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

Short Answer Questions

Question 1. Describe how we can charge a body by friction.
Answer:
Take a glass rod and rub it with a silk cloth (or an ebonite rod with a woollen cloth).
Bring the rod near small bits of paper. They jump towards it, which shows the rod is charged.
The cloth also attracts paper bits after the rubbing, so it is charged too.
In rubbing, some electrons pass from one body to the other. The body that loses electrons becomes positively charged and the body that gains them becomes negatively charged.
Both bodies get equal and opposite charges. For example, glass rubbed with silk becomes positive and silk becomes negative.
In simple words: Rubbing two different materials moves electrons from one to the other, leaving one positive and the other negative.

Teacher's Note:
1. Mention the activity, the cause (electron transfer) and the result (equal and opposite charges).
2. Use the glass-silk example in the answer.

Question 2. Write a short note on lightning conductor.
Answer:
A lightning conductor is a device used to protect tall buildings from the damage caused by lightning.
It is a thick metal strip with a pointed spike at the top, fixed above the highest part of the building.
The strip runs down the outside wall and its lower end is joined to a metal plate buried deep in the earth.
When lightning strikes, the huge charge passes along the strip into the earth, so the building is not harmed.
In simple words: A metal strip from the top of the building to the earth carries the lightning charge safely into the ground.

Teacher's Note:
1. Mention three parts: spike, strip and earth plate.
2. Benjamin Franklin gave the idea of the lightning rod.

Question 3. Describe Leclanché cell.
Answer:
The cell is made in a glass jar containing a solution of ammonium chloride, which is the electrolyte.
A zinc rod dipped in the solution is the negative plate.
A porous pot standing in the jar holds powdered carbon mixed with manganese dioxide, and a carbon rod is dipped in it as the positive plate.
Manganese dioxide acts as the depolariser. The e.m.f. of the cell is about 1.46 V.
The dry cell is a modified form of this cell.
In simple words: A zinc rod and a carbon rod in ammonium chloride give about 1.46 V.

Teacher's Note:
1. Draw the labelled diagram of Fig. 6.10 for full marks.
2. Know the role of manganese dioxide.

Question 4. Write a short note on parallel and series connections of cells.
Answer:
Series. The positive terminal of one cell is joined to the negative terminal of the next. The total e.m.f. is the sum of the e.m.f. of all the cells: E = E₁ + E₂ + E₃ + …
Parallel. All the positive terminals are joined together and all the negative terminals are joined together. For identical cells the total e.m.f. is the same as that of one cell.
Example: three 1.5 V cells give 4.5 V in series and 1.5 V in parallel.
In simple words: Series adds the voltages; parallel keeps the voltage of a single cell.

Teacher's Note:
1. Show a small diagram of each arrangement.
2. Cells in series give a higher voltage; cells in parallel last longer.

Question 5. What are the precautions to be taken in using household electrical appliances?
Answer:
Use only appliances that carry the ISI mark.
For experiments in electricity use only cells, never the mains supply.
Do not put fingers or metal wires into a socket.
Do not touch switches or appliances with wet hands.
Use a tester to check the presence of current.
Do not touch the metal parts of appliances or an electric pole with bare hands.
Always make sure that no part of the body touches a live wire.
In simple words: Water and bare hands can let current pass through our body, so follow these safety rules.

Teacher's Note:
1. Our body conducts electricity, so a shock is dangerous.
2. Pick any four or five points in an exam.

Question 6. How can we charge a body by conduction?
Answer:
Rub a glass rod with silk so that it becomes positively charged.
Touch this charged rod to a neutral metallic body.
Electrons from the neutral body move to the rod, so the neutral body loses electrons and becomes positively charged.
The body charged by conduction gets the same kind of charge as the charged body that touched it.
In simple words: Touching a neutral body with a charged one passes charge to it, and both then have the same kind of charge.

Teacher's Note:
1. Contact is necessary in conduction.
2. Paper bits stuck to a charged rod and then thrown off show the same idea.

Teacher's Note:
1. See the circuit diagrams of Fig. 6.7 and 6.8 in the Notes.
2. A switch changes a circuit from one state to the other.

Question 8. Describe dry cell.
Answer:
The dry cell is a modified Leclanché cell that can be carried about easily.
A zinc container, which is the negative pole, is filled with a moist paste of ammonium chloride, zinc chloride, manganese dioxide and powdered carbon.
A carbon rod with a brass cap stands in the middle and is the positive pole.
Its e.m.f. is about 1.46 V. It is used in torches, radios and clocks.
In simple words: The dry cell is a portable form of the Leclanché cell with a paste in a zinc can.

Teacher's Note:
1. Brass cap means positive, zinc can means negative.
2. Draw the labelled diagram of Fig. 6.11.

Question 9. Write a short note on (a) short circuit and (b) overload.
Answer:
(a) Short circuit. When the insulation on the live and neutral wires wears off and the bare wires touch each other, the resistance becomes very small and a very large current flows.
(b) Overload. When too many appliances are connected to one socket, they together draw more current than the circuit can safely carry.
Both give an excess current, which can cause fire. A fuse or an MCB cuts off the supply in such cases.
In simple words: Short circuit means bare wires touch; overload means too many devices on one socket.

Teacher's Note:
1. Both are the common causes of fire in homes.
2. Fuses and MCBs protect against both.

Question 10. Write the uses of electromagnets.
Answer:
Electromagnetic cranes lift heavy loads of iron.
They separate magnetic materials from scrap.
They are an important part of the electric bell.
Doctors use them to take out tiny iron particles that enter the eye.
In simple words: Electromagnets lift, separate and remove magnetic materials, and ring the bell.

Teacher's Note:
1. Mention at least three uses.
2. Electromagnets can be switched on and off.

Question 11. Describe ‘electroscope’ and explain briefly how it works.
Answer:
The gold leaf electroscope has a metal rod passing through a cork into a glass jar. The rod has a metal cap or disc at the top and two thin metal strips (a brass strip and a gold leaf) at the bottom.
When a charged body touches the cap, the charge flows along the rod to the strips.
Both strips get the same charge, so they repel and move apart. The gap shows the presence of a charge.
To find the nature of the charge, first charge the electroscope with a known charge. A body with the same charge makes the gap bigger and a body with the opposite charge makes the strips come closer.
In simple words: The strips open when charged, and the way they move shows whether a body has charge and its kind.

Teacher's Note:
1. Draw the neat diagram of the electroscope (Fig. 6.5).
2. Same charge increases the gap, opposite charge reduces it.

Question 12. Three cells of 1.5 V, 2.5 V and 3.5 V, respectively, are connected in series to form a battery. Find the e.m.f of the battery.
Answer:
For cells in series, E = E₁ + E₂ + E₃.
E = 1.5 + 2.5 + 3.5 = 7.5 V.
In simple words: Add the three e.m.f. values.

Teacher's Note:
1. Series: add the e.m.f.
2. The unit of e.m.f. is volt.

Question 13. Describe a dynamo.
Answer:
A dynamo (electric generator) is a machine that changes mechanical energy into electrical energy.
In a power plant, flowing water or steam turns a turbine, which is connected to the dynamo and turns it.
The dynamo then produces electricity, which is sent to cities and villages through cables.
In simple words: A dynamo is the part of a generator that makes electricity when it is turned.

Teacher's Note:
1. A generator = turbine + dynamo.
2. It is the opposite of an electric motor.

Question 14. What is an electromagnet? How the strength of the electromagnet can be increased?
Answer:
An electromagnet is a temporary magnet made by winding an insulated wire around an iron rod and passing current through the wire. It is a magnet only while current flows.
Its strength can be increased by increasing the current through the coil.
In simple words: A coil with current around an iron rod acts as a magnet, and more current makes it stronger.

Teacher's Note:
1. Switch the current off and the magnet loses its power.
2. Mention the iron nails activity.

Question 15. Write the major defects in Voltaic cell.
Answer:
Local action. The impurities (carbon and iron) in zinc set up tiny cells on the plate and the zinc gets used up. It is prevented by coating the zinc plate with mercury.
Polarization. Hydrogen bubbles collect on the copper plate and block the flow of current. A depolariser such as potassium dichromate, copper sulphate or manganese dioxide removes them by changing hydrogen into water.
In simple words: Impure zinc wastes itself and hydrogen bubbles cut the current.

Teacher's Note:
1. State the remedy for each defect.
2. Local action – mercury; polarization – depolariser.

Question 16. (a) Two charged bodies, A and B are suspended from a fixed point with the help of silk threads as shown in the figure. If A is positively charged, can we identify the nature of charge on ‘B’?
(b) A negatively charged conducting sphere A and a neutral sphere B are kept on insulating stands and brought close to each other. The charge distribution is as shown in the figure. Is B charged? What type of force exists between A and B?

Answer:
(a) The two bodies move apart, which means they repel each other. Repulsion occurs only between like charges. Since A is positive, B is also positively charged.
(b) The positive and negative charges on B are equal, so the net charge on B is zero and B is not charged. The charges have been redistributed by electrical induction, with the positive charge nearer to A. As a result there is a force of attraction between A and B.
In simple words: (a) Repulsion means the same charge. (b) B is only polarised, not charged, and is attracted.

Teacher's Note:
1. Repulsion is the sure test for the kind of charge; attraction is not.
2. Induction only moves charge around inside B.

Question 17. Raghavan found a metallic rod AB and a glass rod CD both positively charged and placed on an insulating table. He wanted to know what would happen to the charges on AB and CD if he touched both the rods at A and C? Discuss.

Answer:
The human body and the earth are conductors, so touching a charged body lets charge flow to the ground (earthing).
In the metallic rod the charges are free to move, so even if he touches it only at A, the electrons come from the ground through his body and neutralise all the positive charge. AB loses its whole charge.
Glass is an insulator and its charges cannot move. Only the charge at the spot C he touched is neutralised, and the remaining charge on CD stays as it was.
In simple words: A metal rod loses all its charge when touched; a glass rod loses it only at the touched point.

Teacher's Note:
1. Conductors share charge throughout the body; insulators do not.
2. Earthing is the process of passing charge to the ground.

Question 18. Two identical, small metallic spheres A and B are placed side by side on an insulating smooth table as shown in the figure. If A is positively charged and held fixed with the table, discuss the motion of B.

Answer:
B is neutral. When A is brought near, negative charge is induced on the side of B nearer to A and positive charge on the far side. The nearer negative charge is attracted more strongly, so B moves towards A.

When B touches A, positive charge flows from A to B by conduction, so B also becomes positively charged.

Now A and B have like charges and repel, so B moves away from A.

In simple words: B is first attracted to A, touches it, becomes positive and is then pushed away.

Teacher's Note:
1. Attraction by induction comes first; repulsion after contact.
2. The smooth table lets B move freely.