PITE Services: Electricity

ELECTRICITY

Introduction : Electricity is the form of energy resulting from the existence of charged particles either statically or all of charge dynamically as a current. Electricity involves the flow of electrons. All matter is made up of atoms, which has a center called a nucleus. The nucleus contains positively charged particles called protons and uncharged particles called neutrons. The nucleus of an atom is surrounded by negatively charged particles called electrons. The negative charge of an electron is equal to the positive charge of a proton, and the number of electrons in an atom is usually equal to the number of protons.

When the balancing force between protons and electrons is upset by an outside force, an atom may gain or lose an electron. And when electrons are "lost" from an atom, the free movement of these electrons constitutes an electric current.

Various S.I units

USES OF ELECTRICITY

Electricity is an important source of energy. In the modern times electricity is used in our homes for lighting operating fans and heating purposes. In industry, electricity is used to run various types of machines, and in transport sector electricity is being used to pull electric trains.

TWO FUNDAMENTAL LAWS OF ELECTRICITY

1. Opposite charges attract each other.

2. Similar charges repel each other.

Charge:- Fundamental property of matter due to which an object experiences a force when placed in electromagnetic field. The SI unit of charge is coulomb which is denoted by the letter C.

One coulomb :- One coulomb is the quantity of electric charge which exerts a force of 9x10⁹neutons on an equal charge placed at a distance of 1 meter from it.

Conductors:-Those substances through which electricity can flow are called as conductors. e.g Copper, silver etc

Insulators:- Those substances through which electricity cannot flow are called insulators. eg. Plastic, wood etc

Electric potential :- The electric potential at a point in an electric field is defined as the work done in moving a unit positive charge from infinity to that point.

Potential Difference :- The potential difference between two points in a electric circuit is defined as the amount of work done in moving a unit charge from one point to other point. i.e :

POTENTIAL DIFFERENCE=

The SI unit of potential difference is volt.

One Volt: The potential difference between two points is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of electric charge from one point to the other.

Note :- The potential difference is measured by means of an instrument called voltmeter.

Electric current :- The electric current is a flow of electric charges in a conductor flow of charge per unit time is called as current.

Current,

The SI unit of electric current is ampere.

One Ampere:- When one coulomb of a charge flows through any cross-section of a conductor in 1 second, 1 electric current flowing through it is said to be 1 ampere.

Note:- Current is measured by an instrument called ammeter.

Direction of electric current:- The convential direction of electric current is from terminal, through the outer circuit. The actual flow of electrons is however, from negative terminal to positive terminal to positive terminal of a cell which is opposite to the direction of conventional current.

Electric circuit:- A continuous conducting path between the two terminal of a cell or a battery along which an electric current flows, is called a circuit.

simple Electric circuit

Diagram of Electric Components

Representation of Electric Components

Resistance of conductor:- The property of a conductor due to which it opposes the flow of current through it is called resistance.

Resistance=

The resistance of a conductor depends on length, thickness, nature of material and temperature of the conductor.

The SI unit of resistance is Ohm which is denoted by the symbol Omega.

One Ohm:- One Ohm is the resistance of a conductor such that when a potential difference of 1 volt is applied its end, a current of 1 ampere flows through it.

Factors on which resistance depend :- The resistance of a conductor depends upon

1.Length of the conductor (RαL): It has been found by experiment that on increasing the length of the wire, its resistance also increases

2.Area of the cross section of the conductor (Rα 1/A

): The resistance of a conductor is inversely proportional to its area of cross-section. It means that with increase in area of cross section resistance decreases and vice versa.

3. Temperature of the conductor(Rαt): The resistance of all pure metals increased on raising the temperature, and decreases on lowering the temperature.

4. Nature of the material of the conductor :The electric resistance of a conductor depends on the nature of the material of which it is made some material have low resistance whereas other have high resistance.

Ohm’s Law

Ohm Law: In 1827, a German Physicist George Simon Ohm (1787-1854) found out the relationship between the current. ‘I’ flowing in a metallic wire and the potential difference ‘V’ across its terminals.

He stated that "the electric current flowing through a metallic wire is directly proportional to the potential difference ’V’ across its ends provided its temperature and pressure remains the same" .

i. e: IαV

or VαI

v=R x I (Where R is a constant called ‘resistance’ of the conductor)

Experiment Verification:

Complete a circuit which consist of a battery (B), or a switch (S), a rheostat (Rh), an ammeter (A), a voltmeter (V) and some connecting wires. Using all these and the conductor ‘R’ we make a circuit as shown in figure

To start the experiment, the circuit is completed by pressing the switch (S) on pressing the switch, a current starts flowing in the whole circuit including the conductor ‘R’ Note down the reading in ammeter and voltmeter for different values of current by adjusting with the help of rheostat. Plot the graph between the V and I, which is the straight line and hence prove our Ohm’s Law.

Resistivity:

We know that

RαL________________(1)

Rα 1/A

________________(2)

Combining (1) & (2)

Rα l/A

R = pl/A

Where p, is the constant of prop. and is called as Resistivity or specific resistance.

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.

The SI unit of resistivity is Ohm meter which is written in symbol as Ω m.

Note :- The resistivity of a substance does not depend on its length or thickness. It depends on the nature of the substance and temperature.

Resistor:- A device which can produce some resistance called resistor.

Eg: Bulb, Fuse wire etc.

Resistor in series:- When two (or more) resistance are connected end to end consecutively; they are said to be connected in series.

Diagram

Resistors Connected in Series

Let R_1,R_2,and R₃be the three resistances connected in series such} that current I flows through each of them and v_1,v₂and v₃be the potential difference through R_1,R₂and R₃ respectively.

Now

V= V₁+ V₂ + V₃____________ (1)

Using Ohm Law

V = IR

V₁= IR₁

V₂= IR_{2 ___________________(2)}

V₃ = IR₃

Put (2) and (1)

V = IR₁+ IR₂ + IR₃

IR = I [ R₁ + R₂ + R₃] [ V= IR]

R_s = R₁ + R₂ + R₃

When these are ‘n’ resistance

We have

R_s= R₁ + R₂ + R₃........R_n

The combined resistance of any number of resistance connected in series equal to the sum of the individual resistances.

Resistor in Parallel:- When two (or more) resistance are connected between the same two points, they are said to be connected in parallel.

Diagram

Resistors in parallel

Let R₁, R₂ and R₃be the three resistance connected in parallel such that current I₁, I₂ and I₃ flows through R₁, R₂ and R₃ resp. Also V is the potential difference across its end.

I = I₁ + I₂ + I₃ _________₍₁₎

Now

V = IR

Hence, the reciprocal of the combined resistance of a no. of resistance connected in parallel is equal to the sum of the reciprocals of all the individual resistance.