Chapter 22 Elec Fields

Question 1

What direction does an electric field act

Answer 1

Shows the direction a POSITIVE test charge would move when placed in the field

This is from POSITIVE TI NEAGRIVE

Question 2

Where do electric fields come from

Answer 2

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

Question 3

Remmeber hat equations to use when radial field vs unfimrom field?

Answer 3

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

Question 4

Why do we use distance r as distance to centres?

Answer 4

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

Question 5

If firce is a negative sign, vs a positive one, then what does this mean

Answer 5

This means attraction for neagrive and repulsion if positbe

Question 6

What is capacitance

Answer 6

It’s a measure of how much charge can be stored per unit voltage

Question 7

So for a parallel plate capacitor, what do you think the charge stored depends on

Answer 7

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

Question 8

Why does e = e0 x r

Answer 8

The dielectric separating is ALWAYS bigger than the permitivity of air , so just some number r can be used to show this

Question 9

What is permitivity

Answer 9

It’s a measure of how strict an electric field can be be made in the material,

Question 10

How ti di experiment to find electric permitivity if ANY material using parallel plates

What piece of equipment used to measure charge?

Answer 10

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

Question 11

Just to clear niw, if you have two parallel plates with different voltages, which side is the positibe one?

Answer 11

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

Question 12

How can an electric field ACCELERATE an electron?

Answer 12

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

Question 13

When an electron enters a uniform field what is the firce

So why can we use SUVAT

Answer 13

Firce is uniform so accelrwtion if irmom so we can use SUVAT

Question 14

2 scenarios

Electron moved from positive plate to negative

Electron moved across field and projectile motion

Answer 14

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

Question 15

So what is definition for electric potential energy

Acc definition
- if energy negative/ positbe ?
Alternative way of thinking
- if energy negative / positbe

Answer 15

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

Question 16

Units for potential ?

Answer 16

This is energy / charge so JC-1
Also volts
Also can be Cm-1 too

Question 17

How does a sphere become a capacitor

Answer 17

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

Question 18

So why is earth a calcitonin

Answer 18

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?

Question 19

When an electron is fired in a parallel plate or whenever, what is the work done on the electron remember electron gun

Answer 19

EQ= 1/2mv2

Question 20

Equipotential lines

Answer 20

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

Question 21

Work done remember equation and thus meaning

Answer 21

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

Question 22

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

Answer 22

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

Question 23

How to add potentials and fields

2) also rules for like what energy is dependent on etc

Answer 23

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

Question 24

Don’t get confused mass and elec fields ptoentisl and energy ideas

Answer 24

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

Question 25

IMPORTANT for identifying if OVERALL work has been done?

Answer 25

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