## Uniform Electric Fields - A level Physics

hi guys today I'm going to give you a

presentation on electric fields we're

going to be looking at the types of

charge the laws of electrostatics

electric fields that are uniform some of

the equations to do with electric fields

and the motion of charged particles

within electrical fields now unless you

had your head in the clouds during the

IGCSE course you should be aware that

there are in fact two types of charge

there's positive and negative charges

positive charges here well the electric

field radiates out of positive charges

and in a negative charge the electric

field goes towards the negative charge

another way of thinking about it is

imagine if there was a particle here and

it was positively charged well it want

to repel from the positive charge it

would feel a force in this direction

whereas if it had a positive charge here

it would be attracted towards the

negative charge so that's one way of

thinking about the fields around

positive and negative charges okay let's

just go over the laws of electrostatics

but you should again be familiar with

them from the ITCC course if you've got

a positive and negative charge they will

attract towards each other because

opposite charges attract

whereas if you've got a negative and a

negative charge they'll repel and

likewise a positive and a positive

charge will also repel electric field

strength now electric field strength is

a vector quantity so it's got a

direction and it's defined as the force

per unit positive charge and you can

calculate electric field strength by

using this equation here where we've got

the electric field strength which is in

Newton's per Coulomb and that's equal to

the force on a charge divided by the

charge of the charge and if you've got a

uniform field so imagine if I had two

plates here a positively charged plate

and a negatively charged plate I'd have

an electric field running from the

positive towards the negative the field

strength of a uniform field to calculate

the field strength of a uniform field

you're going to need to

use you need to know the potential

difference between these two plates so

maybe your upper plate might be charged

at say positive maybe five thousand and

normally the negative is probably going

to be zero probably connected to earth

so you know the potential difference

between the plates and also you need to

know the distance or the separation

between the two plates has to be in

meters but usually in an exam they might

give you millimeters or centimeters and

you're going to have to convert so this

equation here helps us find the

electrical field strengths between the

plates and it's equal to the electric

field strength which we saw on a

previous slide again in Newton's per

Coulomb is equal to the potential

difference or the voltage divided by the

plate separation in meters the motion of

charged particles in an electric field

if I've got a positive charge here and

it's moving through my electrical field

it's going to feel a force in this

direction towards the negative plate

which is here so I could have a particle

maybe a beam of protons coming through

as soon as it gets into the electric

field it's going to feel an acceleration

towards the bottom plate here and when

it leaves I'll carry on going straight

so the movement of the charged particle

would be something like this go straight

accelerate towards the negative and then

carry on the street again and it's

important to note that the acceleration

is uniform that as your little particle

is moving accelerating towards the

negative plate that acceleration doesn't

actually change and that's a bit

counterintuitive because you kind of

expect as the positive charge got closer

and closer to the negative charged plate

that it would feel more acceleration

because it's getting closer and closer

but that just isn't true so it's just

something important to note that in a

uniform field the the force and

therefore the acceleration anywhere

within the field is uniform

it doesn't change okay

to finish off I just want you guys to be

aware of a famous experiment called

Millikan's drop experiment and it was

performed around 1909 and as the first

measurement of the charge of an electron

it's quite a clever idea using uniform

electrical fields what Milliken did he

got a oil drop and he dropped it through

a hole here and the oil drop had a

charge in it and there the charge or the

electric field was set up in a very

clever way and gravity is trying to pull

the charge the oil drop all the way down

but Milliken just got the correct

voltage in order to provide a force

going upwards that would balance with

the force going down the force of weight

the oil drop is stationary and yet the

forces to balance this is very very

clever because what he could do is he

knew the weight and he knew the the

electric field strength so he'd work out

the distance between the plates and he

knew the voltage between the plates so

he knew the electrical field strength

here but he didn't know the charge so he

could rearrange this equation here so

these two would be equal so mg would

equal at the electric field strength

times the charge and the only thing he

wouldn't really know would be the charge

so just by rearranging the equations he

could find the electrical charge of an

electron and 100 years ago he was about

2% out which is really quite remarkable

anyway bye for now

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