Chapter 12

*Electricity!

Electricity has an important place in the modern society. It is a controllable and convenient form of energy for a variety of uses in homes ,schools, hospitals, industries and so on…

The Most

Dangerous

City…

*Electric current

*An electric current is a flow of electric charge. Electric charge flows when there is voltage present across a conductor.

*The rate at which charges move in a conductor is called electric current i.e. quantity of charges crossing a point in unit time.I = q/twhere I is current in amperes (A)q is total charge in Coulombs (C)t - denotes time in seconds (s).

∴  1 A = 1 C/s

*Electric current

*The direction of conventional current is in the direction of the flow of positive charges.

*The flow of electrons is opposite to conventional current.

*How to Measure Current?

*The SI unit of current is ampere (A)

*Smaller currents are measured in milli amperes (mA).

*Different electrical components and appliances require different sizes of current to turn them on.

1 A = 1,000 mA

1 mA = 0.001 A

1 A = 1,000 mA

1 mA = 0.001 A

*Circuit

*A continuous and closed path of an electric current is called an electric circuit.Electric circuits are made up of electrical components

*These components must be joined together without any gap in between to form a closed circuit.

∴ a circuit should be in a closed path.

*Electric circuits and its components.

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*Electric potential

*In electricity potential is like height, level, temperature.

*Some work has to be done by charging a body.

*This work done is stored in a body as its electrical potential energy.

*Electrical potential difference

*Electrical potential difference between 2 points A & B on a conductor through which the current is flowing is defined as amount of work done to move a unit charge from point A to point B in a closed circuit.

*If work is done to move a charge from one a point A to point b on a conductor then the potential difference between A & B is V=w/q

*Where V= potential difference

W=Work done

q=Charge

*Unit of potential is volt

*Potential difference is said to be one volt of one joule of work is done to move a charge(q) of one coulomb from one point to another in a given circuit.

*1 v =1j/1c

*Electrical potential difference

*Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance, i=v/r

*where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

*Ohm’s law

*Resistance

*Resistance is a property that resists the flow of electrons in a conductor. It controls the magnitude of the current. The SI unit of resistance is ohm (Ω).

*Resistance is measured in ohms (Ω) using an ohmmeter.

*An ohmmeter is a device for measuring resistance.

*Resistance

*Conductors, such as metals, allow electrons to flow freely through them and have low resistance values.

*Insulators resist electron flow greatly and have high resistance values.

*Factors on which resistance of a

conductor depends

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

*Factors on which resistance of a

conductor depends

*Resistors and Potential

Difference

*Resistors can be used to control current or potential difference in a circuit.

*In a circuit, electrons have a higher potential difference as they enter a resistor compared to when they leave the resistor because they use up some energy in passing through the resistor.

*Resistance of a system of Resistors

*There are two methods of joining the resistors together.

(a) Resistors in series

(b) Resistors in parallel

*Resistors in series

*The current through each of the resistors is the same. As ….

*V = V1+ V2 + V3

*Applying Ohm’s law to the entire circuit we have.. V = I R, then

*On applying ohm’s law to the three resistors seperately,we further have

*V=IR1

*V=IR2

*V=IR3 from that we have I R = IR1 + IR2 + IR3

Or Rs = R1 + R2 + R3

*Resistance in parallel

*The reciprocal of the equivalent resistance of a group of resistance joined in parallel is equal to the sum of the reciprocals of the individual resistances. As….

*I = I1 + I2 + I3

*I = V/Rp (by ohm’s formula to parallel combination of

resistors we have)

*I1 = V/R1 ; I2 = V/R2 ; and I3 = V/R3 (by ohm’s

law to each resistor)*V/Rp = V/R1 + V/R2 + V/R3

Heating effect of electric current

*As some power is converted into heat energy, this phenomenon can be described by Joules law, which states that H = i2.r.t, where H is the generated heat in calories,

*When an electric current flows through a wire, the wire heats up. Electrical energy has been converted into heat energy.

*The greater the resistance of the wire, the greater the amount of heat produced. This heating effect is used in common electrical appliances.

*Electric power

*Electric power is the rate at which  electric energy is transferred by an electric circuit.

*The power (p) is given by

*P=VI Or P= I2R= V2/R

* The SI unit of electric power is watt(W).

1W = 1 volt.1ampere = 1 V A

The commercial unit of energy is kilowatt hour(kWh),commonly known as ‘unit’

1 kW h = 1000 watt. 3600 second

= 3.6 . 106 watt second

= 3.6 . 106 joule (j)

Chapter 12

*ENDA presentation by :-

Suraj Kakkad