Electricity

Electricity

ELECTRIC CURRENT AND CIRCUIT

A continuous conducting path consisting of wires and other resistance (like Bulb) and a switch, between the two terminals of a cell or a battery along with an the electric current flows, is called an electric circuit.

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

In electric circuits this charge is often carried by moving electrons in a wire.

The SI unit for measuring an electric current is the ampere, which is the flow of electric charges through a surface at the rate of one coulomb per second. I=Q/T

Electric current can be measured using an ammeter. The conventional symbol for current is (I) .

Potential Difference Potential Difference (p.d) between two point s in

an electric circuit is defined as the work done in moving a unit charge from one point to the other point.

It is represented by the symbol ‘V’. Potential difference= Work done/Quantity of

charge moved V=W/Q 1V=1W/1C The p.d is measured by an means of an

instrument called voltmeter. It has a high resistance so that it takes negligible current from the circuit.

It is connected parallel to the circuit.

Common symbols used in a circuit diagram

Ohm`s Law

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points at a constant temperature. Introducing the constant of proportionality, the resistance, one

arrives at the usual mathematical equation that describes this relationship: V=IR

I V ; V I ; I=V/R∝ ∝ If p.d across the ends of a conductor is doubled then the I also

doubles & if the p.d is halved then the current also becomes half.

If the resistance is doubled then the current gets halved, and if the resistance is halved the current gets doubled.

It is represented by a symbol omega, Ω. V-I graph is a straight line graph.

Factors effecting the resistance of a conductor Electric resistance of a conductor depends on the following factors: Effect of length of the conductor R l (where l is the length of the conductor) when the length of the wire ∝is doubled then the R is doubled and vise-versa. Effect of area of cross –Section of the conductorR 1/A(where A is the area of cross-section of conductor)when the area ∝of cross section is halved the resistance if doubled and vise-versa . Effect of nature of material of the conductorSome material have high resistance and some have low resistance. Effect of temperatureThe resistance of a pure metal increases on raising the temperature. But the resistance of Nichrome remains almost unaffected.

ResistivityDerive the formula for resistivity:R∝l –(1) ; R ∝1/A–(2)

From (1) &(2)

R ∝l/A ; R=; where

The resistivity of a substance is numerically equal to the resistance of a rod of that substance which is 1 metre long

and 1 metre in cross section. 2metre

Resistivity The resistivity of a substance does not depends upon its

length & thickness. It depends on the nature & the temperature. Good conductors of electricity have low resistivity and a poor

conductor has high resistivity. Silver has the lowest resistivity, which means that silver

offers the least resistance to the flow of current through it.

The resistivities of alloys are much higher than those of pure metals

(from which they are made). The resistivity of semi-conductor like silicon is in-between

those of conductors and insulators, and decreases on ≤temp.

Combination of resistors

• The resistances can be combined in two different ways:

In series- The p.d across the ends of all the resistances in series is equal to the voltage of the battery.

When the no. of resistance are connected in series, then the same current flows through each resistance.

The combined resistance is more than the biggest individual resistance

In parallel- The reciprocal of the combined resistance of a no. of resistances connected n parallel is equal to the sum of the reciprocal of all individual resistances.

The combined resistance is less than the smallest individual resistance.

The p.d across each resistance is the same which is equal to the voltage of the battery applied.

Series combination

When resistances are connected in series then the total resistances is: Req =R1 +R2 +R3 +….+Rn

Disadvantages of series combination:1. In series circuit, if one appliance stops working due to some defect, then all

other appliances also stop working because the whole circuit is broken.2. In series circuit, all the appliances have only one switch due to which they

cannot be turned on or off separately.3. In series circuit, the appliances do not get the same voltage (220V) as that of

the power supply because the voltage is shared by all the appliances because the voltage is shared by the appliances. The appliances get less voltage and do not work properly.

4. In the series connection of electric appliances, the overall resistances of the circuit increase too much due to which the current from the power supply is low.

Parallel Combination

When resistances are connected in parallel then the total resistances is:

Advantages of parallel combination are:1. In a parallel circuit, if one electrical appliance stops working due to some defect

then all the other appliance keeps working normally.2. In a parallel circuit , each electric appliance has its own switch due to which it

can be turned on and off independently, without affecting the other appliances .3. In a parallel circuit , each electric appliance gets the same voltage (220V) as that

of the power supply line. Due to this all appliance will work properly4. In the parallel connection of electrical appliances, the overall resistance of the

household circuit is reduced due to which the current from the power supply line is high.

Electric Power Electric power is the electric work done per unit time. Power = : P=W/t SI unit of electric power is Watt (W). The power of 1 watt is a rate of working of 1Joule per second. Other formulae: P=V I -(1) Electric power =p.d Current 1W= 1Volt 1 Ampere By Ohms law P=I2R -(2) P in terms of V and R P=V2/R -(3) Power is inversely proportional to the resistance. Commercial unit of Power is Kilowatt-hour.

Heating effect of electric current When an electric current is passed through a high resistance wire, like

Nichrome, the resistance wire becomes very hot and produces heat. This is called the Heating Effect Of Electric current.

It is obtained by the transformation of electric energy into heat energy H=I2R t implies that the heat produced in a wire is directly proportional

to : square of current; resistance of wire ; Time… Applications: 1.Heating effect of electric current is used in bulbs to

produce light. It is utilized in electric fuse for protecting household wiring and electrical

appliances. It is utilized in the working of electrical heating appliances such as

electric iron, Kettle, toaster and many more….

Made By: Chhavi Bansal Of :X-D

Roll No.-12