MULTIPLE CHOICE QUESTIONS13.2 Do bends in a wire affect its electrical resistance? Explain. Ans: As...

[CHAPTER-13 CURRENT ELECTRICITY] www.profaminz.com

BY: Prof. Muhammad Amin 1

MULTIPLE CHOICE QUESTIONS (1) In carbon resistor the gold band indicates tolerance of

(a) 5% (b) 2%

(c) 20% (d) 10% (2) The work done to move a positive charge from one point to another point given by

(a) W Q V (b) Q

WV

(c) V

WQ

(d) 1

WV Q

(3) Which one is used to determine the internal resistance of a cell (a) ammeter (b) voltmeter (c) galvanometer (d) potentiometer

(4) Slide wire bridge is practical form of (a) ammeter (b) Wheatstone bridge (c) galvanometer (d) potentiometer

(5) The current flowing towards a point is taken as (a) positive (b) negative (c) zero (d) neutral

(6) For an open circuit (a) E = Vt (b) E > Vt (c) E < Vt (d) E = Vt + Ir

(7) 1 J/C = (a) volt (b) farad (c) ampere (d) ohm

(8) A thermistor is heat sensitive (a) resistor (b) capacitor (c) inductor (d) diode

(9) The temperature coefficient of resistance of a material is expressed as

(a) o t

o

R R

R t

(b) o t

o

R R

R t

(c) t o

o

R R

R t

(d) t o

oR t

(10) If the resistances R1, R2 and R3 are connected in series then their equivalent resistance is given by

(a) 1 2 3eqR R R R (b) 1 2 3

1 1 1 1

eqR R R R

(c) 1 2 3

1 1 1 1

eqR R R R (d) 1 2 3eqR R R R

(11) Which instrument measure the potential difference correctly

(a) CRO (b) potentiometer

(c) voltmeter (d) both a and b

(12) Two resistors each of value 10 ohm are connected in series, the equivalent resistance will be

(a) 10 Ω (b) 20 Ω

(c) 5 Ω (d) zero Ω

(13) The resistance of silicon decreases with

(a) increase in temperature (b) decrease in temperature

(c) increase in viscosity (d) decreases in velocity

(14) Whenever current is drawn from a cell, its terminal potential difference and e.m.f. become.

(a) Different (b) Same

(c) Zero (d) Negative

(15) A rheostat can operate as a:

(a) Transformer (b) Amplifier

(c) Oscillator (d) Potential divider

(16) The value of temperature coefficient of resistance of most thermistors is:

(a) Positive (b) Negative



(c) Zero (d) Infinity

(17) If the conductivity of a material is small then it is:

(a) A conductor (b) A poor conductor

(c) A good conductor (d) An insulator

(18) The power output of a lamp is 6 W. How much energy does the lamp give out in 2 minutes?

(a) 3 J (b) 12 J

(c) 120 J (d) 720 J

(19) Electrical energy is measured in (a) Watt (b) Horse power (c) Kilowatt (d) Kilowatt hour

(20) Maximum power delivered by battery is

(a) 2

max

EP =

4r (b) Pmax = 4rE2

(c) Pmax = VI (d) unlimited

(21) Power dissipation is expressed by

(a) V

P =R

(b) P = RV

(c) P = RV2 (d) P = I2R

(22) The algebraic sum of potential changes for a closed circuit is: (a) Positive (b) Negative (c) Zero (d) greater than unity

(23) Free electrons are: (a) tightly bound (b) fixed (c) loosely bound (d) tightly fixed

(24) Which one is used to determine the internal resistance of a cell? (a) ammeter (b) voltmeter (c) galvanometer (d) potentiometer

(25) Siemen is the unit of

(a) resistance (b) conductance (c) resistivity (d) conductivity

(26) The resistivity of a conductor increases with (a) increase in its length (b) increase in its area (c) increase in its temperature (d) decrease in its length

(27) Current flows in the gases due to (a) electrons only (b) electrons and ions

(c) positive and negative ions (d) electrons and holes

(28) The ‘emf’ is always _______ even when no current is drawn through the battery of cell. (a) Zero (b) Present (c) Absent (d) Maximum

(29) The graphical representation of Ohm’s law is (a) Hyperbola (b) Ellipse (c) Parabola (d) Straight line

(30) A student has five resistances each of value1

5. The minimum resistance that can be obtained by

combining them in parallel is

(a) 1

50 (b)

1

25 (c)

1

10 (d) 5

(31) The substance having negative temperature co-efficient is:

(a) Carbon (b) Iron (c)Tungsten (d)Gold

(32) Resistivity at a given temperature depends upon :

(a) Area of cross-section (b) Length

(c)Nature of material of conductor (d) Both length and area

(33) Temperature co-efficient of Resistivity permeability is

(a) k (b) m (c) 1k (d) k

(34) The drift velocity of an electron in a conductor in the presence of electric field is of the order of

(a) 2 110 mS

(b) 3 110 mS

(c) 6 110 mS

(d) 7 110 mS

(35) Specific resistance of a material depends upon:

(a) Length (b) Area



(c) Temperature (d) Both A & B

(36) If the resistance of 500 have forth band of silver colour then its upper maximum resistance

will be:

(a) 600 (b) 550 (c) 450 (d) 400

(37) One Ohm is equal to:

(a) VC-1 (b) CV-1 (c) AC-1 (d) VA-1

(38) The temperature co-efficient of resistance for Ge is

(a) Positive (b) Negative

(c) All of these (d) None of these

(39) Resistivity is reciprocal of

(a) Conductance (b) Conductivity

(c) Induction (d) None of these

(40) Kirchoff’s first rule is manifestation of law of conservation of

(a) Mass (b) Energy

(c) Momentum (d) Charge

(41) Maximum power delivered by battery is

(a) 2

max

EP =

4r (b) 2

maxP = 4rE

(c) maxP = VIt (d) Unlimited

(42) The drift velocity of electrons in a conductor is of the order of

(a) 10-3ms-1 (b) 10-2ms-1

(c) 103ms-1 (d) 10-4ms-1

(43) The heat produced by the passage of current through a resistor

(a) H = I2Rt (b) H = IR2t

(c) 1

H =Rt

(c) 2I

H =Rt

(44) A wire of uniform area of cross – section A and length L is cut into two equal parts. The

resistance of each part becomes

(a) Double (b) Half

(c) 4 time (d) ¼ time

(45) A 100 watt bulb is operated by 200 volt, the current flowing through the bulb is

(a) 0.5 A (b) 1 A

(c) 2A (d) 2.5A

(46) One coulomb of charge is created by

(a) 10 Electrons (b) 1.6 10-19 electrons

(c) 6.25 1018 electrons (d) 6.25 1021 electrons

(47) The electrical analog of mass in electricity is

(a) Capacitance (b) Inductance

(c) Charge (d) Resistance

(48) If the conductivity of a material is high, then it is

(a) An insulator (b) A semi conductor

(c) A good conductor (d) A super conductor

(49) For an open circuit, the current flowing through circuit will be:

(a) Infinite (b) Finite (c) Maximum (d) Zero

(50) The colour of strips on a carbon resistor from extreme left are yellow, black and brown

respectively its resistance is

(a) 4K (b) 400 (c) 40 (d) 40 K

(51) The terminal potential difference of a battery of internal resistance “r” and emf “” is

(a) V= + Ir (b) V = Ir (c) ε r

VI

(d)

IV

ε r

(52) If the current through a resistance is halved, then the

(a) Power is halved (b) Heat dissipated is halved

(c) Potential difference is halved (d) Resistance is doubled



(53) Electromotive force is most closely related to:

(a) Electric field (b) Potential difference

(c) Potential energy (d) Magnetic field

(54) Charge carriers in electrolytes are:

(a) Protons (b) Electrons

(c) Holes (d) Positive and Negative ions

(55) Ampere second stands for the unit of:

(a) charge (b) emf

(c) energy (d) power

(56) The unit of electromotive force is:

(a) Newton (b) Pascal

(c) Volt (d) Ampere

(57) The SI unit of temperature coefficient of resistivity is.

(a) Ohm–m (b) K–1 (c) K (d) Ohm

(58) The terminal potential difference of a short-circuit battery of emf E is equal to

(a) 2E (b) E

(c) E

2 (d) 0

(59) Substances like germanium, Silicon have:

(a) Positive temperature coefficient (b) Negative temperature coefficient

(c) Both ‘a’ and ‘b’ (d) None of these

(60) Three resistances 5000, 500 and 50 Ohms are connected in series across 550 V mains. The

current flowing through them will be:

(a) 1 A (b) 100 mA

(c) 10 mA (d) 1 mA



SHORT QUESTIONS 13.1 A potential difference is applied across the ends of a copper wire. What is the effect

on the drift velocity of free electrons by.

(i) Increasing the potential difference.

(ii) Decreasing the length and the temperature of the wire

Ans: (i) As V = Ed or V Eµ if d = constant.

Above relation shows that by increasing the potential difference V, E will also increases.

,

,

As F qE

or F eE

so F E

As E increases, force F on the free electrons will also increase Hence, the drift velocity

will increase.

(ii) As V = E d or V 1

d= d E E

µ (if V is constant)

If the length d of the wire is decreased, Electric field intensity E increases. So the drift

velocity will increase.

Decreasing temperature decreases the amplitude of vibration of the atoms. Then the

chance of collision of free electrons with atoms decreases. Hence the drift velocity

increases

13.2 Do bends in a wire affect its electrical resistance? Explain.

Ans: As L

RA

Electrical resistance of a wire depends upon its (i) nature (ii) length (iii) area of cross section

Any bend in a wire do not have any affect on the length, area of cross section or nature of

wire. So the resistance will not be affected.

13.3 What are the resistances of the resistors given in the figures A and B? What is the

tolerance of each? Explain what is meant by the tolerance?

Ans:-

ST

ND

RD

TH

1 Band = Yellow = 4

2 Band = White = 9

3 Band = Orange = 00 o

4 Band = Silver = 10%

So, R = 49000 10%W±

Tolerance: Tolerance means the possible variation of a resistance from the marked

value. For example, a 1000resistance with a tolerance of 10%± will have an actual

resistance anywhere between 900 and 1100

First band = Brown = 1

2ND band = Green = 5

3RD band = Red = 00

4TH band = Gold = 5 %

So, R = 1500 5%



13.4 Why does the resistance of a conductor rise with temperature?

Ans: As the temperature of the conductor rises, amplitude of the vibration of atoms of the

lattice increases. As a result, collision of free electron with themselves & with the

vibrating atoms increases and so resistance of the conductor increases.

13.5. What are the difficulties in testing whether the filament of a lighted bulb obeys

Ohm’s law?

Ans: When current is passed through the filament of a bulb, its temperature increases, so

resistance increases. Hence filament of a bulb does not obey ohm’s law, ohm’s law is

only obeyed if the resistance is kept constant.

13.6 Is the filament resistance lower or higher in a 500W, 220V light bulb than in a

100W, 220V bulb?

Ans: According to given data

P1 = 500W V1 = 220V

P2 = 100 V2 = 220 V

We know that 2

2

2

500

2

100

VP=

RV

R=P

(220)in first bulb R = =96.8Ω

(500)(220)

in second bulb R = =484Ω(100)

Therefore resistance of 500W bulb is less than that of 100W bulb.

13.7 Explain why the terminal potential difference of a battery decreases when the

current drawn form it is increased?

Ans: We know that terminal potential is given as:

Vt = E – Ir

When current drawn I increases then factor Ir” increases. Hence terminal P.D decreases.

13.9. What is Wheatstone bridge? How can it be used to determine

an unknown resistance?

Ans: Wheatstone Bridge consist of four resistances R1,R2,R3 andR4

connected in such a way to form a mesh ABCDA. When the

bridge is balanced electrically, no current flows through the

galvanometer.

In this situation. 31

2 4

RR

R R If one of the resistance R4 = X is

unknown resistance and the other resistances R1, R2, and R3, are

known, then

31 23

2 1

RR Ror X R

R X R

So X can be found.

13.8 Describe a circuit which will give a continuously varying

potential.

Ans: Potential divider circuit can provide continuously varying

potential. The circuit is shown in Fig. voltage V is applied

across A and B of the rheostat with the help of a battery. The

current passing through R is given by VI=

R. The potential

difference between the points BC is

tanBC

BC

BC

BC

V current resis ce

V I r

V rV r V

R R

rV V

R



This circuit can provide a varying potential difference from 0 to V.

As the sliding contact C is moved towards the end B, The length and hence the resistor r

of the portion of wire decrease.

As BC

rV = V

R

Hence, VBC decreases

If C is moved towards the end A, r increases hence output voltage increase

ADDITIONAL SHORT QUESTIONS (1) Differentiate between EMF and P.D?

EMF POTENTIAL DIFFERENCE

It is a potential difference across the source when no current flows

It is a potential difference across the source when some current flows.

emf is the cause. Potential Difference is a effect of emf.

It can never be zero. It may be zero

It is given by the relation. E = V + Ir

It is given by the relation. V = E – Ir

Voltmeter connected with source measures emf with open circuit.

Voltmeter connected with source measures Potential Difference with closed circuit.

(2) How current flows through a metallic conductor.

Ans: In the absence of electric field, the free electrons in conductor are in random motion,

which depends upon temperature.

when electric field is set up in a conductor by a source which establishes constant

potential difference across the conductor, the free electrons modify their random motion

in such a way that they drift slowly opposite to field. Thus a net directed motion of

charges takes place along the wire and a current begins to flow through it.

(3) What is series and parallel combinations of resistors. Ans:

SERIES COMBINATION PARALLEL COMBINATION

Resistances connected end to end

are called series combination of

resistance. i.e.

Resistances connected side by side are

called parallel combination of resistance.

i.e.

There is single path of current. There are more than one path of current.

Current through each resistance is

same i.e. 1 2 3I I I I

Sum of currents through all resistances is

equal to the net current of the circuit. I.e.

1 2 3I I I I

Sum of potential difference across

each is equal to the potential

difference of the battery i.e.

1 2 3V V V V

Potential difference across each resistance

is same as that of battery i.e.

1 2 3V V V V

Equivalent resistance of series

combination is

eq 1 2 3R R R R .........

Equivalent resistance of parallel

combination is

eq 1 2 31 R 1 R 1 R 1 R .........

Equivalent resistance is greater

than the maximum of all

resistances combined in series i.e.

eq 1 eq 2 eq max.R R ,R R ...........R R

Equivalent resistance is less than the

minimum of all resistance is less than the

minimum of all resistances combined in

parallel i.e.

eq 1 eq 2 eq max.R R ,R R ...........R R



(4) What is difference between ohmic and non-ohmic substances? Ans:

OHMIC SUBSTANCES NON OHMIC SUBSTANCES Ohmic substances obey the Ohm’s law. Non ohmic substances do not obey

ohm’s law.

Graph between V and I for ohmic substances is straight line or linear.

Graph between V and I for non ohmic substances is not straight line i.e. it is a curve.

Slope of graph remains same at all points.

Slope of the graph is not same at all points.

Examples of ohmic substances are pure metals.

Examples of non ohmic substances are tungsten filament and semi conductor diode.

Graphs of ohmic substances for V vs I are as:

Graphs of none ohmic substances for V vs I are as:

(5) What is the temperature co-efficient of resistivity?

Ans: The fractional change in resistivity per Kelvin is known as temperature co-efficient of

resistivity.

Mathematically it can be written as

t o

o

ρ -ρα =

ρ t

Where oρ is the resistivity of the material at 0o C and tρ is the resistivity at toC, and t is

the given change temperature.

Unit:- The SI unit of temperature co-efficient of resistivity is K-1

(6) What is the difference between conventional current and electronic current.

Ans: Conventional current:-

It is defined as that current which passes from a point at higher potential to a point at a

lower potential as if it represents the movement of positive charges.

Electronic current:-

The current due to motion of negative charges (or electrons) that flows from the negative

terminal of the battery to the positive terminal in the electrical circuit is called electronic

current.

(7) State Kirchhoff’s 1st and 2nd rule.

Ans: Kirchhoff’s First Rule

It states that algebraic sum of currents meeting at a point is zero. This is called

Kirchhoff’s current law. Kirchhoff’s 1st rule is also known as Kirchhoff’s point rule.

∑I = 0

Kirchhoff’s Second Rule

It is stated as:

Algebraic Sum of voltage changes across a closed circuit or loop

must be equal to zero.

According to law of conservation of energy, the total change in

energy of our system is zero.

Thus +E1 Q - IR1 Q - E2 Q - IR2 Q = 0

Or E1 – IR1 – E2 – IR2 = 0

(8) Name two different effects of current.

Ans: Effects of Current:

Heating effect

When current passes through a conductor due to resistance some of electrical energy

is converted into heat.



i.e H = I2 R t

where I = current, R = resistance and t = time

Magnetic effect

It is intrinsic property that when a current passes through a conductor a magnetic field

is produced around it.

Chemical effect

Certain liquids such as dilute sulphuric acid or copper sulphate solution conducts

ekctric current due to some chemical reactions, this process is called electrolysis.

(9) How can you compare emf of two cells with the help of a potentiometer?

Ans: Suppose E1 & E2 are emf’s of two cells to be compared. For this purpose potentiometer is

balanced by connecting E1 & E2 separately. Let l1 is the balancing length for E1 & E2 is

the balancing length for E2. These two emf’s will be compared by the following formula:

1 1

2 2

E

E

l

l

That is emf is proportional to balancing length in potentiometer.

(10) What is resistivity and how it depend upon temperature.

Ans: Resistivity: It is defined as

“The resistance of a meter cube of a material.”

RA

L

It unit is m

Dependence on Temperature: Within certain limits:

(i) Resistivity increases with increase of temperature of conductor

(ii) Resistivity decreases with increase of temperature of semiconductor.

(iii) Resistivity remains constant with change of temperature of insulator.

(11) Write down the value of equivalent resistance for three resistors R1, R2 and R3 when

joined in (a) series, and (b) parallel.

Ans: (a) Series:

In case of series joining, the equivalent resistance R will be

e 1 2 3R R R R

(b) Parallel:

In case of parallel joining, the equivalent resistance R is found as follows:

e 1 2 3

2 3 1 3 1 2

e 1 2 3

1 2 3e

1 2 2 3 3 1

1 1 1 1

R R R R

R R R R R R1

R R R R

R R RR

R R R R R R

MULTIPLE CHOICE QUESTIONS13.2 Do bends in a wire affect its electrical resistance? Explain. Ans: As...

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