Les 1 current and charge_12

Electric current is the rate of flow of charges

I=

I=Electric current ; Q=charge ;t=time

Electric current is measured in amperes

1Ampere= 1coulomb/1second; A=C/S

One ampere is the current through a wire, if charge flows at the rate of 1coulomb/second

Charge of an electron=1.6× 10¯19 c

Icoulomb= charge of 6.25×1018 coulombs

t

Q

Conventional currentDirection of conventional current is

opposite to the direction of electron flow

The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field.

V=

Where ,v=drift velocity I= current

n= free electrons per m³

A= cross sectional area

e= charge of electron

nAe

I

The potential difference between two points is the electrical potential energy transferred to other forms, per coulomb of charge that passes between the two points.

Potential difference is measured in volts

Define 1 volt

V=W/Q; v=1volt if W=1joule and Q=1coulomb

The p.d between two points is 1volt if 1joule of electrical energy is transferred when 1 coulomb of charge flows between the two points

ENERGY TRANSFERRED

RESISTANCERESISTANCE is the opposition to the flow of electrons

Tungsten filament has more resistance .But copper wire has less resistance. Why?

The electrons make many collisions with the tungsten ions as they move through the filament. But electrons move more easily through Cu wire as they make fewer collisions

Ohm’s LawThe current (I) flowing through a wire is directly proportional to the p.d between its two ends if the temperature remains constant.

Q)Define resistance

The resistance of a conductor is the ratio of the potential difference applied across it to the current passing through it.

Unit of resistance- Ohms

Q)Define 1 ohm

R=V/I ;R=1ohm if V=1volt and I= 1Ampere

The resistance of a wire is 1ohm if a current of 1Ampere flows through it when the p.dbetween its two ends is 1volt.

How to measure the resistance of a wire?

CURRENT-VOLTAGE CHARACTERISTICS

• Graph is a straight line• I is directly proportional to V.• V/I is a constant • The wire has constant resistance

FILAMENT LAMP

• V is not proportional to I

• V/I is not a constant but increases with current.

• V/I=R increases with current

• As the current rises the tungstenfilament gets hotter and resistance of the lamp rises

• The graph is symmetrical for bothpositive and negative voltages

DIODE

• Diode conducts only when it is forward biased.

• When reversed biased, thereis no current

• In the forward direction thecurrent increases very rapidly

• The forward biased diode has very low resistance

• There is no current when thevoltage is applied in the reverse direction.

• Reverse diode has very highresistance.

EFFECT OF TEMPERATURE ON RESISTANCE

1. METALS : As temperature rises, the resistance of metalsincreases because the ions vibrate faster with greater amplitudeand it is more difficult for electrons to pass through the lattice.Metals have +ve temperature coefficient

2. INSULATOR: Resistance decreases with temperature.insulators have few free electrons available for conduction at roomtemperature. At high temperatures some electrons gain enough Energy to escape from their atoms and the insulator is able to Conduct.They have –ve temperature coefficient

3. SEMICONDUCTORS: Resistance decreases with Increase in temperature. At low temperatures, semiconductors arePoor conductors. As temperature rises, more and more electrons Break free to make it a better conductor.It has –ve temperature coefficient.

Eg: Thermistor

RESISTIVITY

1. R α l R α 1/A

=> R α l/A

=> R = ℓ l/A

Where ℓ is a constant . This is equal to the resistivity of theMaterialUnit: Ohm meter (Ωm)

ℓ = R x A/l

Resistivity is defined as numerically equal to theresistance of a sample of the material of unit lengthand unit area of cross-section.