XB ELECTRICITY PART II

•             Electricity

Electricity is a controllable and convenient form of energy used widely in our daily use. 

Flowing of water=current of water

Flow or electric charges (electrons/ions) through a conductor produce electric current. 

In a torch Cell provide the

In an electric circuit, the direction of electric current is taken as opposite to the direction of the flow of electrons.

•             Electric potential and potential difference

  Electric charges flow through the conductor only if there is a potential difference between the two points.

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

Potential difference (V) between two points= work done (W) / charge Q,

i.e.,

   V = W/Q

SI unit of electric potential differences is volt V, named after Alessandro Volta.

One volt is the potential difference between two points in a current carrying conductor when one joule of work is done to move a charge of one coulomb from one point to the other.

Therefore, one volt = one joule/one coulomb

1V = 1 J C-1.

The potential difference is measured by an instrument called voltmeter.

(Study symbols of some commonly used components in circuit diagrams from page 203).

•             Ohm’s Law

Refer Activity 12.1 NCERT

Electric current flowing through a metallic wire is directly proportional to the potential difference V across its ends provided its temperature remains the same. This is called Ohm’s law,

i.e., V ∞ I

or V/ I = constant

            = R

Or V    = IR

Different components offer different resistance for the flow of electric current. 

Again, flow of electrons is restrained by the attraction of atoms among which they move.  So the motion of electrons through a conductor is retarded by its resistance.

A component of a given size that offers a low resistance is called a resistor.

A component of same size that offers a higher resistance is a poor conductor. 

A component of same size offering even higher resistance will be an insulator.

Resistance of a conductor depends on its

a.            Length

b.            Area of cross-section

c.             Nature of material

         Resistance of a uniform metallic conductor is directly proportional to its length (l) and inversely proportional to the area of cross-section (A). 

That is,      

                   R ∞l

           and  R ∞ l/A

Combining the above equations

                   R ∞ l/A

Or,              R = ρ l/A

Where ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor.

The SI unit of resistivity is Ωm.  It is a characteristic property of the material.

The metals and alloys have very low resistivity in the range of 10-8 Ωm to 10-6 Ωm.

They are good conductors of resistivity.

Insulators like rubber and glass have resistivity of the order of 1012   to  1017  Ωm.

Both the resistance and resistivity of a material vary with temperature.

•             Resistance of a system of resistors

Ohm’s law can be applied to different combination of resistors. 

Resistors can be joined in two ways, i. e., in series and in parallel.

1.Resistors in series:   

Refer fig.12.6, page 210. 

In this figure, three resistors having resistance R1, R2 and R3 are joined end to end, i.e., the resistors are connected in series in a circuit.

Here, the value of current in the ammeter is the same independent of its position. 

If a voltmeter is inserted across the ends xy of the series combination of three resistors, the potential difference V will be equal to the sum of potential differences V1, V2 and V3. That is the total potential difference across a combination of resistors in series is equal to the sum of potential difference across the individual resistors. That is V = V1 + V2 + V3.

In the electric circuit shown in fig. 12.8, let I be the current through the circuit.  The current through each resistor is also I.  It is possible to replace the three resistors joined in series by an equivalent single resistor of resistance R such that the potential difference V across it and the current I through the circuit remains the same.  If the Ohm’s law is applied to the entire circuit, we have V= IR.  On applying Ohm’s law to the three resistors separately, we further have

V1= IR1

V2 = IR2

V3=IR3

Therefore, IR = IR1 + IR2  + l IR3 or

RS = R1 + R2 + R3

In conclusion, when several resistors are joined in series, the resistance of the combination RS equals the sum of their individual resistances R1, R2, R3, and is thus greater than individual resistors.

2.Resistors in parallel

(Refer fig. 12.10, page 213 , refer Activity 12.6 and derivation)

 When we join three resistors having resistance R1, R2, R3 respectively joined in parallel with a combination of cells, the reciprocal of equivalent resistance was found to be equal to the sum of reciprocal of the individual resistances.

•             Heating effect of electric current

Whenever electric energy is consumed for a useful work, a part of the energy is expended in heat, raising the temperature of the gadget.  If the electric current is purely resistive, i.e., a configuration of resistors only connected to a battery; the source energy continually gets dissipated entirely in the form of heat. This is known as the heating effect of the electric current. 

This effect is utilized in devices such as electric heater, iron etc.

For a steady current I, the amount of heat H produced in time t is H = VIt.

Applying Ohm’s law we get

H = I2 Rt

This is known as Joule’s law of heating.

•             Practical application of heating effects of electric current

Heating effect of electric current has many applications.

Electric oven, electric laundry iron, electric toaster, electric heater etc. are some of the devices.

Electric heating is also used to produce light as in electric bulb.

A fuse protects circuits and appliances by stopping the flow of any unduly high electric current.

•             Electric Power

We know that rate or doing work is power.  It is also the rate of consumption of energy and is termed electric power.  The power P is given by

     P = VI

Or P =I2R = V2/R  

The SI unit of electric power is watt. 

1W = 1 volt x 1 ampere = 1 V A

The unit watt is very small.  Therefore, a much larger unit called kilowatt is used.   1 kilowatt = 1000 watts

Since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt hour (Wh).  1 watt hour is the energy consumed when 1 watt of power is used for one hour.  The commercial unit of electric energy is kilowatt hour (kWh), commonly known as ‘unit’.

1 kWh = 1000 watt x 3600 second

            = 3.6 x 106 watt second

            = 3.6 x 106 joule  (J)

Questions

1 – mark questions

1. What is an electric circuit?

A continuous enclosed path of an electric current is called electric circuit.

2. Mention SI unit of electric charge.

A.Coulomb (C)

3.Name the instrument used for measuring electric current in circuit.

  A. Ammeter

4.            What is the SI unit of potential difference?

         Volt (V)

5.            Name the instrument used for measuring potential difference. 

         Voltmeter

6.            Name a device that helps to maintain a potential difference across a conductor. 

         Battery

7.            What is the SI unit of resistivity?  

Ohm

8.            Name the material used as filament in the electric bulb.  

Tungsten

9. What is the SI unit of power? 

          Watt

10.What is the commercial unit of electric energy? 

         1Kilowatt hour or 1KWh                               

    11.What is the smallest unit of electric current?  

     Milliampere(10-3A) or Microampere(10-6 A).

   12. What is the formula used for finding the resistance when a circuit is      connected in a series?

     R = R1+ R2+R3

13. What is the formula used for finding the resistance when a circuit is connected in a parallel? 

1/R = 1/R1+ 1/R2+1/R3

2 – marks questions

1. What is meant by 1 ampere?

1 ampere is constituted by the flow of one coulomb of charge per second. i.e., 1 ampere = 1C/1sec.

2.  What is meant by volt? 

Volt is the potential difference between two points in a current carrying conductor when one joule of work is done to move a charge of one coulomb from one point to another.

3.  How is a fuse connected in a device? 

The fuse is connected in the series with a device, so that current flows through the device first.

4.  Why is tungsten used in the electric bulb? 

It has high melting point which helps in getting the filament heated to red hot without getting burned up.

5.  What is meant by one Watt power? 

It is the power consumed by a device that carries one ampere of current when operated at a potential difference of one volt.

6.  What is meant by series connection? 

A series is a connection in which all the components of the electric circuit are placed in a series and constant current flows through the circuit.

7.  What is an insulator? 

It is a device which does not allow electric current to pass through it.

8.  What is meant by potential difference two points one volt? 

It means that one joule of work is done to move one coulomb of charge from one point to another point.

9.  What is Ohm’s law? 

The potential difference across the ends of the resistor is directly proportional to the current flowing through it provided its temperature remains the same.

10. Draw a closed circuit and label various parts.

N.C.E.R.T figure 12.2 Page no. 204.

3 Mark questions

1.  What is ammeter?  How is it connected?

Ammeter is a device used to measure  the electric current in a circuit and it is connected near  the source of electricity and is always in a series connection.

2.            What is electric power?  Mention its SI units and also write the units of its SI unit. 

The rate at which energy is consumed.  SI unit is watt (W).  Unit of Electrical energy  is Wh.

3.            Why is series arrangement not used for domestic circuit? 

        Different electrical Appliances need entirely different range of current.    If all are connected in series, then one component fails, the entire circuit is broken   and circuit cannot be made again.

4.            What is electric and circuit?  Draw a schematic diagram of electric circuit comprising of cell, electric bulb, ammeter and plug key. 

A continuous and closed part of electric current is electric circuit.

 Label the diagram.

Flow of electric charge through a conductor.                         

   5 – mark questions

1.  Mention three good effects and two ill effects of heating. 

Used in posters, produces light and used to run machines.  Ill effects  : excessive  heating alters the properties of objects.  Small leakages cause electric shocks.

2.  How does an electric fuse work? 

Electric fuse is a wire of very low melting point.  It melts in excess of current and cuts the circuit and helps prevent accidents.

3.            Draw the symbols of a battery, electric bulb, ammeter, wire crossing without joining, variable resistance or rheostat

Extended information

AN ELECTRIC FUSE  fuse works on heating effect of current.a fuse is connected in series in the electrical circuits.

  an electric fuse is represented by this symbol. "  different types of fuses

MISCONCEPTIONS

In metals, electric current is a flow of electrons. Many books claim that these electrons flow at the speed of light. This is incorrect. Electrons in an electric current actually flow quite slowly; at speeds on the order of centimeters per minute. And in AC circuits the electrons don't really "flow" much at all, instead they sit in place and vibrate. It's the energy in the circuit which flows fast, not the electrons.

1. All electric currents are flows of electrons? Wrong.

Electric currents are not always flows of electrons, they can be flows of any type of electric charge. It depends on the type of conductor. Both protons and electrons possess exactly the same amount of 'electricity.' If either the protons or the electrons flow, that flow is an electric current. In solid metals, electrons do the flowing. But this isn't true for other types of conductor. For example, in salt water, in fluorescent bulbs, and in battery acid, atoms with extra protons can flow along, and this flow is a genuine electric current. And in fuel cell membranes and in solid ice, the electric current is actually a flow of positive hydrogen ions, also called single protons.

2. "Electricity" is made of electrons, not protons? Nope.

Charges of "electricity" are carried both by electrons and by protons. These two types of particles have very different weights (mass), but both have exactly the same amount of electric charge. Electrons are easily removed from atoms, while protons usually are stuck to other protons, but that doesn't affect the amount of charge protons carry. If we remove an electron from a neutral atom, that atom is left with too many protons, and that's the only reason why the atom has an excess of positive electric charge. Positive charge isn't "really just a lack of electrons." Instead, *all* positive charges in objects and in circuits are created by protons. Removing an electron just reveals the positive charge which was already hidden in the atom.

Quantities of "Electricity" (meaning charge) have weight because charge is part of matter particles.

Positive charge is not made of "missing electrons." Positive charge is a genuine type of charge in its own right. Take a neutral hydrogen atom, remove one electron, and we now have positive charge: a proton, not "missing electrons.

Positive charge cannot flow? Totally  wrong.

Electric Current and Ohm's Law MCQ

1. Resistivity of a wire depends on

(A) length(B) material(C) cross section area (D) none of the above.

2. When n resistances each of value r are connected in parallel, then resultant resistance is x. When these n resistances are connected in series, total resistance is

(A) nx(B) rnx(C) x / n(D) n2 x.

3. Resistance of a wire is r ohms. The wire is stretched to double its length, then its resistance in ohms is

(A) r / 2 (B) 4 r (C) 2 r (D) r / 4.

4. Kirchhoff's second law is based on law of conservation of

(A) charge (B) energy (C) momentum (D) mass.

5. The diameter of the nucleus of an atom is of the order of

(A) 10 -31 m (B) 10 -25 m(C) 10 -21 m  (D) 10 -14m.

6. The mass of proton is roughly how many times the mass of an electron?

(A) 184,000  (B) 184,00 (C) 1840 (D)184.

7. The charge on an electron is known to be 1.6 x 10-19 coulomb. In a circuit the current flowing is 1 A. How many electrons will be flowing through the circuit in a second?

(A) 1.6 x 1019  (B) 1.6 x 10-19 (C) 0.625 x 1019(D) 0.625 x 1012.

8. Two bulbs marked 200 watt-250 volts and 100 watt-250 volts are joined in series to 250 volts supply. Power consumed in circuit is

(A) 33 watt  (B) 67 watt   (C) 100 watt  (D) 300 watt.

9. Ampere second could be the unit of

(A) power

(B) conductance

(C) energy

(D) charge.

10. Which of the following is not the same as watt?

(A) joule/sec

(B) amperes/volt

(C) amperes x volts

(D) ( amperes )2 x ohm.

11. One kilowatt hour of electrical energy is the same as

(A) 36 x 105 watts

(B) 36 x 10s ergs

(C) 36 x 105 joules

(D) 36 x 105 B.T.U.

12. An electric current of 5 A is same as

(A) 5 J / C

(B) 5 V / C

(C) 5 C / sec

(D) 5 w / sec.

13. An electron of mass m kg and having a charge of e coulombs travels from rest through a potential difference of V volts. Its kinetic energy will be (A) eV Joules

(B) meV Joules

(C)me / V Joules

(D)V / me Joules.

14. The value of the following is given by 100 (kilo ampere ) x ( micro ampere ) 100 milli ampere * 10 ampere

(A) 0.001 A (B) 0.1 A(C) 1 A(D) 10A.

15. A circuit contains two un-equal resistances in parallel

(A) current is same in both  (B) large current flows in larger resistor  (C) potential difference across each is same (D) smaller resistance has smaller conductance.

46.The element of electric heater is made of

(A) copper

(B) steel

(C) carbon

(D) nichrome.

 47. 5xl016 electrons pass across the section of a conductor in 1 minute 20 sec. The current flowing is

(A) 1mA

(B) 0.1 mA

(C) 0.01mA

(D) 10 mA.

48.Which of the following figures represents the effect of temperature on resistance for copper ?

(A) figure A

(B) figure B

(C) figure C

(D) figure D.

 49.Three elements having conductance G1, G2 and G3 are connected in parallel. Their combined conductance will be

(A) 1/( 1/G1 + 1/G2 + 1/G3)

(B) (G1G2 + G2G3 + G3G1)/(G1 + G2 + G3)

(C) 1/(G1 + G2+ G3)

(D) G1 + G2 +G3