dc electricity presentation

By: Maizura Daud

Nur Atiqah Hasbullah

Nor Husna Mohd Rosli

DC ELECTRICITY

Originated from Greek word – ‘amber’ . Modern word – ‘electron’

Du FayDiscover that there are two types of charge. Like charges repel while unlike charges attract

Michael Faraday and Joseph HenryBelieve that electricity is some kind of fluid.Michael Faraday – Discover theoretical basis of electricityJoseph Henry – Invents practical applications (eg: telegraph)

History of Electricity

Term ‘emf’ only used in electrical circuit. Term ‘pd- potential difference’ is used in all energy fields (eg: gravitational, electric etc)

EMF – electrical potential difference generated by a source (eg: generator and battery)

Potential difference can be measured between any two points

BUTEMF is measured between two ends of an energy source

Difference: EMF and pd

DEFINITION:The resistance of a conductor is the ratio of the pd applied across it, to the current passing through it

Resistance, R = pd across the conductor, V (Volt)

current through the conductor, I (Amps)

Resistance

V = Voltage (Unit: V)I = Current (Unit: A)R = Resistance (Unit: Ω

Ohm)

Equation : Ohm’s Law

V = IR

R = V/IOhm’s Law stated that:

Provided the temperature and other physical factors remain constant, the current through a wire is proportional to the potential difference across its ends.

Quiz30 volt lamp carries a current of 0.7 amperes. What is the resistance of the lamp?

R = V/I = 30/0.7 = 43 Ω

Obey Ohm’s Law V =

IR

1) More current flows2) Metal Filament gets hotter3) Filament gains energy, then the

atoms vibrate faster 4) Collision with the travelling

electron increases, thus resist their motion

Two resistances depends on its polarityWhich way it connected

Forward reaction = low resistanceReverse direction = High resistance

More current flow -> Temperature increases -> Thermistor makes available more free electrons to carry the current

Length (l)Cross-sectional area (A)

Material of which it is made

R = ρ /Aℓρ = resistivity (Unit: Ωm)

Resistance depends on :-

Length Long wire - more resistance Short wire – less resistance

Cross-sectional ->Thin wire- more resistance Thick wire- less resistance

Temperature -> in metals, a hot wire has more resistance than a cold wire.

POINTS!Metal has large number of free electronsWhen the electron move through the metal

lattice, electrons collide with the vibrating metal atoms

The collision oppose the flow of electrons. This causes metal to have resistance

Metal get heats up, thus metal atoms vibrate faster with higher amplitude. This causes electrons to become more difficult to pass through the lattice.

This increases the resistance of metal

Effect of Temperature on Resistance

Is the condition where: conductor is a perfect conductor -> completely

loses its resistivity Continue to carry current (persistent current)

when it’s kept below critical temperature, Tc.

The phenomenon occur in certain materials, usually at very low temperatures.

Occurs in a wide variety of materialsFor example, tin, aluminium and various metallic alloys

SUPERCONDUCTIVITY

For superconductor, the resistance drops abruptly to zero when material is cooled below its critical temperature.

Graph of superconductor

The transport Equation

I = current through conductorn = number of free electronsA = cross-sectional areav = drift velocitye = electron charge

From the information, we may deduce that:Volume of conductor = AℓNumber of free electrons - nAℓTotal charge on free electrons = Q = nA eℓ

I = Q/t * v, drift velocity of the electronsI = nA e / tℓ * 1 electron = 1.6 x 10 ^-19

C

v = /tℓ

I = nAve Transport Equation

Consider a piece of copper and a piece of silicon with

exactly the same cross-sectional area of 3 x 10^-6 m². If

both the copper and the silicon carry a current of 100µA,

what is the drift velocity of the electrons in each?

Assume that n (copper) is 10^29 m^-3 and n (silicon) is

2.6 x 10 ^18 m^-3

I = nAve v (copper) = (10^-4)

v = I/nAe (10^29) x (3 x 10^-6) x (1.6x10^-19)

= 2 x 10^-9 ms^-1

v (silicon) = (10^-4)

(2.6x10^18) x (3x10^-16) x (1.6 x 10^-19)

= 80 ms^-1

Quiz

Page 127 - Textbook

Potential Divider

V1 = IR1V2 = IR2

V1 = R1

V2 R2

Quiz

What is potentiometer?It uses the principle of potential divider circuit.

Potentiometer

Internal Resistance = Resistance to electric current within the source of emf.

Effects of internal resistance of a source of emf

Results in:- 1) Voltage across the terminals (potential

difference) of the source dropping as a current flows through it

2) Source being less than 100% efficient as energy is dissipated in the internal resistance as current flows through it.

Sources of emf – Internal Resistance

Formulae of Internal Resistance

Diagram – Battery with Internal Resistance

Graph: Power Against Load R