (b), pure water (without any dissolved acid or salt) is a poor electrolyte, so it will not produce the required current to light the LED.
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?

Ans: Yes. The coiled wire alone will still produce a magnetic field when current flows and so will deflect the compass needle. However, the deflection will be less than before because the iron nail inside the coil concentrates and strengthens the magnetic field; removing the nail makes the electromagnet weaker.

Q11: 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.

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

Ans: Option (iv) In all four circuits.

The compass needles will show deflection in all four cases because a current through a coil always produces a magnetic field. The amount of deflection will differ: coils with iron cores (or magnetic materials) will produce stronger fields and larger deflections, while coils with non-magnetic cores or without cores may produce smaller deflections.

Discover, design, and debate (Page 61)

Q1: 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. 
Ans: Activity and Observations: Coils of 25, 50, 75 and 100 turns are each connected to the same cell in turn. A compass is placed at the same position near each coil and its deflection is observed. The compass needle deflects more as the number of turns increases.

Q2: 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. 
Ans: Part 1: Same Length, Different Thickness
Two nichrome wires of the same length but different thicknesses (for example, 0.3 mm and 0.6 mm) are connected one by one and current is passed for 30 s.

Result: The thinner wire (0.3 mm) becomes hotter. A thinner wire has higher resistance and therefore produces more heat for the same current.

Part 2: Same Thickness, Different Lengths

Two wires of the same thickness but different lengths (for example, 10 cm and 20 cm) are tested in the same way.

Result: The longer wire gets hotter because a longer wire has greater resistance than a shorter wire of the same thickness. Brief Report: Nichrome wires with higher resistance (either by being thinner or longer) heat up more when current flows. For safety, avoid touching wires immediately after current has passed through them.

Q3: Try to make an electric cell using various fruits and vegetables. Also try with electrodes of different metals. Prepare a brief report.
Ans: Activity: Use fruits like lemons, potatoes or tomatoes and insert two different metal strips (for example copper and zinc or copper and iron) into each fruit or vegetable. Connect several such fruit cells in series and attach an LED or a small bulb to test if electricity is produced.
Observations:

• With a copper and zinc combination in a lemon, the LED glows dimly.
• Connecting more fruits in series makes the LED glow brighter because the voltages add up.
• Lemons often work better than plain water because their juice is acidic and conducts ions well, acting as a good electrolyte.
• Using metals that differ more in their chemical activity (for example zinc and copper) gives a stronger cell.
• Plain water without dissolved salts or acid gives almost no electricity.

Brief Report: Fruits and vegetables can be used to make simple electric cells because their juices act as electrolytes. Different metal pairs produce different voltages; combinations such as zinc and copper are commonly effective. Multiple fruit cells connected in series increase the output so they can light an LED or a small bulb, but such cells are much weaker than commercial batteries.