Chapter 13 Electrostatics (Long Questions)

Q.13.1 How can you show by simple experiments that there are two types of electric charges? 

13.2. Describe the method of charging bodies by electrostatic induction.
13.3. How does electrostatic induction differ from charging by friction?
13.4. What is gold leaf electroscope? Discuss its working principle with a labelled diagram.
13.5. Suppose you have a glass rod which becomes positively charged when you rub it with wool. Describe how would you charge the electroscope

(i) negatively (ii) positively.
13.6. With the help of electroscope how you can find presence of charge on a body.
13.7. Describe how you would determine the nature of the charge on a body by using electroscope.
13.8. Explain Coulomb’s law of electrostatics and write its mathematical form.
13.9. What is meant by electric field and electric intensity?
13.10. Is electric intensity a vector quantity? What will be its direction?
13.11. How would you define potential difference between two points? Define its unit.
13.12. Show that potential difference can be described as energy transfer per unit charge between the two points.
13.13. What do you mean by the capacitance of a capacitor? Define units of capacitance.
13.14. Derive the formula for the equivalent capacitance for a series combination of a number of capacitors.
13.15. Discuss different types of capacitors.
13.16. What is difference between variable and fixed type capacitor?
13.17. Enlist some uses of capacitors.
13.18. Discuss one application of static electricity.
13.19. What are hazards of static electricity?

CONCEPTUAL QUESTIONS

13.1. An electrified rod attracts pieces of paper. After a while these pieces fly away! Why?
13.2. How much negative charge has been removed from a positively charged  electroscope, if it has a charge of 7.5 × 10^-11 C?
13.3. In what direction will a positively charged particle move in an electric field?
13.4. Does each capacitor carry equal charge in series combination? Explain.
13.5. Each capacitor in parallel combination has equal potential difference between its two plates. Justify the statement.
13.6. Perhaps you have seen a gasoline truck trailing a metal chain beneath it. What purpose does the chain serve?
13.7. If a high-voltage power line fell across your car while you were in the car, why should you not come out of the car?
13.8. Explain why, a glass rod can be charged by rubbing when held by hand but an iron rod cannot be charged by rubbing, if held by hand?

Q.13.1 How can you show by simple experiments that there are two types of electric charges? 

Ans: Take a plastic rod which is rubbed with fur and suspended horizontally as shown in figure. Another plastic rod already rubbed with fur is brought close to the suspended rod. The suspended rods show repulsion. It means during rubbing they get same kind of charges.

Now take a glass rod which is rubbed with silk. It is brought near another plastic rod (already rubbed with fur) and suspended. It also shows attraction. These two experiments confirm the presence of two types of electric charges.

13.2. Describe the method of charging bodies by electrostatic induction.

Ans: Electrostatic Induction:

The process of charging an insulated conductor develops positive charge at one end and negative charge at the other end in the presence of a charged body is called electrostatic induction.

13.3. How does electrostatic induction differ from charging by friction?

Ans: In case of electrostatic induction the body is not physically in contact with the other body to be charged. The charged body is brought near to the body to be charged. But in case of friction the surfaces of the two bodies are rubbed with each other, so there is mutual transfer of electrons.

13.4. What is gold leaf electroscope? Discuss its working principle with a labelled diagram.

Ans: Gold Leaf Electroscope :
It is a simple device to detect the presence of charge on any body.
Construction :
It consists of a metal rod which is fitted in an insulating box. Metal rod has a metal knob at its top. Two gold leaves are also attached at the bottom end of the rod.
Working :
Since electroscope is used to detect the presence of charge. So through it we can find whether a body is charged or uncharged.
Therefore the body to be detected is brought close enough to the metal knob. When a charged object touches the knob at the top of the rod, charge flows through the rod on to the leaves.
Both the gold leaves will have same charge and hence as a result they will repel and diverge.
The degree of divergence is an indicator of the amount of charge i.e., more the charge, more will be the divergence.

13.5. Suppose you have a glass rod which becomes positively charged when you rub it with wool. Describe how would you charge the electroscope  (i) negatively (ii) positively.

Ans:  An electroscope can be charged using glass or rubber rods rubbed with silk or wool. When the glass rod is rubbed with silk, the silk strips electrons from the rod, leaving it a positive charge. When the hard rubber rod is rubbed with wool, it gains electrons from the wool, gaining a negative charge.

Since the glass rod losses electrons, it becomes rich in positive charge. Similarly, the silk cloth becomes negatively charged. This phenomenon is called the tribo electric effect. For example, when glass rod is rubbed with hair, glass acquires a negative charge and hair acquires a positive charge.
13.6. With the help of electroscope how you can find presence of charge on a body.

Ans: Detecting the presence of charge: If a neutral rod is brought near the disk of electroscope, there is no deflection on the leaves of the electroscope.

When a positively or negatively charged rod is brought near the disk of electroscope then in either case the gold leaves diverge or distance between the two increases.

13.7. Describe how you would determine the nature of the charge on a body by using electroscope.

Ans: We take a charged electroscope. The sign of charge positive or negative on its leaves is already known. Consider the electroscope is positively charged.

Now, a charged body is brought near the disk of the positively charged electroscope. If the divergence of the leaves increases then the body carries positive. On the other hand if the divergence decreases then the body carries negative charge.

13.8. Explain Coulomb’s law of electrostatics and write its mathematical form.

Ans: It states that the force of attraction or repulsion between two charges IS directly proportional to product of the magnitudes of the point charges.

Mathematically F ∝ q₁q₂

and this force is inversely proportional to the square of distance between these two charges.

Mathematically F ∝ 1/r²

Combining these two facts we get F ∝ q₁q₂ /r²

F = k q₁q₂ /r² 

where k = 1/4πε = 9 x 10⁹ Nm²C^-2

The constant of proportionally, k depends upon the nature of medium between the two point charges, and the system of units of in which F, r, q_1, q₂ are measured.

13.9. What is meant by electric field and electric intensity?

Ans: An electric field is the region around any electric charge q within which it can exert coulomb’s force on any other electric charge when placed in it.
Electric Field intensity: E

The strength of electric field at any point in space is known as electric field intensity. It is represented by E.

13.10. Is electric intensity a vector quantity? What will be its direction?

Ans: Electric intensity is a vector quantity. Its direction is the same as the direction of Coulomb’s force.
13.11. How would you define potential difference between two points? Define its unit.

Ans: Electric potential 

Electric potential at a point in electric field is equal to the amount of work done in bringing a unit positive charge from infinity to that point.

SI Unit of Electric Potential:

The electric potential is said to be one volt, if one joule work is done in bringing a one coulomb positive charge.

13.12. Show that potential difference can be described as energy transfer per unit charge between the two points.

Ans: If W is the amount of work done in moving a charge q from infinity to a certain point against in the field.

So that  W / q =  work done on unit charge

It is converted into P.E. The electric potential is denoted by symbol V where V = W / q

This shows that “electric potential is equal to electric potential energy per unit charge. It is a scalar quantity. Its SI unit is JC^-1(joule per coulomb). It is called
Volt.

13.13. What do you mean by the capacitance of a capacitor? Define units of capacitance.

Ans: Capacitance of capacitor is the ability to store the charge. By the capacitor equation: Q = CV    ⇒  C = Q / V

SI Unit of Capacitance: The SI unit of capacitance is “farad”. One farad is the capacitance of that capacitor which can store an electric charge of one coulomb when the potential difference applied across the capacitor is one volt.

13.14. Derive the formula for the equivalent capacitance for a series combination of a number of capacitors.

Ans:

13.15. Discuss different types of capacitors.

Ans: The different types of capacitors are following.

1. Electrolytic Capacitor

Generally, the electrolyte capacitors are used when the large capacitor values are required. The thin metal film layer is used for one electrode and for the second electrode (cathode) a semi-liquid electrolyte solution which is in jelly or paste is used. The dielectric plate is a thin layer of oxide, it is developed electromagnetically in production with the thickness of the film and it is less than the ten microns.

2. Mica Capacitor

This capacitor is a group of natural minerals and the silver mica capacitors use the dielectric. There are two types of mica capacitors which are clamped capacitors & silver mica capacitor. Clamped mica capacitors are considered as an obsolete because of their inferior characteristic.

3. Paper Capacitor

The construction of paper capacitor is between the two tin foil sheet and they are separated from the paper, or, oiled paper & thin waxed. The sandwich of the thin foils and papers then rolled into the cylindrical shape and then it is enclosed into the plastic capsule. The two thin foils of the paper capacitors attach to the external load.

4. Film Capacitor

The film capacitors are also capacitors and they use a thin plastic as the dielectric. The film capacitor is prepared extremely thin using the sophisticated film drawing process. If the film is manufactured, it may be metalized depend on the properties of a capacitor. To protect from the environmental factor the electrodes are added and they are assembled.

5. Non-Polarized Capacitor

The non polarized capacitors are classified into two types plastic foil capacitor and the other one is the electrolytic non polarized capacitor. The plastic foil capacitor is non-polarized by nature and the electrolytic capacitors are generally two capacitors in the series, which are in the back to back hence the result is in the non-polarized with half capacitance. The non polarized capacitor requires the AC applications in the series or in parallel with the signal or power supply.

6. Ceramic Capacitor

The ceramic capacitors are the capacitors and use the ceramic material as a dielectric. The ceramics are one of the first materials to use in the production of capacitors as an insulator. There are many geometries are used in the ceramic capacitors and some of them are the ceramic tubular capacitor, barrier layer capacitors are obsolete because of their size, parasitic effects or electrical characteristics. The two common types of ceramic capacitors are multi layer ceramic capacitor (MLCC) and ceramic disc capacitor.
13.16. What is difference between variable and fixed type capacitor?

Ans: There are two major types of capacitors: fixed and variable. Fixed capacitors have set capacitance because the parallel sheets of metal are at a fixed distance apart. Variable capacitors have the ability to change the based on manipulation of the parallel plates.
13.17. Enlist some uses of capacitors.

Ans: Capacitors are devices which store electrical charge. They are a basic component of electronics and have a host of various applications. The most common use for capacitors is energy storage. Additional uses include power conditioning, signal coupling or decoupling, electronic noise filtering, and remote sensing.
13.18. Discuss one application of static electricity.

Ans: Static electricity is used in pollution control by applying a static charge to dirt particles in the air and then collecting those charged particles on a plate or collector of the opposite electrical charge. Such devices are often called electrostatic precipitates. They give the smoke an electric charge.
13.19. What are hazards of static electricity?

Ans: Here are some examples of dangers associated with static electricity: It is dangerous when there are flammable gases or a high concentration of oxygen. A spark could ignite the gases and cause an explosion. It is dangerous when you touch something with a large electric charge on it.

CONCEPTUAL QUESTIONS

13.1. An electrified rod attracts pieces of paper. After a while these pieces fly away! Why?

Ans: When an electrified rod brings close to a piece of paper, opposite electric charges appear on piece of paper due to electrostatic induction. After attraction the positive charge body attracts the electrons from other so become neutral. Force finishes and paper flies away.

13.2. How much negative charge has been removed from a positively charged  electroscope, if it has a charge of 7.5 × 10^-11 C?

Ans: Quantization rule of charges we know  q = ne

n = q / e putting values we get

n = 7.5 x 10^-11 / 1.6 x 10^-19

n = 4.69 x 10⁸

Hence 4.69 x 10⁸ number of negative charges have been removed.

13.3. In what direction will a positively charged particle move in an electric field?

Ans: An electric field is directed from positive (higher potential) to negative (lower potential). So a positive charged particle will move in the direction of electric field.

13.4. Does each capacitor carry equal charge in series combination? Explain.

Ans: Yes in series combination each capacitor carries equal charges. The extreme end plates are charged by battery and the middle plates are being charged by electrostatic induction by equal amount.

13.5. Each capacitor in parallel combination has equal potential difference between its two plates. Justify the statement.

Ans: Yes in parallel combination all the capacitors get same potential difference because all the capacitors are connected across the same terminals of
battery.

13.6. Perhaps you have seen a gasoline truck trailing a metal chain beneath it. What purpose does the chain serve?

Ans: Yes there is metal chain beneath a gasoline truck. The tiers of the truck are made up of rubber due to friction with road they gain charges. It is dangerous if they accumulate to large amount because there is fuel in the truck, it may explode. To avoid this situation, charges go back to road by the conducting (metal) chain.

13.7. If a high-voltage power line fell across your car while you were in the car, why should you not come out of the car?

Ans: If a high-voltage power line (about 12/24 kV) falls across your car, you are safe until you are in car because the tiers of car are made up of rubber (insulator). The loop for the flow of current does not complete. Don’t come out until the line is taken off your car because region around the car is in the electric field.

13.8. Explain why, a glass rod can be charged by rubbing when held by hand but an iron rod cannot be charged by rubbing, if held by hand?

Ans: Glass rod is insulator can be charged by rubbing and the charge remains on its surface. In case of iron that is a conductor can be charged by rubbing but instantly the charges flow towards the earth by human body or flow towards the iron from earth to make it neutral. Hence the iron can be charged in this way.

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