Electric fields Flashcards
What is a free electron
An electron that can move about and isn’t attracted to a singular atom
First step to charge an object
- Must first be isolated from the earth - otherwise any charge gained is neutralised by electrons transferring between the object and the earth
Why are certain insulators easy to charge
Their surface atoms can easily gain/lose electrons e.g. perspex and polythene
Electric field line
Path a positive test charge follows
Field lines for oppositely charged points
Concentrated at the points - positive test charge flows along curved path to negative
Field lines for point to oppositely charged plate
Concentrated at the point, but at right angles where they meet the plate. Field is stronger where lines are concentrated
Field lines for oppositely charged plates
Run parallel (slightly eccentric)
meet plates at right angles (apart from edges)
Field is uniform between plates as field lines are parallel
Electric field strength
Force per unit charge on a positive test charge at that point
(Newtons per coulomb)
Properties of electric field strength
In same direction as electric field if object is positive (viceversa)
A vector
+Test charge must be negligible compared to 1 coulomb as it would affect the charges that cause the field - affecting electric field strength
Proportionalities of E between parallel plates
depends on concentration of charge on surface of plates, proportional to charge per unit area
Does work done in moving a particle in a field depend on the path it takes?
No
Using eq to see if force is attractive or repulsive
Multiplying 2 positive charges gives a positive force - repulsive as it acts in the same direction as increasing separation
Adding field strength vs potentials
Field - vector sum as field strength is a vector, potential is normal sum as is a scalar
Why insulators can’t conduct
Electrons are attached to individual atoms (not free).
What should thread be made out of in shuttling ball exp
Insulator, otherwise wld divert charge
Calculating current in shuttle ball exp
I=Qf where f is frequency ball shuttles back and forth at (1/time taken to go there AND back)
Electric force on a neutron
Zero - chargeless
Air as an insulator
Can be treated as an insulator (gases are poor conductors of heat+lack of free e). However in strong electric fields, molecules can be ionised by having es removed.
Lighting where a cloud becomes more and more charged and E in air becomes stronger - charge discharges through air to Earth.
How a lightning rod works
Provides path of least resistance for charged cloud - tallest and most conductive thing, as opposed to discharging through air to the ground - less conductive + needs to travel greater length.
When a charged cloud is overhead, strong E at the tip of rod - ionises air molecules around it, discharge charge from cloud to rod then through earth.
Uniform field
Same magnitude + direction everywhere in field
Field factors
E field exists near any charged body, greater the charge, stronger the field. More concentrated charge is on surface, stronger E at surface.
Field between 2 parallel plates depends on concentration on charge at the surface, spread evenly across. Can be shown that E is proportional to Q/A where A is the area.
Q/A = Eε0 - Don’t need to kno eq
How a Van de Graff generator works
Charge created when rubber belt rubs against a pad - is carried up to metal dome. Potential difference between dome and earth builds up as charge accumulates until sparking occurs.
Spark transfers energy from dome, work must be done to charge the dome since a force is required to move charge up the belt to the dome (has same charge).
In general, work must be done to move a charged object closer to a like charge.
Electric potential
For 2 like charges X and Y
Electric potential increases from 0 as X is moved from infinity towards Y. Work done to overcome repulsion.
At a point in field, defined as work done per unit positive charge on another positive test charge in moving charge from infinity to that point.
By definition, position of 0 potential is infinity
W=Q(V2-V1)
Equipotentials
Surfaces of constant potential, no work done moving along them for a test charge, at right angles to lines of force of field
Potential gradient
Change in potential per unit distance.
In non uniform field, gradient varies with position - closer together equipotentials are, greater potential gradient (at right angles to equipotentials)
If field is uniform, equipotentials are equally spaced
E=-V/d
Why point charges are convenient
Where distances under consideration are much larger than diameter of object
same idea as a distant star - diameter negligible compared to distance
Summing electric field strengths
On a line
Et = E1+E2 (prove)
perpendicular
Et^2=E1^2+E2^2
Concentration of charge on objects
For a curved object/irregular, charge concentrated at sharpest/most curved points.
For a plate, concentrated at surface
Why charge gathers at the tip of a lightning rod when a charged cloud is overhead. Also why fatal accidents can occur if conducting rod is held near a high voltage cable
What makes an object a perfect conductor
If the electric field strength inside is 0
F and E def
F arrow = qE arrow
Force and E in same direction
Why equipotentials are negative for a negative point charge
Electric potential is taken to be zero at infinity.
A positive test charge would gain potential
energy as it was moved away from the negative charge and towards infinity.
Why an electron getting repelled from a negative changes its Ek, as well as de broccoli wavelength
- The electron loses potential energy as it
moves away from Q. - As it moves away its speed will increase.
- Its de Broglie wavelength will decrease…
- because its momentum will increase.
repulsive force
accelerates the electron, causing its
kinetic energy to increase and its
potential energy to decrease.
