Electricity Made by :-

Uttam Kumar.X - A

Types of charges

• There are two types of charges :-• Positive charge :- These are made of

sub atomic particle proton.• Negative charge :- These are made

of negative sub atomic particle electron.

S.I. unit of charge

• The S.I. unit of charge is coulomb.• An electron posses a negative charge

of 1.5 x 10-19.• The S.I. unit of one coulomb is

equivalent to the charge containing 6.25 x 10-18.

Conductors and InsulatorsConductors • These substance

have the property to conduct electricity through them.

• These have free or loosely held electrons which helps in conducting electricity.

• Example – copper.

Insulators • These substance have

the property to obstruct the flow of electricity.

• These do not have free electrons present in them.

• Example – Rubber Insulation.

Electric potential

• When a small electric charge is placed in the electric field due to another charge, it experiences a force. So, work has to be done on the positive charge to move it against this force of repulsion.

• The electric potential is defined as the work done in moving a unit positive charge fro infinity to that point.

Potential Difference

• The concept of electric potential is closely linked to that of the electric field. A small charge placed within an electric field experiences a force, and to have brought that charge to that point against the force requires work. The electric potential at any point is defined as the energy required to bring a unit test charge from an infinite distance slowly to that point.

•  It is usually measured in volts, and one volt is the potential for which one joule of work must be expended to bring a charge of one coulomb from infinity.

Potential difference =

or, V = . where W = work done. and Q = quantity of charge moved.

S.I. unit of potential difference is volt. thus 1 volt =

Voltmeter

• A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit.

• Voltmeter has a high resistance so that it takes negligible current.

Electric Current• The movement of electric charge is known

as an electric current, the intensity of which is usually measured in amperes. Current can consist of any moving charged particles; most commonly these are electrons, but any charge in motion constitutes a current.

• 1 ampere = .

Ammeter

• An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name.

• An ammeter should have a very low resistance so that it may not change the value of current flowing in the circuit.

Circuit Diagram

• We know that an electric circuit, as shown in Fig. 12.1, comprises a cell(or a battery), a plug key, electrical component(s), and connecting wires. It is often convenient to draw a schematic diagram, in which different components of the circuit are represented by the symbols conveniently used. Conventional symbols used to represent some of the most commonly used electrical components.

Screen clipping taken: 14-Jun-14 11:43 AM

Georg Ohm• Georg Simon Ohm (16 March

1789 – 6 July 1854) was a German physicist and mathematician. As a school teacher, Ohm began his research with the new electrochemical cell, invented by Italian scientist Alessandro Volta. Using equipment of his own creation, Ohm found that there is a direct proportionality between the potential difference (voltage) applied across a conductor and the resultant electric current. This relationship is known as Ohm's law.

Ohm’s Law

• Ohm’s Law explains the relationship between voltage (V or E), current (I) and resistance (R)

• Used by electricians, automotive technicians, stereo installers.

• According to Ohm’s law : At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its end.

• According to Ohm’s law: V ∝ I or, V= R x I. where R is constant “resistance” of the conductor. This can also be written as – or, I = . So, Current, I = .Therefore,i. The current is directly proportional to

potential difference.ii. The current is inversely proportional to

resistance.

Resistance

• An electron traveling through the wires and loads of the external circuit encounters resistance. Resistance is the hindrance to the flow of charge. For an electron, the journey from terminal to terminal is not a direct route. Rather, it is a zigzag path that results from countless collisions with fixed atoms within the conducting material. The electrons encounter resistance - a hindrance to their movement. 

• The S.I. unit of resistance is ohm’s (Ω).

Factors affecting Resistance

i. Length of conductor.ii. Area of cross section of the

conductor (or thickness of the conductor).

iii. Nature of the material of the conductor, and

iv. Temperature of conductor.

Resistivity

• It has been found by experiments that :• The resistivity of a given of a given conductor

is directly proportional to its length. R ∝ l ……………..(1)• The resistivity of a given conductor is inversely

proportional to its area of cross section. R ∝ 1/A …………… (2)Combining (1) and (2), we get : R ∝ l/A R = ………………….(3)

• Where p(rho) is a constant known as resistivity of the material.

• The resistivity of a substance is numerically equal to the resistance of a rod of that substance which is 1 meter long and 1 square meter in cross section.

• Resistivity, p = .• The unit of resistance R is ohm.• The unit of area of cross-section A is (meter)2.• The unit of length l is meter. putting these unit in the above equation – p = . p = ohm-meter.The S.I. unit of resistivity is ohm-meter (Ωm)

Resistivity of some common substances (200 C )

• The resistivity of alloys are much more than those of pure metals (from which they are made).

• For example the resistivity of maganine (which is an alloy of copper, manganese and nickel)is about 25 times more than that of copper.

• Alloys are used in making heating a materials as –

i. Alloys have very high resistivity (due to which heating elements produce a lot of heat on passing current).

ii. Alloys do not undergo oxidation easily even at high temprature.

Combination of Resistors

• Resistors can be combined in two ways –

i. In series.ii.In parallel.

Resistors in Series

• When two (or more) resistors are connected end to end consecutively, they are said to be connected in series.

• According to the law of combination of resistance in series: The combined resistance of any number of resistances connected in series is equal to the sum of the individual resistances.

R= R1 +R2 +R3+………..

I. When a number of resistors connected in series are joined to the terminal of a battery, then each resistance has a different potential difference across its ends (which depends on the value of resistance). But the total potential difference across all the ends of all the resistors in series is equal.

II. When a number of resistors are connected in series, then the same current flows through each resistance.

Resultant of Resistances connected in Series

• The figure shows three resistances R1,R2,R3 connected in series. Now suppose potential difference across resistance R1 is V1 , R2 is V2 and R3 is V3. Let potential difference across battery be V, then :

V = V1+V2+V3.

Applying Ohm’s law to the whole circuit : V = IR. ………..(1)Applying Ohm’s law to the three resistors separately, we get:V1 = I x R1. ………………….. (2)

V2 = I x R2. ………………….. (3)

V3 = I x R3. ………………….. (4)

Substituting (2), (3), (4) in (1)IR = IR1 + IR2+ IR3

OR, IR= I (R1+R2+R3)

Or, R = R1+R2+R3 .

Therefore we conclude that the sum total resistance in a series resistance connection is equal to the sum of all the resistances.

Resistors in Parallel

• When two (or more) resistors are connected between the same points, they are said to be connected in parallel.

• According to the law of combination of resistance in parallel: The reciprocal of the combined resistance of any number of resistances connected in parallel is equal to the sum of the reciprocals of the individual resistances.

1/R= 1/R1 +1/R2 +1/R3+………..

• When a number of resistances are connected in parallel then their combined resistance is less than the smallest individual resistance.

• When a number of resistance are connected in parallel, then the potential difference across each resistance is same which is equal to the voltage of battery applied.

• When a number of resistances connected in parallel are joined to the two terminals of a battery, then different amounts of current flow through each resistance (which depend on the value of resistance). But the current flowing through each parallel resistance, taken together, is equal to the current flowing in the circuit as a whole. Thus, when a number of resistance are connected in parallel, then the sum of current flowing through all the resistances is equal to the total current flowing in the circuit.

Resultant of Resistances connected in Parallel

• The figure shows three resistances R1,R2,R3 connected in series. Now suppose currant across resistance R1 is I1 , R2 is I2 and R3 is I3. Let total current in the circuit be I, then:

I = I1+I2+I3.

Applying Ohm’s law to the whole circuit : I = V/R. ………..(1)Applying Ohm’s law to the three resistors separately, we get:I1 = V / R1. ………………….. (2)

I2 = V / R2. ………………….. (3)

I3 = V / R3. ………………….. (4)

Substituting (2), (3), (4) in (1)V/R = V/R1 + V/R2+ V/R3

OR, V/R= I (1/R1 +1/R2 + 1/R3)

Or, 1/R = 1/R1+1/R2+1/R3 .

Therefore we conclude that the sum total resistance in a parallel resistance connection is equal to the sum of reciprocal of all the resistances.

Parallel and Series connection

Parallel connection• If one electric appliance

stops working due to some defect, then all other appliances keep working normally.

• In parallel circuits, each electric appliance has its own switch due to which it can be turned on or off independently.

• Each appliance gets same voltage as that of power source.

• Overall resistance of household circuit is reduced due to which the current from power supply is high.

Series connection• If one electric appliance stop

working due to some defect, then all other appliances stop working.

• All the electric appliances have only one switch due to which they cannot be turned on or off separately.

• In series circuit, the appliances do not get same voltage (220 V) as that of the power supply line.

• In series circuit the overall resistance of the circuit increases due to which the current from the power source is low.

Heating effect of electric current

• When electricity passes through a high resistance wire like a nichrome wire, the resistance wire becomes very hot and produces heat. This is called the heating effect of current.

James Prescott Joule

James Prescott Joule (24 December 1818 – 11 October 1889) was an English physicist and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work. This led to the law of conservation of energy, and this led to the development of the first law of thermodynamics. The SI derived unit of energy, the joule, is named for James Joule. He worked with Lord Kelvin to develop the absolute scale of temperature. Joule also made observations of magnetostriction, and he found the relationship between the current through a resistor and the heat dissipated, which is now called Joule's first law.

Joule’s law of heating

LetAn electric current I is flowing through a resistor having resistance equal to R.The potential difference through the resistor is equal to V.The charge Q flows through the circuit for the time t.Thus, work done in moving of charge Q of potential difference V = VQSince, this charge Q flows through the circuit for time t,

• The heat produced in wire is directly proportional to

i. Square of current.ii. Resistance of wire.iii. Time for which current is passed.

Applications of heating effect of electric current

There are many practical uses of heating effect of current. Some of the most common are as follows.• An incandescent light bulb glows when the filament is heated by heating effect of

current, so hot that it glows white with thermal radiation (also called blackbody radiation).

• Electric stoves and other electric heaters usually work by heating effect of current.• Soldering irons and cartridge heaters are very often heated by heating effect of current.• Electric fuses rely on the fact that if enough current flows, enough heat will be generated

to melt the fuse wire.• Electronic cigarettes usually work by heating effect of current, vaporizing propylene

glycol and vegetable glycerin.• Thermistors and resistance thermometers are resistors whose resistance changes when

the temperature changes. These are sometimes used in conjunction with heating effect of current(also called self-heating in this context): If a large current is running through the nonlinear resistor, the resistor's temperature rises and therefore its resistance changes. Therefore, these components can be used in a circuit-protection role similar to fuses, or for feedback in circuits, or for many other purposes. In general, self-heating can turn a resistor into a nonlinear and hysteretic circuit element.

Electric Energy• H = I2 Rt gives the rate at which electric energy is dissipated or consumed in

an electric circuit. This is also termed as electric power. The power P is given by

P = VIOr P = I2R = V2/R

• The SI unit of electric power is watt (W). It is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V. Thus,

1 W = 1 volt × 1 ampere = 1 V A• The unit ‘watt’ is very small. Therefore, in actual practice we use a much

larger unit called ‘kilowatt’. It is equal to 1000 watts. Since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt hour (W h). One watt hour is the energy consumed when 1 watt of power is used for 1 hour. The commercial unit of electric energy is kilowatt hour (kW h), commonly known as ‘unit’.

1 kW h = 1000 watt × 3600 second= 3.6 × 106 watt second

= 3.6 × 106 joule (J)