CBSE Class 7 Science Advanced Physics Electricity Short Answer Questions Set 02

Short Answer Questions

Question 1. Observe the circuit and state whether the cells are connected in series or parallel. If each cell is of e.m.f 1.5 V, then find the total e.m.f applied across the bulb connected.

Answer:
The positive terminal of each cell is joined to the negative terminal of the next cell, so the cells are connected in series.
Total e.m.f. = E₁ + E₂ + E₃ = 1.5 + 1.5 + 1.5 = 4.5 V.
In simple words: Cells joined end to end are in series, so the e.m.f. adds up to 4.5 V.

Teacher's Note:
1. Look at which terminals are joined.
2. Series: add; parallel: same as one.

 

Question 2. Three identical cells of e.m.f 1.5 V are given. Find how we can arrange them to get a total e.m.f of (a) 1.5 V (b) 3 V (c) 4.5 V.
Answer:
(a) 1.5 V: join all three cells in parallel. The e.m.f. is the same as that of one cell.
(b) 3 V: join two cells in series and connect the third across them in parallel, as shown. The two cells in series give 3 V.
(c) 4.5 V: join all three cells in series, 1.5 + 1.5 + 1.5 = 4.5 V.
In simple words: Parallel gives 1.5 V, two in series give 3 V and three in series give 4.5 V.

Teacher's Note:
1. The arrangement in (b) follows the book; the series pair alone supplies the 3 V.
2. Series adds e.m.f.; parallel does not.

 

Question 3. Prahar took two identical heating elements and connected them to a battery in two different ways as shown in the figure. When he switched ON the switch, is the heat developed in each element the same or different in both the cases?

Answer:
The heat produced in an element depends on the current through it: more current, more heat.
In (a) the elements are in series, so each carries a smaller current.
In (b) the elements are in parallel, so each is joined directly to the battery and carries a larger current.
So the heat developed is different: more in case (b), the parallel connection.
In simple words: Parallel elements get more current, so they heat more than elements in series.

Teacher's Note:
1. Compare the current in each element.
2. Parallel connections give more heat.

 

Question 4. Compare the nature of materials used to make the ‘filament’ of an electric bulb and the ‘fuse wire’ used in electric fuse?
Answer:
Filament: it must glow without melting, so it is made of a metal with a high melting point and high resistance, usually tungsten.
Fuse wire: it must melt quickly and break the circuit when the current is too high, so it is made of an alloy with a low melting point and fairly high resistance, usually an alloy of lead and tin.
In simple words: A filament has to survive a high temperature, whereas a fuse wire has to melt easily.

Teacher's Note:
1. High melting point for the filament, low for the fuse.
2. Tungsten for filaments, lead-tin alloy for fuses.

 

Question 5. Represent the connections shown below by a circuit diagram.

Answer:
The figure shows three cells A, B and E, a bulb C and a switch D joined with wires. Cells are joined end to end and so are in series. The circuit diagram uses the standard symbols as shown below.

In simple words: Replace each picture with its symbol: cells in series, then the bulb and the switch in the same loop.

Teacher's Note:
1. Join the long line (+) of one cell to the short line (−) of the next.
2. Use a clear symbol for each component.

 

Question 6. We observe the bulb glowing when the switch is ‘ON’. Can we conclude that charges instantly move from the ‘switch’ to the ‘bulb’? Discuss.
Answer:
No. A wire is already full of free charges that move in all directions at random.
When the switch is turned ON, all the free charges in the whole circuit begin to move in one direction at once.
The charges at the end of the wire connected to the bulb start moving through the bulb immediately. This is why the bulb glows at once; the same charges do not travel from the switch to the bulb.
In simple words: The charges are already present in the wire and all of them start moving together when the switch is turned on.

Teacher's Note:
1. The bulb glows without delay because the whole circuit starts at once.
2. The effect travels quickly, not an individual charge.

 

Question 7. What is a depolarizer and how does it work?
Answer:
When a voltaic cell works for some time, a thin layer of hydrogen forms around the copper electrode and blocks further flow of charge. This defect is called polarization.
A depolariser is a substance used to remove this defect. It is an oxidising agent that reacts with the hydrogen and changes it into water.
Examples: potassium dichromate, copper sulphate and manganese dioxide.
In simple words: A depolariser turns the hydrogen on the plate into water so that the current keeps flowing.

Teacher's Note:
1. Link the depolariser to the defect of polarization.
2. Name two examples.

 

Question 8. Gourang made an electric circuit with three bulbs B₁, B₂ and B₃, a cell and two switches S₁ and S₂ as shown in the figure. He noted his observation when (a) S₁ is closed and S₂ is open, (b) S₂ is closed and S₁ is open. He also found that a bulb whose failure made the circuit ‘open’ irrespective of the status of S₁ and S₂. What are his observations?

Answer:
(a) With S₁ closed and S₂ open, current flows through B₁ and through the branch of B₂. So B₁ and B₂ glow and B₃ does not.
(b) With S₂ closed and S₁ open, current flows through B₁ and the branch of B₃. So B₁ and B₃ glow and B₂ does not.
The bulb whose failure opens the circuit whatever the switches is B₁, because it is in the main path in series with both branches.
In simple words: B₁ is in series with the branches, so it glows whenever any branch is on; B₂ and B₃ glow only with their own switches.

Teacher's Note:
1. Trace the path of the current through each switch.
2. A bulb on the main line decides whether the circuit is open.

 

Question 9. Saket was repairing an electric bell. By mistake, instead of an electromagnet, he placed a permanent magnet in the electric bell. Explain, what changes do you observe.
Answer:
In a bell the electromagnet pulls the iron strip, the strip leaves the contact screw, the circuit breaks, and the strip springs back when the magnetism disappears.
A permanent magnet cannot switch its magnetism off. Once it pulls the iron strip, the strip stays attracted to it and the strip cannot return to touch the contact screw.
So the circuit stays open and the bell cannot ring continuously; it may make just one sound.
In simple words: A permanent magnet holds on to the strip, so the on-off action needed for ringing cannot happen.

Teacher's Note:
1. The electromagnet's strength is switched off when the circuit opens.
2. That repeated switching makes the bell ring.

 

Question 10. Ravinder was inspired by reading the autobiography of Thomas Alva Edison and changed a portion of his room into a laboratory. In his laboratory he performed an activity that demonstrates both ‘heating effect’ and ‘magnetic effect’ of electric current. He connected different components with different wires and the connections are represented by the following circuit diagram. To his disappointment, he was not successful in his effort. Observe the circuit and suggest modifications to make his effort successful.

Answer:
Modification 1: replace the silk thread that joins one of the bulbs to the cell with a conducting wire, because silk is an insulator and does not carry current.
Modification 2: replace the fused bulb with a good bulb so that the circuit is complete.
Modification 3: switch ON (close) the switch.
Then the bulbs glow (heating effect) and the electromagnet attracts iron objects (magnetic effect).
In simple words: Make sure every part of the loop is a conductor and that the switch is closed.

Teacher's Note:
1. An open path cannot carry a current.
2. Check wires, bulbs and the switch in that order.

 

Question 11. Describe the electrostatic force that exists between A and B in the following cases.

(a)

(b)

(c)

(d)

(e)

(f)

Answer:
(a) Both A and B are neutral, so no electrostatic force acts between them.
(b) A is charged and B is neutral: a force of attraction exists, because the charged body induces opposite charge on the nearer side of the neutral body.
(c) B is charged and A is neutral: again a force of attraction due to induction.
(d) A is positive and B is negative, so they are oppositely charged and attract each other.
(e) Both A and B are positively charged, so there is a force of repulsion.
(f) Both A and B are negatively charged, so there is a force of repulsion.
In simple words: Neutral-neutral gives no force, charged-neutral attracts, unlike charges attract and like charges repel.

Teacher's Note:
1. Decide first which bodies carry charge and which signs they have.
2. Induction explains the attraction between a charged and a neutral body.

 

Question 12. We know that (a) like electrical charges repel each other and (b) substances which allow charges to move through them are called conductors. In the light of these facts, find where the charge placed on a conductor resides?
Answer:
Take a spherical conductor and place some negative charge on it.
The charges repel each other, and because the body is a conductor they are free to move. They go as far from each other as possible.
So all the charge is pushed to the outer surface of the conductor, and none stays inside.

In simple words: Charges on a conductor repel one another and end up on the outer surface.

Teacher's Note:
1. Inside the body there is no extra charge.
2. Left figure: wrong; right figure: correct distribution.

 

Question 13. A and B are two identical conducting rods. A is given positive charge and B is connected to a cell as shown in the figure. Will there be any change in the electrostatic force between A and B if the switch is closed?

Answer:
With the switch open, B is an uncharged conductor, so the positively charged rod A attracts it by induction.
When the switch is closed, current flows through B. The amount of charge entering B equals the amount leaving it, so B stays electrically neutral.
Because the charges in B are in motion, the attraction due to induction becomes negligible. So the electrostatic force disappears (practically zero) when the switch is closed.
In simple words: Once current flows in B, the induction effect becomes negligible and the force vanishes.

Teacher's Note:
1. The answer follows the book's reasoning.
2. Compare the two cases: switch open and closed.

 

Question 14. Three identical cells E₁, E₂, E₃ and three identical bulbs B₁, B₂ and B₃ are connected as shown in the circuit.
(a) Find how E₁, E₂, E₃ and B₁, B₂, B₃ are connected, whether in parallel or in series?
(b) Compare the brightness of the bulbs.

Answer:
(a) The three cells E₁, E₂ and E₃ are connected in series. The bulbs B₁ and B₂ are in series, whereas bulb B₃ is in parallel, as it is connected across the two cells E₂ and E₃.
(b) B₁ and B₂ are in series and carry the same current, so they have the same brightness. B₃ is connected across two cells and is brighter than B₁ and B₂.
In simple words: B₁ and B₂ share the energy and glow equally, while B₃ is brighter.

Teacher's Note:
1. The answer assumes that the switch next to B₁ is closed, as the book does.
2. Series bulbs are equally bright; the bulb connected across two cells gets more.

 

Question 15. Usually we use two different types of sockets (a) two-pin socket and (b) three-pin socket. What purpose does the third pin in a three-pin socket serve?
Answer:
(a) In a two-pin socket one pin is joined to the live wire and the other to the neutral wire.
(b) In a three-pin socket the third pin, called the earth pin, is connected to the earth.
The earth connection is essential because it carries any leakage current that accidentally flows through the metal cover of an appliance to the ground, so we do not get a shock.
In simple words: The third pin takes any leaking current safely to the ground.

Teacher's Note:
1. Heavy appliances such as refrigerators and irons need the earth pin.
2. The earth pin is longer and thicker.

 

Question 16. Pranith received an electric bill for the month of April. He started verifying it by calculating the electrical energy consumed by a 2000 W electrical appliance that is used for two hours every day in his workshop. In his house he used bulbs of 100 W for 4 hours every day in the month of April, which consumes the same electrical energy as that of the electrical appliance in the work shop. Find the number of bulbs Pranith uses in his house.
Answer:
Energy used by the appliance in one day = 2000 W × 2 h = 4000 W h = 4 units.
In April (30 days) = 4 × 30 = 120 units.
One 100 W bulb used 4 h every day uses 100 × 4 = 400 W h = 0.4 unit per day, i.e. 0.4 × 30 = 12 units in the month.
Number of bulbs = 120 ÷ 12 = 10 bulbs.
In simple words: The appliance uses 120 units in 30 days, and each bulb uses 12 units, so he has 10 bulbs.

Teacher's Note:
1. Convert W h to units by dividing by 1000.
2. Check: 10 bulbs × 100 W × 4 h × 30 = 120 000 W h = 120 units.

 

Question 17. Which effect of electricity is used in electrotype setting? Discuss.
Answer:
Electrotype setting uses the chemical effect of electric current.
In a printing press a page is first composed with ordinary types. A mould of the letters is made in wax and graphite powder is spread over it so that it conducts electricity.
This mould is the cathode and a piece of copper is the anode in the electrolysis of copper sulphate solution. A thick plating of copper forms on the mould.
The wax is then melted off. The copper letters are used to print thousands of copies of the newspaper page.
In simple words: Copper is deposited on a wax mould by electrolysis to make strong printing plates.

Teacher's Note:
1. Electrotype setting and electroplating both use electrolysis.
2. Mention the electrolyte, copper sulphate.

 

Question 18. Do all bulbs we use work on the principle of heating effects of electric current? Discuss.
Answer:
No. Filament bulbs work on the heating effect: the filament gets hot and glows, and a lot of the energy is wasted as unwanted heat.
Fluorescent tube lights work on a different principle and give out much less heat, so they waste less electricity.
Compact fluorescent lamps (CFLs) also reduce wastage and can be fitted in ordinary bulb holders.
In simple words: Only filament bulbs rely on heating; tubes and CFLs use another method and save energy.

Teacher's Note:
1. Switching to tubes and CFLs conserves electricity.
2. The ordinary bulb gives more heat than light.