Chapter 22 electric Fields Flashcards
What is a field in physics
A region where an object will experience a force at a certain distance due to some property they have
For example mass or vharge
Where do electric fields come from
Any charged particle will have its own electric field, and thus if another charged particle was placed in this field a force would be entered on it
What is the electric field show (the firce or direction what charge will face)
Electric field shows the direction of force experience by a POSITIVE CHARGE
Negative field lines thus pointing in to them and will go reverse
What is the equation for electric field strength
E=F/Q
The force felt per unit charge at that point shows you the electric field strength at that point
Electric fuel strength is a vector quantity so what direction
Same direction as firce
, it’s the direftuojn a positive charge would move when placed at the point
R,ember what are basic facts about electric field lines
Closer they are stringer field, if parallel then uniform field
How can we model the electric field on uniform spheres or point charges?
As RADIAL fields, thus we can use equations to show how the electric field strength decreases with increase of radius
What does coloumbs law describe
The firce experienced by BOTH charged particles due to electric fields
Experienced by both due to newtons 3rd law , force exerted on each other will be equal in MAGENTIC use but opposite in direction
What is Coulomb’s law
F = Qq/r2 (4pi episilom 0)
Essentially the firce experience by two POINT CHARGES (has ti be radial fields, can model Sphered as point charges too) is proportional to the product of their charges and INVERSELY proportional to the square of their separation from the centre .
Thus we have a proportional constant, and this is 1/ 4PI x permitivity of a vacuum
Why can we model spheres uniform as POINT CHARGES? (And all of newtons laws were point )
Looking at it from afar looks like a point charge that’s why
So what’s Columbia equation in full
F = Qq/ 4piE0r2
How else can we now write down equation for electric field
E=f/q
The electric field strength at a distance r is equal to the firce field by a POSITIVE TEST CHARGE , divided by the charge of it , thus qcancel
(Sub in f =Qq formula)
So E= Q/ 4piE0r2
Simlaotiy to grav fields?
g = F/M
F= GMm/r2
So g = GM/r2
Same kinda thing
Both are assuming you have POINT MASSES of radial FIELD LINES
Key difference between grav field and elec field
Is that grav field is always attractive but elec can be repulsive too
For a system of two parallel plates where one is charged positive and other negative with a potential difference, what is the electric field strength between them shape?
Parallel lines = thus uniform field between them
What’s the electric field strength between two PARALLEL. Plates OR ANY UNIFORM FIELD charged by a potential difference
How to derive
E=V/d
All you need is voltmeter and measure the separation
This formula comes from v = wd/q, and wd = fxd, and sub in equation for f from e=F/Q
What even is a capacitor
A capacitor is an electric,a component that essentially stores charge when a pod is applied across it, and then can discharge this when attached ti something else
How does a capacitor store charge or in ither words what happens when the circuit first starts
Electrons go from one terminal to one plate and build up as that’s all they can do . At this point it repels electrons from surface if other player making it positive . As electrons can’t go anywhere they stay here until discharged.
This is possible by connecting wire to some other component and electrons will flow back as they want to be stable
This happens in a flashlight from a phone
What is capacitance then
A measure of how much charge can be stored on a parallel plate based on how much voltage is applied
C= Q/V
This is a constant on the Q against v graph as gradient
And depends on conditions of plate and insulator in between
What is the unit for cappacistsnce
This is Farrad , but CV-1
And seen as tho C is the gradient of Q against v graph what’s the energy stored by a cappacsit
And what is this energy is a fraction of total energy in ANY CIRCUIT , and why is that?
Well v = energy/Q, so energy = V x Q, which is precisely the area under our graph
So energy = 1/2VQ
2) if total energy is vQ it means capacitor always takes half energy, and other half is always loss in resistance and wires
How else write energy equation for cappacitorn
If E = 1/2VQ, and C= Q/V, then Q= CV
And E= 1/2cV2
So with a cappator again where is half the energy always gone
Lost to heat always and half energy taken by cappator
Back to electric field topic and cappaticne , we now know capacitance is a measure of how much charge can be stored on a parallel plate compared to voltage supplied,
what other factors would increase the amount of charge with one volt? (3)
1) The greater the area of the plate means MORE CHARGE can be held
2) the less the distance means more charge can be held too as further away harder ti hold
These are thus proportional, but we must also consider the “material “ we are in, as for different materials we will have different values
Thus this gives us a proportionality constant of the permitivity of the material capacitance is happening through, called a DIELECTRIC
