Electricity

electric current?

flow of charged particles

unit of charge?

coulomb

charge of an electron?

1.6 x 10^-19

you cannot move a smaller charge than this and any charge that moves must be a multiple of this, ions are the same

in solids...

ions and protons cannot move, only electrons move

current flows from..

a positive potential to a negative potential

electrons move from a negative to positive potential

charge is defined as..

numerically equal to the current that flows per unit time

coulomb?

the charge that passes a point if one ampere flows for one second

mean drift velocity?

if an electric field is placed across the wire, the electrons will start to move. the mean drift velocity is the speed with which the electrons move through the wire

why are some materials good conductors?

they have a large number of conduction electrons available

number density?

the number of conduction electrons per unit volume

total number of conduction electrons =?

n x volume

derivation of I = nALe/t

total number of conduction electrons = n x volume

consider of a wire of length L and cross sectional area A

the number of conduction electrons will be equal to n x L x A

the charge of an electron is given the symbol e and is equal to 1.6 x 10^-19

the total charge of electrons in the section of the wire will be nALe

if all the electrons leave the wire in time t. Then the current in the wire will be given by

I = nALe/t

derivation of I = nAve or nAvq

if we start timing as the first electron leaves the wire and stop and stop timing when the last electron leaves the wire then we can have the velocity for the last electron = L/t

if we presume all the electrons move with the same speed we have the velocity of the electrons to be v=L/t

I = nAve or nAvq

q is the charge on the moving charge carrier

the actual velocity of electrons is about..

10^5 m/s

but the mean drift velocity is less than 1mm/s

if the current increases..

then the drift velocity increases

if the wire is thinner..

the drift velocity for a given current increases

if n is smaller..

then for a given current the drift velocity increases

in conductors n is about..

10^29

in semi conductors n is about..

10^23

the mean drift velocity in a semi conductor is about a million times greater than in a conductor

definition of e.m.f?

total work done (total energy transferred) when unit charge flows round a complete circuit

it refers only to the energy source as it indicated now much energy it is able to provide

potential difference?

refers to the difference in potential between one side and another of any component/circuit

potential difference =

work done/charge

power =

work/time

what is p.d and e.m.f measured in?

voltage

P=

VI

I^2R = V^2/R

energy =

power x time

E=VIt

definition of resistance?

the ratio of the potential difference across the component to the current passing through it

R=VI , measured in ohms

one ohm?

resistance of a resistor if there is a current of one amp when the potential difference it is one volt

when the diode is forward biased..

it does not allow any current in until the threshold voltage is reached. This is usually very small - less than 0.6 volts

once this voltage is reached the resistance becomes almost zero and the diode will allow an infinite current through

a diode must always be put in..

series with another component or the current through it would get very large and would burn out the diode and possibly the power source

if the diode is negatively biased..

it will not allow any current through

if the voltage gets large enough it will break down the material of the diode and it will become a conductor

what is the current and and voltage like in a filament lamp?

as the voltage increases the current increases at a slower rate and so the resistance is increasing

although a filament lamp is made up of a metal wire the resistance does not stay constant because the temperature of the wire increases

the particles are vibrating more rapidly and stopping the electrons moving as easily through the material

the resistance of the wire increases with increasing current as the temperature is increasing

ohmic conductor?

the current through a resistor is proportional to the potential difference across it if the external conditions remain constant eg.temp.

metallic conductors are ohmic conductors

how do you find the resistance of a given ohmic resistor?

R = pL/A

p is resistivity

A is cross sectional area

L is length

units of resistivity?

p = AR/L

light dependent resistor?

the resistance of an LDR depends on the light intensity falling onit

as the light intensity increases the resistance decreases

a typical LDR has a resistance of 100 ohms in bright sunlight and a resistance of 100000 ohms in the dark

thermistor?

a device which has resistance that changes when the temperature changes

most have negative temperature coefficient which means that as the temperature increases the resistance decreases

all power sources have their own internal ..

resistance

some of the energy they produce is used in driving the charge through their internal resistance

therefore the energy available to drive the charge through the external circuit is less than the energy produced

often we can show the internal resistance

terminal potential difference?

a voltmeter is placed across the terminals of the power supply will measure the potential difference across the external circuit

this potential is known as the terminal potential difference

it gives the potential difference that is available to drive the charge around the external circuit

the power provided by a source is greatest when the ..

internal resistance = external resistance

the emf gives..

the total potential difference the source can provvide

derivation of V = E-Ir?

termal pd = v

V=IR where R is the effective resistance of external circuit and I is the current through it

E= IR +Ir = V+Ir

therefore V=E-Ir

Ir increases as current increases

a large resistance voltmeter will allow a very small current through it and Ir will be almost zero so the voltmeter gives a good indication of the emf