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Physics10

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Chapter 14 Current Electricity (Long Questions)

14.1. Define and explain the term electric current.

14.2. What is the difference between electronic current and conventional current?

14.3. What do we mean by the term e.m.f? Is it really a force? Explain.

14.4. How can we differentiate between e.m.f. and potential difference?

14.5. Explain Ohm’s law. What are its limitations?

14.6. Define resistance and its units.

14.7. What is the difference between conductors and insulators?

14.8. Explain the energy dissipation in a resistance. What is Joule’s law?

14.9. What is difference between D.C and A.C?

14.10. Discuss the main features of parallel combination of resistors.

14.11. Determine the equivalent resistance of series combination of resistors.

14.12. Describe briefly the hazards of household electricity.

14.13. Describe four safety measures that should be taken in connection with the household circuit.

14.14. Design a circuit diagram for a study room that needs the following equipment’s in parallel:

(a) One 100 W lamp operated by one switch.

(b) One reading lamp fitted with a 40 W bulb which can be switched ON and OFF from two points.

(c) What is the advantage of connecting the equipment’s in parallel instead of series?

CONCEPTUAL QUESTIONS

14.1. Why in conductors charge is transferred by free electrons rather than by positive charges?

14.2. What is the difference between a cell and a battery?

14.3. Can current flow in a circuit without potential difference?

14.4. Two points on an object are at different electric potentials. Does charge necessarily flow between them?

14.5. In order to measure current in a circuit why ammeter is always connected in series?

14.6. In order to measure voltage in a circuit voltmeter is always connected in parallel. Discuss.

14.7. How many watt-hours are there in 1000 joules?

14.8. From your experience in watching cars on the roads at night, are automobile headlamps connected in series or in parallel.

14.9. A certain flash-light can use a 10 ohm bulb or a 5 ohm bulb. Which bulb should be used to get the brighter light? Which bulb will discharge the battery first?

14.10. It is impracticable to connect an electric bulb and an electric heater in series. Why?

14.11. Does a fuse in a circuit control the potential difference or the current?

14.1. Define and explain the term electric current.

Ans. Statement: “The rate of flow of electric charges through cross-section area of a conductor is called electric current”.

The flowing electric charge may be positive or negative. In case of conducting metal wires the electric current is due to flow of free electrons which carry negative charge.

Formula: If “Q” denotes the amount of charges flow, through any cross section, in time “t” then value of current “I” is given by I = Q/t (Ampere)

14.2. What is the difference between electronic current and conventional current?

Ans. Electronic current: The current due to negatively charged particles (free electrons) that is from lower potential to higher potential is called electronic current. Conventional current: The current due to positively charged particles that is from higher potential to lower potential is called conventional current.

Current in metals is electronic current but we take conventional current.

14.3. What do we mean by the term e.m.f? Is it really a force? Explain.

Ans. e.m.f. stands for electromotive force. Basically it is the potential energy provided by the source which causes to accelerate the charges (electrons)

to move that is current. There is not force but energy.

14.4. How can we differentiate between e.m.f. and potential difference?

Ans. e.m.f. is the total amount of energy supplied by the battery or the cell in moving in a one coulomb of positive charge from the negative to positive terminal of the battery, The current passes’ through the external resistor(bulb, appliance) due to which potential drops across it. By Ohm’s law V = IR It is called the potential difference’ across the resistor. e.m.f. is greater than potential difference.

14.5. Explain Ohm’s Law. What are its limitations?

Ans. Statement: It states that potential drop (voltage) across a resistance is directly proportional to value of electric current passing through it provided the temperature and the physical state of the conductor does not change.

Consider V is the potential difference between the two ends of a conductor and I is the current is flowing through it. By ohm’s law we can write:

Mathematically:

V ∝ I

V = (Constant) I

The constant of proportionally is denoted by R called electrical resistance of the conductor, so that

V = IR Mathematical form of Ohm’s law.

Limitations:

The temperature of the conductor does not change.

2. The physical state of the conductor does not change.

14.6. Define resistance and its units.

Ans. Electrical Resistance :(R)

The property of a substance which offers opposition to the flow of current through it is called its resistance,

Unit of Resistance: ( Ohm Ω)

The SI unit of resistance is ohm, its symbol is Ω.

By Ohm’s law V=IR ⇒ R = V/I

The resistance is said to be one ohm if a potential difference of one volt across the conductor produces an electric current of one ampere through the conductor.

14.7. What is the difference between conductors and insulators?

Ans. Conductors: Materials offer low resistance to pass current through them are called conductors. For example: AI! metals are good conductors of electricity. Metals have excess of free electrons which are responsible for the flow of current in metals.

Insulators: Materials, offer very high resistance and negligible current passes through them, are called insulators.

For example: Materials like rubber, glass, wood etc. have not free electrons to move.

14.8. Explain the energy dissipation in a resistance. What is Joule’s law?

Ans. As a charge q moves through a resistor, it loses a potential energy qV where V is the potential drop across the resistor. This energy goes into heat, much like the way a ball of putty that falls off a cliff converts its potential energy to heat when it hits the ground. We refer to this conversion of potential energy into heat as dissipation.

The power dissipated in a resistor is the energy dissipated per time. If an amount of charge Dt moves through the resistor in a time Dt, the power loss is

where I is the current through the resistor and V is the voltage drop across it.

The formula P = I V also gives the power generated by a battery if I is the current coming from the battery and V is its voltage.

Joule’s Law:

This law governs the heating effect of current as heat energy released by a conductor when current passes through it. If the conductor is having resistance R and current I passes through it for time t. The mathematical expression of Joule’s law is as explained below.

Q = I2 R T

Where,

Q indicates the amount of heat

I show electric current

R is the amount of electric resistance in the conductor

T denotes time

14.9. What is difference between D.C and A.C?

Ans. Direct Current:(D.C)

The current which does not change its direction of flow is known as direct” current or D.C. The positive and negative terminals of D.C battery have fixed polarity so the voltage of D.C remains constant with time.

Alternating Current: (A. C) The current which changes its direction of flow after regular intervals of time is known as alternating current or A.C.

14.10. Discuss the main features of parallel combination of resistors.

Ans. Characteristics:

1. If there are more than one path for the flow of current in a circuit then the combination of resistances is called Parallel Combination.

2. In parallel combination current through each resistor is different.

3. Potential difference across each resistor is constant.

4. Equivalent resistance of circuit is always less than either of the resistances included in the circuit.

ADVANTAGE:

In parallel combination of resistors, if one component of circuit (resistor) is damaged then rest of the component of the circuit will perform their work without any disturbance. It is due to the presence of more than paths for the flow of electric current.

EQUIVALENT RESISTANCE IN PARALLEL COMBINATION:

Consider three resistances R1 , R2 & R3 connected in parallel combination with a power supply of voltage V.

Now

I = I1 + I2 + I3

According to Ohm’s law

V/R = I

Therefore,

V/Re = V/R1 + V/R2 + V/R3

V/Re = V(1/R1 + 1/R2 + 1/R3)

V/ReV = 1/R1 + 1/R2 + 1/R3

OR

14.11. Determine the equivalent resistance of series combination of resistors.

Ans.

Consider three resistances R1, R2, & R3 connected in series combination with a power supply of voltage.

Potential difference of each resistor is V1, V2, & V3 respectively. Let electric current I is passing through the circuit.

Now

V = V1 + V2 + V3

According to Ohm’s law V = IR

thus

IRe = IR1 + IR2 + IR3

IRe = I(R1 + R2 + R3)

IRe/I = R1 + R2 + R3

Re = R1 + R2 + R3

This shows that in series combination equivalent resistance of circuit is always greater than individual resistances.

14.12. Describe briefly the hazards of household electricity.

Ans. The major hazards associated with electricity are electrical shock and fire. Electrical shock occurs when the body becomes part of the electric circuit, either when an individual comes in contact with both wires of an electrical circuit, one wire of an energized circuit and the ground, or a metallic part that has become energized by contact with an electrical conductor.

Examples:

Electrical hazards come in a variety of forms, but all have the potential to cause serious injury. Common types of electrical hazard include:

Contact with live wires resulting in electric shock and burns,

Fires due to faulty wiring,

Exposed electrical parts,

Ignition of fires or explosions due to electrical contact with potentially flammable or explosive materials,

Inadequate wiring,

Improper grounding, sometimes caused by workers deliberately removing the ground pin on an electric plug to fit a two-prong extension cord,

Interaction with overhead power lines,

Damaged wire insulation, causing electrical conductors to make contact with each other, tools, or a worker’s body,

Overloaded circuits,

Wet conditions.

14.13. Describe four safety measures that should be taken in connection with the household circuit.

Ans. (i) To avoid shocks from electrical appliances use proper-earthing arrangement.

(ii) Replace old worn out and damaged wires with new set

(iii) Put the main switch off when removing any fault in electric circuits.

(iv) Wear rubber shoes and gloves while dealing with replacement of any appliance.

(v) Work with electric circuits in proper light.

(vi) Do not put your hand inside water being heated with immersion rod, when the rod is inside and on.

14.14. Design a circuit diagram for a study room that needs the following equipment’s in parallel:

(a) One 100 W lamp operated by one switch.

(b) One reading lamp fitted with a 40 W bulb which can be switched ON and OFF from two points.

(c) What is the advantage of connecting the equipment’s in parallel instead of series?

Ans. (a) Draw a simple circuit with one switch.

(b)

(c) There are two advantages of connecting the equipment’s in parallel instead of series combination. All the appliances get the same input voltage and if one fuses or faulty then the other appliances can work properly.

CONCEPTUAL QUESTIONS

14.1. Why in conductors charge is transferred by free electrons rather than by positive charges?

Ans. Conductor means the material having large number of free electrons. In all materials positive charges are bound to their respective nuclei. When potential difference is applied across a conductor the free electrons are mobile charges move under the influence of electric field but positive charges are bound.

14.2. What is the difference between a cell and a battery?

Ans. Both are the sources of emf (means provide electrical potential energy) to flow the charges. Cell is the specific name of source of emf in which chemical energy is converted into electrical energy.

Battery is more general name of source of emf in which any kind of energy ( heat, solar, chemical, mechanical) energy is converted into electrical potential energy.

14.3. Can current flow in a circuit without potential difference?

Ans. A conductor has large number of free electrons. They move randomly like gas particles. They constitute current of very small value but the net current is zero. It means net current is zero without potential difference.

14.4. Two points on an object are at different electric potentials. Does charge necessarily flow between them?

Ans. The charges will flow even if there is potential difference is present between two points. Unless these two points are connected with some conducting material having free charged particles.

14.5. In order to measure current in a circuit why ammeter is always connected in series?

Ans. In order to measure/detect current ammeter / galvanometer is always connected in series because the whole current which is to be measured should be passed through the ammeter.

14.6. In order to measure voltage in a circuit voltmeter is always connected in parallel. Discuss.

Ans. Voltmeter is a high resistance galvanometer. The voltmeter is connected in parallel across the resistor whose potential difference is to be measured.

The voltmeter will draw very small amount of current for measuring purpose without changing the current in the circuit.

14.7. How many watt-hours are there in 1000 joules?

Ans. As we know I watt-hour = 3600 joules. As 1 h = 3600 sec.

So E = 1000/3600 = 0.28 watt-hour

14.8. From your experience in watching cars on the roads at night, are automobile headlamps connected in series or in parallel.

Ans. It is common experience, sometime one head light of the car is working. It means they are in parallel combination. If one fuses or faulty the other can work properly.

14.9. A certain flash-light can use a 10 ohm bulb or a 5 ohm bulb. Which bulb should be used to get the brighter light? Which bulb will discharge the battery first?

Ans. A bulb of 5 ohm gives brighter light because low resistance gives rise high current according to the equation

V = IR

Formula for power is P = I2R .

High current and low resistance of 5 ohm bulb will discharge the battery first.

14.10. It is impracticable to connect an electric bulb and an electric heater in series. Why?

Ans. There are two reasons due to which it is impracticable to connect an electric bulb and heater is series. Heater and bulb having different resistances and the potential will drop according to the resistance. If one appliance fuses the other will not run.

Physics10

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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 ∝ 12

Combining these two facts we get F ∝ q1q2 2

F = k q1q2 2

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, q1, 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:

Let the capacitance of each capacitor be C1​, C2​ and C3​ and their equivalent capacitance be Ceq​.

As these capacitors are connected in series, thus charge across each capacitor is same as Q.

When some electrical components, let say 3, are connected in series with each other, the potential difference of the battery V gets divided across each component as V1​, V2​ and V3​ as shown in the figure.

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.

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