CHAPTER # 13
ELECTROSTATICS

Multiple Choice Questions

 

                    i.            Repels other positive charge

                  ii.            Neutral

                iii.            Becomes positively charged

                iv.            The rod and the paper are oppositely charged

                  v.            Decreases

                vi.            Stationary and point charges

              vii.            16 times larger than before

            viii.            0.5V

                ix.            +2q and -2q

                  x.            Never cross each other

                xi.            Q/V

REVIEW QUESTIONS 

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

·         Plastic rod rubbed with fur and glass rod rubbed with silk attract each other.

·         Two plastic rods rubbed with fur repel each other.

13.2 Describe the method of charging bodies by electrostatic induction.

When a charged body is present, an insulated conductor develops a positive charge at one end and a negative charge at the other. This is known as electrostatic induction.

13.3 How does electrostatic induction differ from charging by friction?

During friction charging, bodies come into contact with one another. However, bodies are not in contact with each other during electrostatic induction.

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

The gold leaf electroscope is a sensitive charge detection instrument. The electrostatic induction principle governs the operation of the gold leaf electroscope.

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

Bring a negatively charged body close to the electroscope's disc to produce a positive charge on it. Positive charges will appear on the electroscope's disc, while negative charges will move to the leaves and be grounded by a conducting wire.

Bring a positively charged body close to the electroscope's disc to generate a negative charge on it. Positive charges will appear on the electroscope's disc, while negative charges will shift to the leaves and be grounded by conducting wire.

13.6 With the help of an electroscope how you can find the presence of charge on a body?

Bring the body close to the disc of an uncharged electroscope to detect the presence of charge. There will be no deflection of the leaves if the body is neutral. However, the electroscope leaves diverge if the body is positively or negatively charged.

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

The electroscope is first charged either positively or negatively to detect the nature of a charge on a body. Assuming it is positively charged, bring the charged body close to the electroscope's disc. When the divergence of the leaves increases, the body has a positive charge; when it decreases, the body has a negative charge.

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

In two-point charges, the force of attraction or repulsion is inversely proportional to the square of the distance and directly proportional to the product of the magnitude of the charges.

For long question refer to the book on page # 74.

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

Electric Field	Electric Intensity
The term "electric field" refers to the area surrounding a charge where it exerts a force on another charge.	Electric intensity is the measure of how strong an electric field is at any given location in space. It is a vector quantity.

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

Yes, the direction of the electrostatic force on a positive test charge is the same as the direction of the electric intensity as a vector quantity.

13.11 How would you define the potential difference between two points? Define its unit.

The potential difference between two points is the term used to describe the energy that a unit charge provides as it moves from one point to another in the direction of the field.

The measure of potential difference is the volt. If one joule of work is done to bring a one-coulomb charge, the potential difference is one volt.

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

If we release a point charge between two points say A and B in an electric field then the potential difference will be qV_a and qV_b. The energy supplied per unit charge will be qV_a - qV_b.

Energy supplied = q(V_a – V_b)

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

Capacitance of capacitor	The capacitance of a capacitor refers to its capacity to hold a charge.
Unit of capacitance	The capacitance unit is the farad. If one coulomb of charge is held between a capacitor's plates with a one-volt potential difference, the capacitance is one farad.

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.

Parallel plate capacitor	It is a type of fixed capacitor. It has parallel plates. Its dielectric is a flexible material
Paper capacitor	It is a type of fixed capacitor It has a cylindrical shape. Oil or grease and paper sheet are used as its dielectric.
Mica capacitor	It is a type of fixed capacitor. It is enclosed in a plastic case or some insulator. Its dielectric is mica.
Electrolytic capacitor	It is a type of variable capacitor. It is used to store a large number of charges at low voltages. Its dielectric is an electrolyte solution.

13.16 What is the difference between variable and fixed type capacitors?

Variable capacitor	Fixed Capacitor
The capacitor which has variable capacitance is known as a variable capacitor.	The capacitor which has fixed capacitance is known as a fixed capacitor.
Types of Variable Capacitor	Types of Fixed Capacitor
Electrolytic capacitor	Paper capacitor, Mica capacitor

13.17 Enlist some uses of capacitors.

Capacitors are used in different electronic circuits.

1. It is used in radios.
2. It is used in fans.
3. It is used in washing machines.
4. It is used in computers.
5. It is used to differentiate between high and low frequency signals.
6. It is used in resonant circuits.

13.18 Discuss one application of static electricity.

1. Static electricity is used in the painting of automobiles.
2. Firstly, the body of the car is charged.
3. Give an opposite charge to the paint by charging the nozzle.
4. Paint will stick on the body when it comes out from the nozzle in the form of mist.

13.19 What are the hazards of static electricity?

1. Lighting
2. Fire or Explosion
3. Sparking

CONCEPTUAL QUESTION

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

Due to their opposite charges, paper pieces are attracted to an electrified rod. Due to the conduction process, when the rod touches the paper, both receive the same charge. Because of the force of repulsion, the paper fragments flew 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?

A positively charged electroscope has had the same amount of negative charge removed from it.

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

In an electric field, positively charged particles will move in a direction that runs from higher potential to lower potential.

13.4 Does each capacitor carry an equal charge in series combination?

Yes, because they are connected side by side in a series, the capacitors would carry an equal amount of charge.

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

One plate of each capacitor is connected to the positive terminal of the battery and the other plate is connected to the negative terminal of the battery when multiple capacitors are connected in parallel. That’s why they have equal potential difference, same as of battery.

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

To prevent an explosion or fire during filling, this metal chain is used as an earthing device.

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?

We would become the point of earthing if we got out of the car, which could be fatal because the charge from the high-voltage power line would travel through our bodies to the ground. We are secure inside, just as we were in Faraday's cage.

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?

This is because the glass rod is an insulator while the iron rod is a conductor. Thus, when the charge is on the insulator it does not move to the ground while on the conductor it will move to the ground.

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