Ch-3 CH-3 PHYSICS. Electric Current

CLASS 12-CURRENT ELECTRICITY 1

Ch-3

CH-3 PHYSICS

Today, life without electricity can not be imagined. It is one of the most important source of energy which is used at home and in industry for various purposes such as heating, lighting, operating machines. The term current means some sort of motion of electric charges constitutes an electric current. The Branch of physics, which deals with the electric charges in motion, is called current electricity.

|Electric Current|

Flow of current in a conductors:

Before we start discussing current let’s recall that we know that all substances are made up of atoms. Atoms are themselves made up of three tiny particles called electrons, protons, and neutrons. The electrons are negatively charged, protons are positively charged, and neutrons have no charge.

The electrons are present in the outermost orbit of metals are loosely bonded. When these electrons absorb energy they start moving freely or we can say that these electrons detached from the orbit of the atom.

Now consider that we have an electric circuit. The electric circuit is the circuit that provide a continuous path for the flow of electric current. This circuit is made up of metal. In a conductor such as a metal, the conduction electrons are in random motion within the material. It is in an isolated conductor, although free electrons are moving randomly with very high speeds of the order of 105_{m/s. Net charges cross any} cross-section are zero. Therefore the current does not flow in an isolated conductor.

CLASS 12-CURRENT ELECTRICITY 2 When we apply a voltage across the metal, these electrons are energised and start moving freely in the atom. But as we know that electrons are negatively charged, these electrons attracted towards the positive terminal of the supply. Because of these attractions, the continuous flow of electrons occurs in the metal. These electrons create currents in the circuit. So, in short, we can say that the electric current is the continuous flow of electrons in a closed circuit.

Thus we can say that, when the ends of the conductors are connected to the two terminals of a cell, the electrons feel a push and move slowly or drift through the metal. We say that at any instant, there is a net flow of charge through a cross-section of the conductor, or an electric current is flowing through the

conductor. Therefore, the electric current is defined as the flow of net charge through a cross-section of the material in one second. Or simply, the current is the rate of flow of electric charge.

The current is denoted by the symbol ’I’.

Theory of Electric Current:

Let us know how how the movement of the current generates current in the wire.

Suppose here is the piece of wire. For simplicity, consider that the atom of the metal consist of only three electrons i.e., the outermost orbit, contains only one electron. Now suppose we supply a voltage across the wire. Voltage is a kind of pushing force. The voltage energises the electrons and because of that voltage electron starts moving. Now consider that one atom is attached to another atom. Now, these electrons do not settle there permanently, immediately they again start detaching and start moving to another atom. So, this force will move electrons continuously and because of that movement of electrons, the current starts flowing in the metal wire.

CLASS 12-CURRENT ELECTRICITY 3 Electric current- The flow of electric charge is called an electric current and it exits as long as there is an electrostatic potential difference.

Quantitatively- The electric current is defined as the rate of flow of electric charge through any crosssection of the conductor.

If a net charge Q flows through a cross-section of a conductor in a time t, the electric current I is given by

I = Amount of charge passing through a cross−section

Time taken

I =

t ---(i)

So, the time rate of flow of charge through any cross-section is called current. So if through a cross section q charge passes in time t

Iav = q t

Hence, Instantaneous current I = lim t→0 q t

=

Where dq is the small amount of charge that flows through any section of a conductor in small-time dt

Regarding the electric current, it is important to note that:

(i) Current is assumed to be a fundamental quantity in physics with unit ampere and dimension A.

(ii) The conventional direction of the current is taken in the direction of flow of positive charge, i.e., field, and in opposite to the direction of flow of negative charge.

(iii) Though conventionally a direction is associated with current (opposite to the motion of electrons), it is not a vector as:

(a) The direction merely represents the sense of charge flow and is not a true direction.

(b) Current does not obey the law of parallelogram of forces, i.e., if two currents I1 and I2 reach at a point where we always have I = I1 + I2, whatever be the angle between I1 and I2.

(c) Current is also defined as the flux density, i.e., I = ∫ 𝐽.⃗⃗ 𝑑𝑠⃗⃗⃗⃗ and the dot product of the two vectors is scalar.

CLASS 12-CURRENT ELECTRICITY 4 (iv) As the charge is conserved and current is the rate of flow of charge, the

charge entering at one end per second of a conductor is equal to the charge leaving the other end per second. This implies that

(a) For a given conductor, the current does not change with a change in cross-section.

(b) Conductors remains unchanged when the current flows through them. (v) Current in different situations is calculated as follows:

(a) Due to the translatory motion of charges:

(i) In ‘n’ particles, each having a charge q, passes through a given area in time t:

I = Q_t

=

(ii) In n particles, each having a charge q, pass per second per unit area, the current is associated with a cross-sectional area S

I = Q

t

=

(iii) If there are n particles per unit volume. Each having a charge q and moving with velocity v, the current through cross-sectional area S I = Q

t

=

t

=

(vi) According to magnitude and direction current is usually divided into two types:

(a) Direct current (dc): if the magnitude and direction of the current does not vary with time, it is said to be direct current (dc). Cell, battery, or dc dynamo are its sources.

(b) Alternating current(ac): if a current is periodic (with constant

amplitude) and has half -cycle positive and half-cycle negative , it is said to be alternating current(ac), ac dynamo is the source of it.

Measuring Electric Current:

Electric current is measured in SI unit of amperes (denoted by symbol A) after French scientist Andre-Marie Ampere. If 1 C of charge passes through a cross-section of a conductor in 1 s, then we say that a current of 1 A flows through the conductor.

CLASS 12-CURRENT ELECTRICITY 5 1 ampere = 𝟏 𝐜𝐨𝐮𝐥𝐨𝐦𝐛

𝟏 𝐬𝐞𝐜𝐨𝐧𝐝

Definition: When one coulomb charge flows through an electric circuit in one second, then the current flowing through the circuit is said to be one ampere.

Ammeter is used to measure the amount of current flowing in a wire. An ammeter denoted by the symbol

To measure the current, you have to break the wire and insert the ammeter so that the current to be measured passes through the meter. Therefore an ammeter is connected in series with the conductor. Large currents are measured in kiloamperes.

1 kA = 103 _A.

Smaller currents in television circuits are measured in milliamperes or microamperes.

1 mA = 10-3 _A

1 A = 10-6 _A

Even smaller currents in the circuits of mobile phones or computer chips are measured in nanoamperes and picoamperes.

1 nA = 10-9 _A

1 pA = 10-12_A.

The electrical nerve impulses in our body constitute a very little current. A current of about 1 A, generally flows through the filament of an electric bulb. A dangerously large amount of current discharges during lightning. In our solar system, the stream of positive ions, protons and electrons emanating from the sun constitutes the current of the solar wind. In our galaxy, mostly energetic protons constitute the current of cosmic rays.

The direction of current: We know that it is actually the flow of conduction electrons through metals that constitute the electric current in wires. Unfortunately, Franklin's choice of naming the charges made the electrons negative. By the time the mistake was recognised, it was too late to fix. We have had to deal with the current flow as the flow of imaginary positive charges in the opposite direction since then. The flow of positive charges is called conventional current. The flow of electrons is called electronic current. The direction of conventional current is opposite to the direction of electronic current.

The end result of the flow of positive charges is same as the end result of flow of equal negative charges in opposite direction.

CLASS 12-CURRENT ELECTRICITY 6 We can however illustrate by an example that it does not matter which of the two

charge flows constitute the current. For example, in an electric cell, if electrons from left end move to the right end, then they leave that left end with a net positive charge and the right end becomes negatively charged. Similarly, if positive elementary charges drift from the right end to the left end, this leaves the right end with a net negative charge and the left end becomes positively charged. So the end results of movement of both the charges is same. The flow of negative charges has the same result as the flow of positive charges in the opposite direction.

Conventionally, the direction of electric current is taken along the direction of motion of positive charges. When the current is caused by electrons, then the direction of the current is opposite to the direction of electrons.

Flow of electric current through liquids:

Within liquids, no electrons move. However, when a negatively charged and a positively charged electrode is placed in a liquid, it is sets up as an electric current. Under the influence of the electric field, the positively charged ions migrate towards the negatively charged electrode and vice-versa. In the cathode, the positively charged ions gain electrons. At the anode, the negatively charged ions lose the same number of electrons.

CLASS 12-CURRENT ELECTRICITY 7 Thus, in a way number of electrons given by the cathode is equal to the number of electrons accepted by the cathode.

To sum up, we can say that simultaneous movement and discharge of positive and negative ions in the opposite directions constitute the electric current.

|Electric Field |

The electric field is a space in which an electric charge placed at any point in it experiences an electric force.

An electric field may be represented by a number of lines of force, starting from the positive charge and ending at the negative charge.

If a charge of q coulomb is placed at a point and a force of F newton, is experienced, then the electric field E = 𝐅

𝐪

Hence the unit of electric field is 𝐧𝐞𝐰𝐭𝐨𝐧

𝐜𝐨𝐮𝐥𝐨𝐦𝐛 (Nc

-1₎

|Electric Potential|

When a charge is placed in an electrostatic field, it experiences an electrostatic force. Work has to be done in moving a positive charge against the electrostatic force in the field. This work done is stored as potential energy on the charge.

Electric potential at a point is defined as the amount of work done in moving a unit positive charge from infinity to that point against electrical forces.

If W is the work done in moving a charge q from infinity to the point, then potential V at that point is

V = 𝐖

𝐪 The unit of electric potential is volt (

𝐣𝐨𝐮𝐥𝐞 𝐜𝐨𝐮𝐥𝐨𝐦𝐛)

|Potential Difference|

Potential difference between two points is equal to the work done in moving a unit positive charge from one point to the other.

The unit of potential difference is volt.

The potential difference between two points in a circuit is 1 volt if 1 joule of work is done in moving 1 coulomb of charge from one point to other against electric forces. Potential differences can be measured using a voltmeter.

CLASS 12-CURRENT ELECTRICITY 8

|Electromotive force|

In order to maintain a constant current along a conductor, it is necessary to maintain a steady potential difference. The potential difference can be supplied only if some device converts some other forms of energy into electric energy. Such a device is called a source of electromotive force or emf.

A familiar source of emf is the simple cell. Another source of emf is the electric generator. In an electric generator mechanical energy is converted into electrical energy.

Difference between emf and potential difference

EMF Potential Difference

Emf is the potential difference between the terminals of a cell when it is an open circuit i.e., when no current is drawn from it.

A potential difference exists between toe points in c circuit only when current flows through the circuit.

CLASS 12-CURRENT ELECTRICITY 9

Multiple Choice Questions

Q.1. Electric current is the net flow of __________ through a surface. (a) charges

(b) atoms (c) molecules (d) neutrons Ans. (a) charges

Explanation:The electric current is defined as the rate of flow of electric charge through any cross section of the conductor.

Q.2. The unit of electric current is (a) coulomb

(b) volt (c) ampere (d) ohm Ans. (c) ampere

Explanation: An ampere is a unit of measure of the rate of electron flow or current in an electrical conductor. One ampere of current represents one coulomb of electrical charge (6.24 x 1018 _{charge carriers) moving past a specific point in one second. Physicists consider}

current to flow from relatively positive points to relatively negative points; this is called conventional current or Franklin current. The ampere is named after Andre Marie Ampere, French physicist (1775-1836).

Q.3. When 1 coulomb of charge flows through any cross-section of a conductor in one second is known as

(a) 1 ampere of current (b) 1 milli ampere of current (c) 10 ampere of current (d) 0.1 ampere of current Ans. (a) 1 ampere of current

Explanation- The rate of electron flow in an electrical conductor is measured in Ampere. One ampere of current represents one coulomb of electrical charge, i.e. 6.24×1018 _charge

carriers, moving in one second.

Q.4. For measuring smaller amount of current which of the following unit is used? (a) ampere

(b) mega ampere (c) milli ampere (d) kilo ampere

CLASS 12-CURRENT ELECTRICITY 10 Ans. (c) milli ampere

Explanation: Smaller currents in television circuits are measured in milliamperes or microamperes.

1 kA = 103 _A.

1 MA = 106 _A

1 mA = 10-3 _A

Q.5. Only amount of current is measured using a device known as (a) Ammeter

(b) Galvanometer (c) Voltmeter (d) Rheostat Ans. (a) Ammeter

Explanation: The most significant difference between the galvanometer and ammeter is that the galvanometer show both the direction and the magnitude of the current,

whereas the ammeter only shows the magnitude of the current.

Q.6. The actual direction of current is considered as the direction of flow of (a) negative charges

(b) positive charges (c) neutral charges Ans. (a) negative charges

Explanation: In reality, an electric current is nothing but the flow of electrons. Electrons are negatively charged particles and are attracted towards the positive charge. Also, many experiments have revealed that it is free electrons in a conductor that flows. Negatively charged electrons move from the negative terminal to the positive terminal. This is the direction of the actual current flow.

Q.7. Which of the following relations is correct? (a) Q = I × t

(b) Q = I + t (c) Q = I/t (d) Q = t/I

Where Q, I and t represent charge, current and time respectively. Ans. (a) Q = I × t

Explanation- If a net charge Q flows through a cross-section of a conductor in a time t, the electric current I is given by

I = 𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑐ℎ𝑎𝑟𝑔𝑒 𝑝𝑎𝑠𝑠𝑖𝑛𝑔 𝑡ℎ𝑟𝑜𝑢𝑔ℎ 𝑎 𝑐𝑟𝑜𝑠𝑠−𝑠𝑒𝑐𝑡𝑖𝑜𝑛

CLASS 12-CURRENT ELECTRICITY 11

I =

Q.8. Which of the following is correct? (a) 1 A

=

=

=

Explanation: 1 ampere is defined as 1 coulomb of charge per second.

Q.9. Assertion: There is no current in the metals in the absence of electric field. Reason : Motion of free electron is randomly.

A. If both assertion and reason are true and the reason is the correct explanation of the assertion.

B. If both assertion and reason are true but reason is not the correct explanation of the assertion.

C. If assertion is true but reason is false. D. If the assertion and reason both are false. E. If assertion is false but reason is true. Correct Answer: A

Explanation: It is clear that electrons move in all directions haphazardly in metals. When

an electric field is applied, each free electron acquires a drift velocity. There is a net flow of charge, which constitute current. In the absence of electric field this is impossible and hence, there is no current.

Q.10. Assertion : In a simple battery circuit the point of lowest potential is positive terminal of the battery

Reason : The current flows towards the point of the higher potential as it flows in such a circuit from the negative to the positive terminal.

A. If both assertion and reason are true and the reason is the correct explanation of the assertion.

B. If both assertion and reason are true but reason is not the correct explanation of the assertion.

CLASS 12-CURRENT ELECTRICITY 12 C. If assertion is true but reason is false.

D. If the assertion and reason both are false. E. If assertion is false but reason is true. Correct Answer: D

Explanation: It is quite clear that in a battery circuit, the point of lowest potential is the

negative terminal of the battery and the current flows from higher potential to lower potential.

Question 11, 12 and 13 are case study based questions and are compulsory. Attempt any 4-subpart from the questions. Each question carries one mark. Q.11. The current through the acid (i.e., electrolyte of the cell) of the storage cell is opposite direction during charging and

discharging. During charging current flows from positive terminal to the negative terminal through the electrolyte, while during discharging, the current flows from negative terminal to the positive terminal of the cell. The capacity of delivering current by the storage cell, is the product

of the current in ampere that the storage cell can supply and the time in hour during which such supply of current is available without discharging the cell.

(i) The capacity of storage cell is expressed in a) Coulomb hour

b) Ampere hour c) Volt hour d) Watt hour Answer: b) Ampere hour

(ii) The EMF produced between the metallic electrodes dipped in an electrolyte depends on

a) The distance between the electrodes b) The nature of electrolyte

c) The capacity of the electrolyte

d) The position of metals in electro chemical series Answer: d) The position of metals in electro chemical series

(iii) Electrical cell is an arrangement which converts ………. Into electrical energy.

a) Chemical energy b) Radiant energy c) Thermal energy d) Light energy

CLASS 12-CURRENT ELECTRICITY 13 Answer: a) Chemical energy

(iv) A storage cell has to be charged by a d.c. supply. Which terminal of the cell is to be connected to the +ve terminal of the supply?

a) Negative b) Positive

c) Both-positive and negative d) None Answer: b) Positive (v) 1 ampere-hour is equal to a) 3000 coulomb b) 6000 coulomb c) 3600 coulomb d) 1500 coulomb Answer: c) 3600 coulomb .

Q.12. When the starter of a car is operated the lights of the car become dim. This is due to the large current drawn by the starter motor. When a large current is drawn by starter motor, there is a large internal drop in the battery. The internal drop is equal to IR(R being the internal resistance of the battery). Therefore the potential difference across the battery reduces momentarily and the bulb becomes dim.

(i) A 50 volt battery is connected across 10 ohm resistor. The current is 4.5 ampere. The internal resistance of the battery is

a) Zero b) 0.5 ohm c) 1.1 ohm d) 5.0 ohm Answer: c) 1.1 ohm

CLASS 12-CURRENT ELECTRICITY 14 (ii) The emf of a cell is 2 volt; but when a resistor of 10 ohm is connected to it the

potential difference between the terminals of the cell becomes 1.6 volt. The internal resistance of the cell is

a) 2.5 ohm b) 25 ohm c) 0.25 ohm d) 0.025 ohm Answer. 0.25 ohm

(iii) A cell of emf 2V and internal resistance 5 ohm is connected to an external resistance of 15 ohm, the potential difference between the terminal of the cell is a) 0.5 V b) 1 V c) 1.5 V d) 2.0 V Answer. 1.5 V

(iv) A cell of emf E is connected across the resistance r . The potential difference between the terminal of the cell is found to be V. The internal resistance of the cell must be

a)

b)

c)

(v) An external resistance R is connected to a cell of internal resistance r, maximum current flows in the external resistance when

a) R < r, b) R > r c) R = r

d) any other value of R Answer c) R = r

CLASS 12-CURRENT ELECTRICITY 15 Q.13. Electrical power transmitted through AC not DC. Transformer is used for transmitting electrical power from one place to another. Transformer is a device which can increase or decrease the AC voltage (50 cycle per second). We transmit power at high voltage to reduce transmission losses. Therefore, by using step up transformer, we increase the voltage at generating station and step down the voltage at 230 V at the distributing station. No such device available in case of DC. Step-up and step-down is not easily possible in case of DC.

(i) A transformer works on the principle of a) self-induction

b) electrical inertia c) mutual induction

d) magnetic effect of the electrical current Answer. c) mutual induction

(ii) For an ideal-step-down transformer, the quantity which is constant for both the coils is

a) current in the coils b) voltage across the coils c) resistance of coils d) power in the coils Answer: d) power in the coils

(iii) What is the frequency of ac mains in India? a) 120 Hz

b) 60 Hz c) 50 Hz d) 30 Hz

CLASS 12-CURRENT ELECTRICITY 16 Answer: c) 50 Hz

(iv) Which of the following effects is not shown in alternating current? a) Chemical effect

b) Magnetic effect c) Heating current d) All of these Answer a) Chemical effect

(v) Which quantity is increased in a step-down transformer? a) Current

b) Voltage c) Power d) Frequency Answer. a) Current

Q.14. An ammeter measures a current of 1.5 A is an electrical circuit. If the circuit is closed for 20 seconds, what is the amount of electric charge that flows through the circuit? (a) 30 C (b) 10 C (c) 20 C (d) 15 C Ans. (a) 30 C Explanation- Q = It = 1.5 × 20 = 30 C

Q.15. The work done in moving a unit charge from one point to another point in an electric circuit is called

(a) electric potential difference (b) electric current

(c) electrical power (d) electrical resistance

Ans. (a) electric potential difference

Explanation: Potential difference between two points is equal to the work done in moving a unit positive charge from one point to the other.

Q.16. The unit of electric potential difference is (a) coulomb

(b) volt (c) joule (d) watt

CLASS 12-CURRENT ELECTRICITY 17 Ans. (b) volt

Q.17. Which of the following measures the potential difference in an electric circuit? (a) switch (b) ammeter (c) voltmeter (d) galvanometer Ans. (c) voltmeter

Explanation: A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes a negligible current from the circuit.

Q.18. How much work is done in moving a charge of 1 C from infinity to a point in an electric field at a potential of 1 V.

(a) 0.5 joule (b) 0.1 joule (c) 1 joule (d) 10 joule Ans. (c) 1 joule

Q.19. Other name of electric potential difference is: (a) wattage

(b) voltage

(c) potential energy (d) work

Ans. (b) voltage

Q.20. Expression for potential difference is :

(a) Potentail Difference = 𝑊𝑜𝑟𝑘 𝐷𝑜𝑛𝑒

𝑄𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝐶ℎ𝑎𝑟𝑔𝑒𝑑 𝑀𝑜𝑣𝑒𝑑 (b) Potentail Difference = 𝑄𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝐶ℎ𝑎𝑟𝑔𝑒𝑑 𝑀𝑜𝑣𝑒𝑑

𝑊𝑜𝑟𝑘 𝐷𝑜𝑛𝑒

(c) Potentail Difference = Quantity of Charge Moved × Work Done (d) Potentail Difference = Quantity of Charge Moved + Work Done

Ans. (a) Potentail Difference = 𝑊𝑜𝑟𝑘 𝐷𝑜𝑛𝑒

𝑄𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝐶ℎ𝑎𝑟𝑔𝑒𝑑 𝑀𝑜𝑣𝑒𝑑

CLASS 12-CURRENT ELECTRICITY 18 Q.21. Which of the following provides the potential difference required for the flow of current in an electric circuit?

(a) Galvanometer (b) Ammeter (c) Voltmeter (d) Electric cell Ans. (a) Electric Cell

Q.22. Which of these is a correct expression for 1 volt? (a) 1 volt = 1 𝑗𝑜𝑢𝑙𝑒

1 𝑐𝑜𝑢𝑙𝑜𝑚𝑏 (b) 1 volt = 1 𝑐𝑜𝑢𝑙𝑜𝑚𝑏_{1 𝑗𝑜𝑢𝑙𝑒}

(c) 1 volt =1 joule × 1 coulomb (d) 1 volt =1 joule + 1 coulomb Ans. (a) 1 volt = 1 joule 1 coulomb

Q.23. How much work is done in moving a charge of 1 C from one point to another point with a potential difference of 1.5 V?

(a) 1.5 J (b) 0.66 J (c) 1.5 A (d) 1 A Ans. (a) 1.5 J Explanation: W = V × Q

Q.24. Current in a circuit flows:

(a) in a direction from high potential to low potential (b) in a direction from low potential to high potential (c) in a direction of flow of

(d) in any direction

Ans. (a) in a direction from high potential to low potential

Q.25. In a metallic conductor, electric current is thought to be due to movement of: (a) ions (b) amperes (c) electrons (d) protons Ans. (c) electrons

CLASS 12-CURRENT ELECTRICITY 19 Q.26. Assuming that the charges of an electron is 1.6 × 10-19_{coulombs, the number}

of electrons passing through a section of wire per sec, when the wire carries a current of one ampere is :

(a) 0.625 × 105 (b) 1.6 × 10-19 (c) 1.6 × 1019 (d) 0.627 × 105 Ans. (a) 0.625 × 105_.

Theoretical Questions with Answers

Very Short Answer Type Questions: [1 Mark]

Q.1. Name the S.I. Unit of charge. Ans. S.I unit charge is coulomb (C).

Q.2. What is the magnitude of charge of an electron? Ans. Charge on an electron is −1.6 × 10–19 _coulomb. Q.3. Define electric current.

Ans. It is the net flow of electric charge through a cross-section of conductor per unit time.

I = Amount of charge passing through a cross−section Time taken

⇒

I =

Q.4. State the S.I unit of electric current.

Ans. S.I. unit of electric current is ampere or coulomb/second. Q.5. Define ampere (A).

Ans. Ampere is the current when one coulomb of electric charge flows through a cross-section in one second.

Q.6. Express one milliampere and microampere in ampere unit. Ans. 1 milliampere (mA) = 10–3 _A

1 microampere (μA) = 10–6 _A

Q.7. Name the instrument that is used to measure the value of electric current. Ans. Ammeter.

Q.8. In an electric circuit, state the relationship between the direction of conventional current and the direction of flow of electrons.

CLASS 12-CURRENT ELECTRICITY 20 Ans. In an electric circuit, the direction of the electric current is taken as opposite to

the direction of the flow of electrons.

Q.9. Why closed path is required for the flow of current?

Ans. Closed path is required for the flow of current because if the path is

not closed, the circuit will break. Which will mean that between the two ends air is present. It will not allow the flow of charges through it and the current will

not flow.

Q.10. What causes the potential difference between the two terminals of a cell?

Ans: The flow of current or charge causes the potential difference between the two terminals of a cell.

Q.11. Define potential difference between two points in a conductor.

Ans. Potential difference between two points in an electric field is defined as the amount of work done in moving a unit charge from one point to another. Potential difference between two points A or B.

VA – VB = WQ

Q.12. Which physical quantity expressed in the joule/coulomb? Ans. Potential difference is expressed in joule/coulomb.

Q.13. What is the S.I. unit of electric potential? Ans. Volt or joule/coulomb.

Q.14. A charge of 2C moves between two plates, maintained at a potential difference of 1 V. What is the energy acquired by the charge?

Ans. W = Q × V

W = 2 × 1 = 2 joule

Q.15. What is meant by the statement “Potential difference between points A and B in an electric field is 1 Volt”?

Ans. It means 1 J of work is done in moving 1 C of charge from A to B in an electric field.

Q.16. Name the instrument used to measure the potential difference. Ans. Voltmeter.

Q.17. What is the source of electric charge? Ans. The cells or battery.

Short Answer Type Questions [2 Marks] Q.1. What do you understand by static and current electricity?

Answer: Static Electricity: Static electric charges, present on the surface of a conductor constitute static electricity.

CLASS 12-CURRENT ELECTRICITY 21 Current electricity: The motion or dynamics of charges gives rise to current electricity.

Q.2. Calculate the number of electrons constituting one coulomb of charge. Answer. Charge (Q) = no. of electrons (n) × charge on one electron (e–₎

1 coulomb = n × (1.66 × 10–19₎

n = 1 /1.66 ×10-19 _{= 6.25 × 10}18 _{≈ 6 × 10}18

Q.3. A current of 0.5 A is drawn by a filament of an electric bulb for 10

minutes. Find the amount of electric charge that flows through the circuit.

Answer. We are given, I = 0.5 A t = 10 min = 600 s

We have, I = Qt

Q = I × t = 0.5 A × 600 s = 300 C

Q.4. If an electric charge of 1500 C flows through an electric iron for 5 minutes, find the electric current drawn by filament of electric iron. Answer. Given, electric charge (Q) = 1500 C

Time (t) = 5 min = 5 × 60 = 300 s Electric current (I) = ?

We know, I = Q/t

⇒ I = 1500 C/ 300 s = 5 A Q.5. Define the term “volt”.

Answer.The potential difference between two points A and B is said to be one volt if 1 joule of work is done to move 1 coulomb of charge from one point.

Q.6. Electric current was considered to be the flow of positive charge and the direction of flow of positive charge was taken to be the direction of electric current. Why?

Answer. This is because electrons were not known at the time when the phenomenon of electricity was first observed. Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negatively charges.

Q.7. Calculate the work done when a charge of 5 C moving across two points having potential difference equal to 15 V.

Answer. Given, Potential difference (V) = 15 V Charge (Q) = 5 C

Work done (W) = ? We know that, V = W/Q

CLASS 12-CURRENT ELECTRICITY 22 ⇒ W = V × Q

⇒ W = 15 V × 5C = 75 J

Q.8. What potential difference is required to do 100 J of work to carry a charge of 10 C between two points?

Answer. Given, Work done (W) = 100 J Charge (Q) = 10 C

Potential difference (V) =? We know that, V = W/Q Or, V = 100 J/ 10 C = 10 V

Q.9. How much charge is transferred between two points having potential difference equal to 220 V, when 1760 J of work is done?

Answer. Given, potential difference (V) = 220 V Work done (W) = 1760 J

Charge (Q) = ?

We know that, V = W/Q ⇒ Q = W/V

⇒ Q = 1760 J /220 V = 8 C

Q.10. Why is the terminal voltage of a cell less than its e.m.f.?

Answer. Terminal voltage of a cell less than emf because some current however small, may be is drawn to measure terminal voltage due to internal resist ance of the cell.

Q.11. How does the random motion of free electrons get affected when a potential difference is applied across its ends?

Answer. Random motion of free electrons gets directed towards the point at a higher potential.

Short Answer Type Questions

[3 Marks]

Q.1. Define electric current. Give its unit and conventional direction of flow. Answer: Electric current is defined as rate of flow of electric charge or electric charge flowing through a conductor per second.

Electric current = Electric Charge Time Or, I = Q

t

The unit of electric current is ampere (A).

One ampere: if 1 coulomb of charge flows through a point in one second, then the current is one ampere.

CLASS 12-CURRENT ELECTRICITY 23 Or 1 ampere = 1 coulomb_{1 second}

Conventional direction of current- The conventional direction of electric current is taken as the direction of flow of positive charge(i.e. opposite to the direction of flow of electrons).

Q.2. Find the amount of electric charge flowing through the circuit if an electric current of 5 A is drawn by an electric appliance for 5 minutes. Answer. Given, electric current (I) = 5 A

Time (t) = 5 minute = 5 × 60 = 300 s Electric charge (Q) = ?

We know; I = Q/t ⇒ Q = I × t

⇒ Q = 5 A × 300 s = 1500 C

Q.3. (a) Define electric current. What are the units of electric current?

(b) How many electrons pass a point when a current of 0.4 A flows for 800 seconds?

Answer. (a) Electric current is defined as the rate of flow of the electric charge through any cross section of a conductor per unit time.

Electric current = Charge/Time Ampere is the S.I. unit of current.

1 ampere = 1 coulomb/1 second 1 A = 1 C/1s = 1 Cs-1

Other units of electric current are milliampere, microampere 1 milliampere = 1 mA = 10-3 _A

1 microampere = 1 μA = 10-6 _A (b) We know that, I = Qt

o r, Q = I × t = 0 .4 × 800 C

Number of electrons = Total Charge Charge of One Electrons Number of Electrons = 0.4 × 800/(1.6 × 10-19_{)= 2 × 10}21

Q.4. What do you mean by electric potential and potential difference?

Answer. The work done to move a unit charge from infinity to a given point in an electric field is called electric potential or potential at a point. Hence, potential can be measured in term of the work done in moving a charge against the force of repulsion. It is a scalar quantity.

Electric potential difference between two points in an electric circuit is the work done to move a unit charge from one point to the other.

CLASS 12-CURRENT ELECTRICITY 24 Then electric potential difference (V) = W/Q

The SI unit of electric potential difference is volt (V). The potential difference is measured by means of an instrument called the voltmeter.

Q.5. What does an electric circuit mean? Name a device th at helps to maintain a potential difference across a conductor in a circuit. When do we say that the potential difference across a conductor is 1 volt? Calculate the amount of work done in shifting a charge of 2 coulombs from a point A to B having potentials 110 V and 25 V respectively.

Answer. Electric circuit: The close path along which an electric current flows is called an ‘electric circuit’. The device that helps to maintain a potential difference across a conductor in a circuit are —

Electric cell, electric battery, electric generator

1 Volt : The potential difference between two points in an electric field is said to one volt if one joule of work has to be done in bringing a positive charge of one coulomb from one point to another in an electric field.

1 volt = 1 1 Joule Coulomb

Work done = V (Potential difference) × Q (charge) W = V × Q

= 85 × 2 = 170 Joule.

Long Answer Type Questions

[5 Marks]

Q.1. What is meant by electric current? Name and define its SI unit. In a conductor electrons are flowing from B to A. What is the direction of conventional current? Give justification for your answer. A steady current of 1 ampere flows through a conductor. Calculate the number of electrons that flows through any section of the conductor in 1 second. (Charge in electron

1.6 × 10–19 _coulomb).

Ans. Electric Current : The amount of charge ‘Q’ flowing through a particular area of cross section in unit time ‘t’ is called electric current. i.e.

Electric current, I = Q/t

SI unit of electric current is ampere.

One ampere of current is that current which flows when one coulomb of electric charge flows through a particular area of cross-section of the conductor in one second, i.e. 1 A = 1 Cs–1_.

The direction of conventional current is A to B, i.e. opposite to the direction of flow of electrons. In a metal, flow of electrons carrying negative charge constitutes the current. Direction of flow of electrons gives the direction of electronic current by

CLASS 12-CURRENT ELECTRICITY 25 convention, the direction, the direction of flow of positive charge is taken as the direction of conventional current.

Charge, q = It = ne n = It 𝑒

=

=

Q.2. What do you understand by the e.m.f. of a cell? How is it related with current and resistance. Give some mechanical analogy for e.m.f. and current flow.

Answer: The electromotive force (e.m.f.) between two points is defined as the work done in taking unit positive charge from one point to another. The e.m.f. of the source is equal to the potential difference between two terminals, when no current is drawn from the source.

The e.m.f. of a cell or a battery is the force required in moving a unit positive charge from its negative to positive terminal inside the cell. In an electrochemical cell, the work is done by the release of energy in a chemical reaction. This reaction involves charges and takes place in such a way that the energy is released is transferred to the charges. Each charge gets fixed potential energy in this process. The potential acquired by a charge divide by the charge, is the e.m.f. when the electric current through the cell is zero.

It is equal to the work done (W) in taking a test charge q from one electrode of cell to another, divided by the charge q, thus

e.m.f. = Work done Charge = W

q

Mechanical analogy for e.m.f. and current

Let us consider a man dropping iron balls a steady rate through a vertical column of viscus liquid. When balls reach the bottom of the column, they are being picked up he man and are again transferred to the top. The balls thus go through the current at a steady state. This is similar situation as the charges flow through the conductor, making a steady current.

The man consider as a source of e.m.f. ( e.g. of a cell) which exists between the bottom and top of the liquid column.

The balls falling through a viscus liquid are equivalent to the charges drifting through a conductor at a steady speed.

When we are connect a source of e.m.f. V to a conductor, steady current I flows through it. The ratio of V and I i.e. V

I is called resistance of the conductor. The unit of e.m.f. is volt, while the unit of current is an ampere. The ratio of these two is called ohm which is the unit of resistance.