Electricity

Charge (q)

• It is the intrinsic property of matter.

• It is a very small particles present in an atom.

• It can be either negative (electron) or positive (proton).

• The SI unit of charge is “Coulomb”, represented by C.

Net charge (Q) Total charge

• Net charge is equivalent to the charge contained in nearly electrons 6 x10^18.
Q = ne (n is no: of electron)

If Q = 1C, e = 1.6 X 10^–19C (negative charge on electron)

n = Q/e = 1/1.6 X 10^–19C

n = 6.2 X 10^18 electrons.

Current (I)

• Rate of flow of net charge is called current. Denoted by (I).

• I = Q/t

• SI unit of current is “Ampere” rep. by A.

Ampere

• Defined as one coulomb of charge flowing per second.

• 1A = 1C/1s

• One Ampere current is defined as if one coulomb charge is flowing through any conductor in one second.

• In an electric circuit the electric current flow in the opposite direction of the flow of electron (–ve charge) conventionally. It flows from the +ve terminal of battery or cell to –ve terminal.

• Small quantity of current are expressed

mA (milli Ampere) = 10^3 A
uA (micro Ampere) = 10^6 A

Ammeter

• It is an instrument used to measure the electric current in a circuit.

• It is always connected in series in a circuit.

• It is represented by the symbol (A) in an electric circuit. It has low resistance.

Electric Circuit

• It is a closed path along which an electric current flow.

• When electrical components connected each other in a closed loop they make electric circuit.

• The electron can only flow when there is difference of electric pressure. For example “water flowing through a tube” It is only possible when there high pressure at one side and low at another side, then it will move from high pressure to low pressure.

• In case of electric current, the flow of charge is made possible due to chemical action with in a cell that generates the potential difference across the terminals of the cell.

Electric potential

• It is the work done in bringing a unit +ve charge from infinity to a point in an electric fied against force.

Electric potential Difference

• It is defined as the work done in carrying a unit charge from one point to another between the two points of an electric circuits.

• V = W/Q

V – Potential Difference, W – Work, Q – Net Charge.

• SI unit of potential difference – Volts represented by “V”.

• One Volt When 1 Joule of work is done to carry one coulomb (1C) of charge from one point to another of a current carrying conductor then the potential difference is send to be IV.

• IV = 1C/1J.

Voltmeter

• It is an instrument, used to measure the potential difference and represented by the symbol in an electric circuit. It is always connected in parallel across the points between which the potential difference is to be measured. It has high resistance.

• Symbols for some commonly used instrument in circuit diagrams + V –

(1) Cell

(2) Battery

(3) Key (switch) open

(4) Key (Close)

(5) Joint wire

(6) Wires with no join

(7) Bulb

(8) Ammeter

(9) Voltmeter

Georg Simon Ohm (physicist) 1787 – 1854

• Found the relationship between the current (I) flowing through a conductor and potential difference (V) across the terminals of a conductor using the circuit diagram.

Ohm’s Law

• Ohm’s Law stated that the electric current flowing through a conductor is directly proportion to the potential difference across its ends, provided the temperature remain constant.

• V directly proportional to I

• V = IR

• Where “R” is the proportionality constant for the given metal at given temperature and is said to be resistance, the graph between V and I is always straight line.

• Resistance– It is the property of a conductor that opposes the flow of current.It is represented by ‘R’ symbol SI unit of resistance “Ohm”.

• 1 Ohm – The resistance of a conductor is said to be one Ohm, when the potential difference across the conductor is 1V and the current flowing through it is 1A.

• V = IR

• R = V/I

• 1 Ohm = 1V/1A

• Rheostate – As we know that

• V = IR

• Shows that current through conductor resistor is inversely proportional is its resistance. So to increase or decrease the current accordingly in the circuit a component is used is called “Rheostat”, that regulates the current without changing potential difference. Represented by “Rh”.

Factors On Which Resistance Of a Conductor Depends

(1) On its length (l)
(2) On its cross sectional area (A)
(3) On the nature of material

• (Resistance) R directly proportional to l

• R inversly proportional to areal 1/A

• R = rho(l/A).

• Where rho is a proportionality constant known as resistivity of the material of conductor.

Resistivity (rho)

• The resistance offered by a wire of unit length and unit crosssectional area is called resistivity.

• (Directly prop. to length), (inversely prop to cross-sectional area).

• Its SI unit is ohm meter Since R = rho(l/A).

• For a material irrespective of length and area, the resistivity is a constant.Resistantly of a material vary with temperature.

• Resistivity of an alloy (homogeneous mixture of metals) is generally higher than of its constituent metals. Example Constantan (alloy of Cu and Ni).

• Alloys have high resistivity and do not oxidise (burn) readly at high temperature,for this reason they are commonly used in electrical heating devices, like electric iron, heater, toasters etc. For example “Tungsten” as filament of electric bulb.

Resistance in Series (Maximum Effective Resistance)

• Let us take three resistance R1, R2 and R3 that are connected in series in a circuit.

• Ohm’s low stated

• V = IR

• The current (I) flowing through the resistance in series will remain same, where as the potential difference (V) across each resistor will be different.

• V1 = IR1 ,V2 = IR2 , V3 = IR3

• Total potential difference (V) = V1 + V2 + V3

• V = IR1 + IR2 + IR3 Putting the value of V1, V2 , V3 and V .

• I R = I (R1 + R2 + R3 )

• Reff = R1 + R2 + R3

• Thus, we conclude that effective Resistance of the several resistors joined in series is equal to the sum of their individual resistance.

Resistance in Parallel (Minimum Effective Resistance)

• Let us take three R , R and R , that are connected in parallel in the electric circuit.

• I1 = V/R1, I2 = V/R2 , I3 = V/R3

• Total current (I) = I1 + I2 + I3

• substitute the value of I1 , I2 , I3 and I = VR

• I = I1 + I2 +I3

• I = V/R1 + V/R2 +V/R3

• I = V(1/R1 + 1/R2 +1/R3)

• 1/Reff = 1/R1 + 1/R2 +1/R3

• Thus, we conclude that the reciprocal of total effective resistance of the several resistors connected in parallel is equal to the sum of the reciprocals of the individual resistance.

Disadvantage of series connection in on electric circuit

1. In series connection if any of the component fail to work, the circuit will break and then none of the component (ex. TV, bulb, fan..) will work.

2. It is not possible to connect a bulb and a heater in series, because they need different value of current to operate properly.

• Hence, to overcome this problem we generally use parallel circuit.

Heating effect of Electric Current

• Explanation Battery or a cell is a source of electrical energy.

• Battery or Cell (Chemical reaction in it will produce potential difference at its two terminals)

• Electron will come in motion to flow current through resistor Rest of energy of source is converted into heat, that raises the temperature of gadget.

• Part of this energy is consumed in useful work (like rotating of fan) To maintain this current, the source has to keep expending its energy This is known as heating effect of electric current

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

Mathematical Expression

• Let us suppose that current (I) is flowing through a resistor of resistance (R) for the time (t).

• The potential difference across the resistance is (V).

• V = W/Q

• Work done in morning the charge Q will be

• W = VQ

• Then power, P = W/t [Rate of change of work done]

= VQ/t

• P = VI [I = Q/t] – (1)

• Heat energy supplied by the source for time t will be

H = P X t P = Energy or Work X time – (2)

Put equation (i) in equation (2)

H = VIt
= (IR) It

H = I2Rt [ V = IR Ohm’s Law]

• This is known as Joule’s Law

• The law stated that the heat produced in a resistor is

(i) directly proportional to square of the current(I)

(ii) directly proportional to resistance (R) for given current.

(iii) directly proportional to time (t) for which current flow through resistor.

Application of Heating Effect of Electric Current :–

(1) Used in electric iron, toaster, oven, heater etc.

(2) It is also used in bulb to produce light.(Filament of bulb is made of strong metal with high melting point such as
tungsten (m.pt = 3380°C). This filament can retain as much of the heat
generated as possible, to become very hot and emit light)

(3) It is also used in the “fuse connected in an electric circuit {Fuse a safety device, protect the circuits and appliance by stopping the flow of high current. The wire of fuse is made of an alloy of metals for ex Aluminium Copper, Iron lead etc. The alloy should be of low m.pt and high resistivity, fuse is always connected in series circuit. When large current flow through the circuit, the temperature of fuse wire will increase. This melts the fuse wire and break the circuit.

• “ Fuses” used for domestic purposes are rated as 1A, 2A, 3A, 5A, 10A etc. for various operation depending upon the power of appliance using.\nExample- let us consider an appliance “electric Iron” which consume 1KW electric power, at 220V

• P = VI

• I = P/V

• I = 4.54A

In this case a 5A fuse is required.

Electric Power

• In case of electricity, it is defined as the rate of change electrical energy dissipated or consumed in an electric electrical energy dissipated or consumed in an electric circuit.

• P = VI

• or P = I²R ( V = IR Ohm’s Law)

• or P =V²/R ( I = V/R )

• or P = Electrical Energy (E)/time (t)

• SI unit of electric power is “Watt” (W).

• 1 Watt Defined as the power consumed by a device, when 1A of current passes through it at the potential difference of 1V.

• P = VI

• 1 Watt = 1 Volt X 1 Ampere

⇒ Electrical Energy

• P = E/t

• E = P x t

• E – Electrical Energy

• t – time

• SI unit of electrical energy = Ws or J

Important Question

1.  What does an electric circuit mean ?

Ans. An electric circuit is a continuous and closed  path for flow of an electric current. If the electric circuit is complete, electric current can flow through the circuit. If the circuit is broken anywhere or switch of the circuit is turned off, the current  stops flowing.

2. Define the unit of current.

Ans. SI unit of electric current is ampere. Current is said to be 1 ampere (1 A), if 1 couloumb charge flows per second across a cross-section of a conductor.

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

Ans. Potential difference across a conductor can be maintained by means of a battery consisting of one or more cells.

4. What is meant by saying that the potential difference between two points is 1 V ?

Ans. The potential difference between two points is said to be 1 volt, if 1 joule of work is to be done for moving a charge of 1 coulomb from one point to another.

5. Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal ?

Ans. Coils of electric toasters and electric irons are made of an alloy (generally) nichrome) due to the following two reasons :

(i) Resistivity of an alloy is generally higher than that of pure metals, hence for a given resistance we need a coil of lesser length.

(ii) At high temperatures, an alloy does not oxidise (burn) readily. Hence, coil of an alloy has longer life.

6. What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series ?

Ans. Advantages of connecting electrical devices in parallel with the battery are as follows :

(i) Voltage across each electrical device is same and the device is same and the device can take current as per its resistance.

(ii) Separate on/off switches can be applied across each device.

(iii) Total resistance in parallel circuit decreases, hence a greater current may be drawn from the cell.

(iv) If one electrical device is damaged, then other devices continue to work properly.

7. Why does the cord of an electric heater not flow while the heating element does ?

Ans. Cord and the electric heater are joined in series and carry same current when joined to a voltage source. As resistance of cord is extremely small as compared to that of heater element, hence, heat produced H = I²Rt is extremly small in cord but much larger in heater element. So, the heating element begins to glow but cord does not glow.

7. Explain the following :

(a) Why is the tungsten used almost exclusively for filament of electric lamps ?

(b) Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rater than a pure metal ?

(c) Why is the series arrangement not used for domestic circuits ?

(d) How does the resistance of a wire vary with its area of cross -section ?

(e) Why are copper and aluminum wires usually employed for electricity transmission ?

Ans. (a) Tungsten is used exclusively for filament of electric lamps because its melting point is extremely high (3380 degree celcius).

(b) Conductors of electric heating devices are made of an alloy rater than a pure metal due to the following reasons :

(i) Resistivity of an alloy is generally higher than that of pure metals, hence for a given resistance we need a conductor of less length.

(ii) At high temperatures, an alloy does not oxidise (burn) readily. Hence, heating element prepared form an alloy has longer life.

(c) Series arrangement is not used for domestic circuits due to the following reasons :

(i) In series arrangement same current will flow through all the appliances, which is not required.

(ii) total resistance of domestic circuit will be sum of the resistances of all appliances and hence current drawn by the circuit will be less.

(iii) We cannot use independent on/off switches with individual appliances.

(iv) All appliances are to be used simultaneously even if we do not need them.

(d) Resistance (R) of a wire is inversely proportional to its cross-section area (A).

(e) Copper and aluminium wires are usually employed for electricity transmission because these are extremely good conductors having a low value of resistivity. Moreover, these are ductile and can be drawn in the form of fine wires.

8. Define potential difference between two points in a conductor.

Ans. Potential difference between two points A and B, in an electric field, is defined as the amount of work done in order to move unit positive charge from point B to point A. Thus,

9. What is meant by the statement “potential difference between points A and B in an electric field is 1 volt” ?

Ans. Amount of work done to bring 1 C charge from point B to point A in the electric field is 1 joule.

10. Which material is the best conductor ?

Ans. Silver

11. Define resistance of a conductor.

Ans. The electric resistance of a conductor is its property to oppose flow of electric charge through it and is measured by the potential difference being applied across the conductor so as to maintain the flow of one ampere current through it.

12. Keeping the potential difference constant, the resistance of an electric circuit is doubled. State the change in the reading of an ammeter connected in this circuit.

Ans. The reading of ammeter is reduced to one-half of its previous value.

13. The length of a wire is doubled and its cross-sectional area is also doubled. What is the change in its resistivity ?

Ans. There is no change because resistivity of a material depends only on its nature and is independent of its dimensions.

14. On what factors does the resistance of a conductor depend ?

Ans. Resistance of a conductor (i) is directly proportional to its length, (ii) inversely proportional to its cross-section area and depends on the material of the conductor. Resistance also depends on the temperature.

15. What is heating effect of electric current ?

Ans. When electric current is passed through a resistor, electrical energy is dissipated and appears as heat energy. This is known as the heating effect of electric current.

16. State joule’s law of heating.

Ans. As per joule’s law the heat produced in a resistor is (i) directly proportional to square of current flowing through it, (ii) directly proportional to resistance, and (iii) directly proportional to time. Mathematically,

Heat H = I²Rt

17. How many joules are equals to 1kWh ?

Ans. 3.6 x 10^6

18. Out of 60 W and 40 W lamps, which one has a higher electrical resistance when in use ?

Ans. 40 W lamp has a higher electrical resistance because R = V²/P.

19. Would you connect a fuse in series or in parallel to an electric circuit ?

Ans. In series of the electric circuit before appliances present in the circuit.

20. Why do electricians wear rubber gloves while working ?

Ans. Rubber is an electrical insulator. Hence electrician can work safely while working on an electric circuit without a risk of getting any electric shock.

21. what is heating effect of electric current ? Find an expression for amount of heat produced. Name some appliances based on heating effect of current.

Ans. When a current flows through a conducting wire, heat is developed and temperature of wire rises. It is known as the heating effect of electric current.

If V be the potential difference maintained across the ends of a wire then, by definition, the amount of work for flow of 1 C charge through the wire is V.

Therefor, Work done for flow of Q charge W = VQ = VIt

where I is the current flowing in time t. As V = IR

W = VIt = (IR)It = I²Rt

This is known as joule’s law of heating.

Incandescent lamps, electric iron, electric stove, toaster, geyser, electric room heater etc., are the appliances based on heating effect of electric current.