Chap 4 Electricity: Magnetic and Heating Effects

Probe and Ponder

1. 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, there is another way. We can use a magnetic compass. If you place a magnetic compass near a wire in the circuit, its needle will move or deflect when the electric current is flowing. When the current stops, the needle will go back to its original position.

2. Is it possible to make temporary magnets? How can these be made?

Yes, it’s possible to make temporary magnets called electromagnets. You can make one by tightly wrapping an insulated wire around an iron nail. When you connect the ends of the wire to an electric cell, the nail becomes a magnet and can pick up things like paper clips. It stops being a magnet as soon as the electric current is turned off.

3. We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?

Heat is generated in electrical appliances because of the heating effect of electric current. When electricity flows through a wire, it faces some resistance. This resistance converts some of the electrical energy into heat energy, making the wire warm. Appliances like heaters and electric irons have a special part called a heating element that is designed to get very hot when current passes through it.

4. How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?

We know a cell or battery is dead when it can no longer produce electricity to make our devices work. This happens because the chemicals inside it have been used up.

No, not all cells and batteries can be recharged. Single-use cells, like dry cells, must be thrown away after they are used up. However, some batteries are specially designed to be rechargeable. These can be used many times by charging them again.

In-Text and Activity Questions

Activity 4.1: Let us investigate

1. While watching the compass needle, move the switch to ‘ON’ position to allow electric current to flow through the wire. What do you observe?

When the switch is moved to the ‘ON’ position, the electric current starts flowing through the wire. You will observe that the needle of the magnetic compass placed near the wire gets deflected, meaning it moves from its original North-South direction.

2. Now again while watching the compass needle, move the switch to ‘OFF’ position. What do you observe this time?

When the switch is moved back to the ‘OFF’ position, the electric current stops flowing. You will observe that the compass needle returns to its original North-South direction.

3. But why does the compass needle deflect when the current flows through the wire?

The compass needle deflects because an electric current flowing through a wire creates a magnetic field around it. A compass needle is a tiny magnet, and it moves when it feels the magnetic effect of the current-carrying wire.

Activity 4.2: Let us explore

1. Can we use electric current to make a magnet?

Yes, we can use electric current to make a temporary magnet called an electromagnet.

2. Bring the nail close to the iron paper clips and lift up. Do the clips hang to the ends of the nail?

Yes, when the electric current is flowing through the wire wrapped around the nail, the paper clips will be attracted to the nail and hang from it, just like they would with a regular magnet.

3. Disconnect the wire from the cell to stop the flow of electric current in the wire. Do the clips fall down?

Yes, as soon as the wire is disconnected from the cell, the electric current stops. The nail loses its magnetism, and the iron paper clips will fall down.

Activity 4.3: Let us experiment

1. Connect the two ends of the coil with the terminals of the cell and observe the magnetic compasses. Do you find any deflection in the needles of the compasses?

Yes, when the coil is connected to the cell, the compass needles show a deflection. This shows that the current flowing through the coil has created a magnetic field.

2. Disconnect the wire from the cell. Do the needles of the compasses come back to their original positions?

Yes, when the wire is disconnected, the current stops, and the magnetic field disappears. The compass needles will come back to their original positions.

3. Insert an iron nail in the paper cylinder and repeat the steps. Is there any difference in the deflection of the compass needles?

Yes, there is a difference. When the iron nail is placed inside the coil, the deflection of the compass needles is much more than before. This means the iron nail makes the electromagnet much stronger.

4. Are the clips attracted to the ends of the nail?

Yes, when the current is flowing through the coil with the iron nail inside, the iron paper clips are attracted to the ends of the nail.

5. Does an electromagnet also have two poles like a bar magnet?

Yes, just like a regular bar magnet, an electromagnet also has two poles: a North pole and a South pole.

Activity 4.4: Let us investigate

1. Repeat this procedure to find the polarity of end B as well. Did you find that the polarity of end B is opposite to the polarity of end A?

Yes. If end A of the electromagnet is the South pole, then end B will be the North pole. An electromagnet always has two opposite poles, just like a permanent magnet.

Think like a scientist

1. Repeat Activity 4.3 with- (i) 2 and 4 cells with the same coil, (ii) 2 cells but different number of turns of the coil. What do you observe?

(i) When we use more cells (like 2 or 4 cells instead of 1), the electric current becomes stronger. This creates a stronger magnetic field. You will observe that the deflection of the compass needle is greater, and the electromagnet can pick up more paper clips.

(ii) When we increase the number of turns in the coil, the electromagnet also becomes stronger, even with the same number of cells. This will also cause a greater deflection in the compass needle and allow it to attract more clips.

2. What happens when you repeat Activity 4.4 by changing the direction of the current?

When you change the direction of the current (by reversing the connections on the battery), the poles of the electromagnet will also reverse. The end that was the North pole will become the South pole, and the end that was the South pole will become the North pole.

Activity 4.5: Let us observe

1. While doing the activity for the electromagnet, did you also notice that the wire ends got warm? Why would that happen?

The wire ends got warm because of the heating effect of electric current. When electricity flows through a wire, some of the electrical energy is converted into heat energy, which makes the wire feel warm.

2. Touch the nichrome wire. What do you feel?

Before the current is passed through it, the nichrome wire feels cool, at room temperature.

3. Move the switch to ON position for about 30 s and then move it back to OFF. Touch the nichrome wire momentarily. What difference do you feel?

After the current has passed through the nichrome wire for about 30 seconds, it will feel warm or hot to the touch.

Think like a scientist

1. Repeat Activity 4.5 with a battery of 2 cells. What do you notice?

When the activity is repeated with a battery of 2 cells, the wire gets hotter than it did with only one cell.

2. For the same duration, does the wire heat up more with one cell or two cells?

For the same duration, the wire heats up more with two cells. This is because a battery with two cells provides a stronger electric current, which generates more heat.

Activity 4.6: Let us construct

1. Can we also make our own Voltaic cell using easily available materials?

Yes, we can make our own Voltaic cell using simple materials like juicy lemons, copper wires, and iron nails.

2. Connect the LED between the copper wire of the first lemon and the iron nail of the last lemon, using connecting wires. What do you observe? Does the LED glow?

When connected correctly, you will observe that the LED glows. This shows that the lemon battery is working and producing an electric current.

3. If the LED does not glow, reverse its connections. Does the LED glow now?

Yes. An LED allows current to pass through it in only one direction. If it does not glow the first time, reversing the connections will likely make it glow, provided the lemon battery is producing enough electricity.

Exercise Questions (“Keep the curiosity alive”)

1. Fill in the blanks:

(i) The solution used in a Voltaic cell is called _____.

Answer: The solution used in a Voltaic cell is called an electrolyte.

(ii) A current carrying coil behaves like a _____.

Answer: A current carrying coil behaves like a magnet.

2. Choose the correct option:

(i) Dry cells are less portable compared to Voltaic cells. (True/False)

Answer: False. Dry cells are more portable because their electrolyte is a paste, not a liquid, making them easier to carry around without spilling.

(ii) A coil becomes an electromagnet only when electric current flows through it. (True/False)

Answer: True. The magnetic effect is only present when the electric current is flowing.

(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. An electromagnet with a battery of 2 cells will have a stronger current and will be a stronger magnet, so it will attract more paper clips.

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

Answer: (c) Both (i) and (ii) are correct. An electric current produces both a heating effect (making the wire warm) and a magnetic effect (deflecting a compass needle).

4. Match the items in Column A with those in Column B.

• Column A

(i) Voltaic cell

(ii) Electric iron

(iii) Nichrome wire

(iv) Electromagnet

• Column B

(a) Generates electricity by chemical reaction

(b) Best suited for electric heater

(c) Works on magnetic effect of electric current

(d) Works on heating effect of electric current

Answer:

(i) Voltaic cell → (a) Generates electricity by chemical reaction

(ii) Electric iron → (d) Works on heating effect of electric current

(iii) Nichrome wire → (b) Best suited for electric heater

(iv) Electromagnet → (c) Works on magnetic effect of electric 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.

Answer: (ii) generates more heat for a given current. Nichrome has a higher resistance compared to copper, which causes it to convert more electrical energy into heat. While it is a conductor (not an insulator), its main advantage for heaters is its high resistance.

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 heating devices are more convenient for several reasons:

• Cleaner: They don’t produce smoke or ash, which helps keep the air inside homes clean and reduces air pollution.
• Safer: They are generally safer as there is no open flame, which reduces the risk of fire accidents.
• Easier to Use: They can be turned on and off easily with a switch and the heat can often be controlled. There is no need to collect, store, and light firewood or charcoal.
• Better for Forests: Using electricity for heating reduces the need to cut down trees for firewood, which helps protect our forests and environment.

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.

(ii) Explain why the compass needle moves when current flows.

(iii) Predict what would happen to the deflection if you reverse the battery terminals.

Answer:

(i) The electric current flows from the positive (+) terminal of the battery, through the coil of wire, and back to the negative (-) terminal. The arrow should show this path.

(ii) The compass needle moves because the flow of electric current through the coil creates a magnetic field around it. The coil becomes an electromagnet. The compass needle, being a small magnet itself, is affected by this magnetic field and moves to align with it.

(iii) If you reverse the battery terminals, the direction of the electric current will be reversed. This will reverse the poles of the electromagnet. The compass needle will still be deflected, but it will now point in the opposite direction.

8. Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position. 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 electromagnet stopped lifting the clips because the battery has become ‘dead’ or very weak. Leaving the switch on for a long time used up all the chemical energy in the battery. The battery can no longer supply enough electric current to create a strong magnetic field. The wire is still a little warm because even a very weak current can produce some heat, but it’s not enough to make the nail a strong magnet.

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

The LED will glow in the case of Figure (a). This is because the setup in (a) has all the parts of a Voltaic cell: two different metals (iron nail and copper strip) and an electrolyte (lemon juice). This combination will produce electricity.

The LED will not glow in Figure (b) because pure water is not a good conductor of electricity and will not act as an effective electrolyte to complete the chemical reaction needed to produce a current.

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 coil will still deflect the compass because a current-carrying coil creates a magnetic field even without an iron core.

However, the deflection will be less than before. The iron nail acts as a core that greatly strengthens the magnetic field. Without the iron nail, the electromagnet is much weaker.

11. 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

Answer: (iv) In all four circuits. Iron, copper, aluminium, and nichrome are all electrical conductors. When electric current passes through any of them, it will produce a magnetic field and cause the compass needle to deflect.