CBSE Class 7 Science Advanced Physics Electricity Long Answer Questions Set 01

Long Answer Questions

Question 1. Write a note on ‘electrical induction’.
Answer:
A neutral body has equal amounts of positive and negative charge.
When a positively charged body is brought near a neutral body, the negative charges of the neutral body are attracted towards it and the positive charges are repelled away from it.
As a result, the end of the neutral body nearer to the charged body gets an excess of negative charge, and the farther end gets an excess of positive charge. The body as a whole is still neutral.

This shifting of charge in an uncharged body because of a nearby charged body is called electrical induction.
Induction explains why a charged body attracts an uncharged one: the nearer opposite charge is closer and is pulled harder than the farther charge is pushed.
In simple words: A charged body pulls opposite charge towards its side of a neutral body and pushes like charge to the far side.

Teacher's Note:
1. Draw the diagram showing the + and − on the neutral body.
2. Remember that the net charge stays zero.

Question 2. Describe Voltaic cell and write two major defects of it.
Answer:
Construction. A glass vessel holds dilute sulphuric acid as the electrolyte. A copper plate and a zinc plate are dipped in it. When they are joined through a bulb, the bulb glows.

The copper plate is the positive pole and the zinc plate is the negative pole. The e.m.f. of the cell is about 1 V.
Defect 1: Local action. Impurities such as carbon and iron in the zinc plate set up tiny cells and local circuits that waste the zinc. It is prevented by coating the zinc plate with mercury.
Defect 2: Polarization. Hydrogen bubbles formed during the working collect on the copper plate and resist the current. It is reduced with a depolariser, which is an oxidising agent that changes hydrogen into water. Potassium dichromate, copper sulphate and manganese dioxide are depolarisers.
In simple words: Copper and zinc plates in dilute sulphuric acid make a cell; its faults are local action and polarization.

Teacher's Note:
1. Always draw the labelled diagram of Fig. 6.9.
2. Give the remedy with each defect.

Question 3. Describe with an example ‘heating effects of electric current’.
Answer:
When an electric current passes through a wire, the wire becomes warm. This is the heating effect of electric current: electrical energy is changed into heat energy.
The heat produced depends on the length, thickness and material of the wire, the current and the time.
Example: electric heater. It has a heating element of coiled nichrome wire mounted on a frame of kaolin or asbestos. The nichrome resists the current, becomes red hot and gives its heat to the vessel placed on it.
Other examples: electric iron, toaster, filament bulb and electric fuse.
In simple words: A current heats the wire it flows through; a heater uses this heat deliberately.

Teacher's Note:
1. Quote the heater with nichrome wire as the example.
2. Mention the five factors on which the heat depends.

Question 4. Write a short note on electric bell.
Answer:
An electric bell has a coil that acts as an electromagnet. A soft iron strip with a hammer at one end is kept close to the coil, and the strip touches a contact screw when the bell is at rest.

On pressing the switch the circuit is closed, current flows and the coil becomes a magnet. It pulls the iron strip, and the hammer strikes the gong, producing sound.
As the strip moves towards the coil it leaves the contact screw, so the circuit opens and the current stops. The coil loses its magnetism and the strip springs back to touch the screw.
The circuit is closed again and the process repeats, so the bell rings continuously as long as the switch is pressed.
In simple words: The electromagnet pulls the hammer, but the circuit then breaks, the strip springs back and the action repeats, giving a continuous ring.

Teacher's Note:
1. The bell works on the magnetic effect of current.
2. The contact screw makes and breaks the circuit.

Question 5. What are the advantages and disadvantages of connecting bulbs in parallel and in series?
Answer:
Parallel connection.
Advantage: each bulb works independently of the others, so if one bulb fuses the rest keep working. Every bulb is joined directly to the battery, gets the full energy and glows brightly.
Disadvantage: the battery has to supply more current, so it drains faster.
Series connection.
Advantage: a single wire joins everything, so only one switch controls all the bulbs.
Disadvantage: if one bulb fuses the others also go off. The energy of the battery is shared, so the bulbs glow dimly and do not work to full capacity.
That is why houses use parallel wiring.
In simple words: Parallel bulbs are bright and independent; series bulbs are dim and all depend on each other.

Teacher's Note:
1. Draw both circuits for full marks.
2. Link the answer to household wiring.

Question 6. Explain magnetic effects of electric current.
Answer:
A wire carrying current behaves like a magnet and produces a magnetic field around it. This was discovered by Hans Christian Oersted in 1820.
Activity. Wind an insulated wire around a magnetic compass and join its ends to a cell through a switch. The needle normally points north-south. When the switch is closed, the needle turns away from its place. When the switch is opened again, the needle comes back.

The needle is a small magnet, so its turning shows that the wire carrying the current acts as a magnet.
Applications. Electromagnets (cranes, scrap sorting, removing iron particles from the eye) and the electric bell work on this effect.
In simple words: A current-carrying wire deflects a compass needle, showing that it produces a magnetic field.

Teacher's Note:
1. Mention Oersted and the compass activity.
2. Name two devices that use the effect.

Question 7. Describe chemical effects of electric current with a few examples.
Answer:
When current is passed through certain liquids such as copper sulphate solution, chemical changes take place in them. Such a liquid is called an electrolyte.
The molecules of the electrolyte split into ions, and the ions move towards the electrodes of opposite sign. This process is electrolysis.
Electroplating: coating a cheap metal article with a thin layer of gold or silver.
Purification of metals: the impure metal is made the positive pole and pure metal collects on the negative pole, with a salt solution of the same metal as the electrolyte.
Electrotype setting in printing also uses the chemical effect.
In simple words: Current splits an electrolyte into ions, which is used to plate and purify metals and to make printing plates.

Teacher's Note:
1. Give at least two examples.
2. Electrolyte, electrodes and ions are the key words.

Question 8. Explain about Bichromate cell with a neat labelled diagram.
Answer:
Construction. The cell is built in a flat-bottomed glass flask closed with a stopper. The electrolyte is a solution of potassium dichromate in dilute sulphuric acid.

Two carbon plates are the positive poles and a single zinc plate is the negative pole. The zinc plate is fixed through a rod and screw so that it can be raised out of the liquid when the cell is not in use.
Working. Potassium dichromate acts as a depolariser, oxidising the hydrogen that would otherwise collect on the carbon plates.
The e.m.f. of the cell is 2 V.
In simple words: Carbon and zinc plates in potassium dichromate and sulphuric acid make a 2 V cell.

Teacher's Note:
1. Draw the labelled diagram of Fig. 6.12.
2. The zinc plate is raised when the cell is idle to save it.

Question 9. With the help of an electroscope, how can we identify the presence and nature of charge on a body?
Answer:
Presence of charge. Bring the body in contact with the metallic disc (cap) of the electroscope. If the metal strips in the jar deviate (move apart), the body is charged.

Nature of charge. First charge the electroscope with a known type of charge. Then bring the unknown body near its disc.
If the strips diverge more, the body has the same type of charge as the electroscope.
If the strips collapse or come closer, the body has the opposite charge.
In simple words: Open strips show a charge; more opening means the same charge as the known one, and closing means the opposite charge.

Teacher's Note:
1. Mention the two-step method.
2. Charge the electroscope with a known charge first.

Teacher's Note:
1. List a few devices and the conversion in each.
2. Remember the saying: energy saved is energy generated.