Chapter 22 Elec Fields Flashcards
What direction does an electric field act
Shows the direction a POSITIVE test charge would move when placed in the field
This is from POSITIVE TI NEAGRIVE
Where do electric fields come from
Any charged particle will have it, and if another charged particle placed in this presence (as it has its own field), will also feel a force due to coloumbs law, where the firce is the same for bith
Remmeber hat equations to use when radial field vs unfimrom field?
For radial field , F =Qq etc
But for unifrom use E=F/Q
Equal parallel = uniform, closer branching raidal, closer they are the stringer the fields
Model the charges as POINT CHARGED
Why do we use distance r as distance to centres?
Because each time we make assumption that it’s a POIJT MASS CHARGE, so in this case distance would be to centre
In reality it’s not like that
If firce is a negative sign, vs a positive one, then what does this mean
This means attraction for neagrive and repulsion if positbe
What is capacitance
It’s a measure of how much charge can be stored per unit voltage
So for a parallel plate capacitor, what do you think the charge stored depends on
1) distance away , (closer, more charge) so inversely
2) area, bigger area = more charge
So c prop to A/d
And thus there is a constsnt of proportionality which is the ELECTRIC PERMITIVITY in between
Which can be calculated as permitivity of r x one value
So C = E x A/D
Why does e = e0 x r
The dielectric separating is ALWAYS bigger than the permitivity of air , so just some number r can be used to show this
What is permitivity
It’s a measure of how strict an electric field can be be made in the material,
How ti di experiment to find electric permitivity if ANY material using parallel plates
What piece of equipment used to measure charge?
We know c=q/v and EA/D
EQUAting gives us an equation Q= EaV/D
Such thst if we could find different charges stored by different voltages, the gradient = EA/D
Here do this by
1) charging suing flying lead connected ti capacitor , where it will charge ti a V = to emf
2) measure the charge by detaching flying lead and attaching to the COLUMBMETER
Find results, o,otngraoh, find gradient and yiu can now find electric permitivity
Just to clear niw, if you have two parallel plates with different voltages, which side is the positibe one?
Positibe one = MORE POSITIVE VOLTAGE
THink about it
- if more positbe potential, and same distance, the only way for more energy taken to bring test charge for. Infinity is if ORIGINAL CHARGE has POSITIVER CHARGE
Thus more positbe voltsge takes this
How can an electric field ACCELERATE an electron?
In the presence of an electric field, the charged particle electron will feel a FORCE, and thus acceleration
Especially if this field is constsnt
So electrons can’t be accelerated really fast
And since the F is dependent on V, that means based on voltsge it goes faster or slower
When an electron enters a uniform field what is the firce
So why can we use SUVAT
Firce is uniform so accelrwtion if irmom so we can use SUVAT
2 scenarios
Electron moved from positive plate to negative
Electron moved across field and projectile motion
When moves from POSITIBE ti negative, accelrwtion will be against electron so only actd one way
When moved across, horizontal velocity stays same but vertical changes for same reason, get projectile motion,
Find final velocotu using SUVAT, and the angle using tan
So what is definition for electric potential energy
Acc definition
- if energy negative/ positbe ?
Alternative way of thinking
- if energy negative / positbe
The amount of energy needed to bring a charge from infinity to a point serowted by r from another charge
If the energy is negative, it means the two charges are attractive, and so bring from infinity to point actually RELEASES ENERGY Z
IF ENERGY POSITIBE, means the charged repel, so it acc needs EXTERNAL ENERGY to bring it to that point
However also think about energy needed to take from that place to infinity (so if I let go)
If it’d negative , you need to add that much energy to bring it back to ifnintiy
If positbe, then letting go means it will EXPEL THAT MUCH ENERGY BACK
so if q says how much energy needed to separate comoletley (so go to infinity)
Answer negative, means you need that much energy ti take it away
Units for potential ?
This is energy / charge so JC-1
Also volts
Also can be Cm-1 too
How does a sphere become a capacitor
If it is A SPHERE, it can act like a capacitor no matter what
And this equaruon is c = 4Pi E0r
So the capacitance of a sphere is ONLY dependent in its radius and nothing else
So why is earth a calcitonin
This is because it is a sphere with a radius, so will act as a capacitor
So doesn’t even need to have charge, as capacitance just shows how much charge would be stored when it has a potential difference
So all spheres a capacitors?
When an electron is fired in a parallel plate or whenever, what is the work done on the electron remember electron gun
EQ= 1/2mv2
Equipotential lines
Are lines with equal ptoentisl,
As energy = change in ptoentisl x charge, if there is no change in ptoentisl, then that means no energy needed to move it form one place to other
Work done remember equation and thus meaning
Work done = Force x distance travelled in DIRECTION OF FORCE
So for uniform plate, force only acts down, so work done csn only happen going down , not sideways
- think about it using ideas of potential, assuming the distance soerstion and overall ptoentisl the same, at any point the E is constsnt
- so lowering distance increases potential
So there is a change in ptoental , and energy done when change in ptoentisl happens
But moving across, there is nil change in ptoentisl as d stays the same.
- therefore no energy done
Assuming intial V and D stay constsnt, what is always constsnt for jnfirmom plate system, and thus how can you find out the different potentials INSIDE
E ALWAYS CONSANT
The potentials will vary from the INITAL positbe plate until the lower plate, continous spectrum
This is dependent on distance , as I increase distance away , increase ptoentisl , decrease then decrease
How to add potentials and fields
2) also rules for like what energy is dependent on etc
Fields = vector addition
Potentials = scalar addition
For gravity potential can never be 0
But for elec fields as the potential can be bith signs both fields and potential can be 0
2) only dependent on start and end positions, ignore everything else in between
Don’t get confused mass and elec fields ptoentisl and energy ideas
For mass, as it’s ALWAYS ATTRACTIVE (and can both though as positbe)
Going from infinity to a point releases energy and is negative, so going from point to infinity that’s how much energy you need to put in ti decorate
Similar if they are opposite , going from infinity to point will be negative and releases energy , same energy needed to separate
If repulsive, then energy put in to take from infinity to post , same energy released when let go
So it’s consistent, confusing ideas was that mass mass is like positbe positbe but it’s not because it stfatctuve even. Tho positive positive is repulsive
Don’t have ti doubt now
IMPORTANT for identifying if OVERALL work has been done?
First check if there is any change in potential , bearing in mind equipotential
Then check, if work has been done, has work been added going closer now
So if it’s the same magnitude then remember the overall change is probable 0
But yeah look at potential first
