Module 6 Electric Fields Flashcards
State Coulombs law in words
The force between two charges is proportional to the product of the charges and inversely proportional to the square of their separation
For Coulombs law, how do you know the sign of the force calculated?
Positive is repulsive
Negative is attractive
Put actual charge ± in the equations
If a known mass has a charge and is on a string at an angle and attracted by a nearby charge, how can you calculate the tension if both charges are known? (mass at rest)
set up a triangle of forces
F is Coulomb force
W=mg
Use Pythagoras and trigonometry
Equation for electrostatic or Coulomb force between two charges?
In an electric field diagram, which direction do field lines point?
in the direction which a POSITIVE test charge will move
What is an electric dipole?
diagram of electric field lines between two equal and opposite charged particles
Draw a simple diagram for the electric field around a dipole
Draw electric field around a point positive charge
Radial field
Electric fields line spherically symmetrical
Arrows away from charge as positive test charge would be repelled
Draw a simple diagram for the electric field between a positive point charge and a metal plate
Modified radial field .
Field lines distorted by plate and meet the plate at right angles
Describe and explain a diagram for a positive point charge and a metal plate
Metal plate has many free electrons which are attracted to the positive charge
Side closer to the charge becomes negatively charged
Field lines move from the charge and meet the plate at 90º
Features of electric field diagrams
arrow to show the direction of the field = direction that a positive charge will move
The density of the field lines gives an indication of the field strength.
Define electric field strength
Electrostatic force per unit positive charge
Which direction is the electric field in?
same direction as the direction a positive charge will move
When can the equation E=F/q be used?
For any electric field E
F is force on charge q placed in the electric field
When two of the same point charges are placed near each other, how do you find the distances where the resultant electric field is 0?
Plug charges (or factor of charges) into E=Q/4πε0r^2 for each charge
Replace r^2 for x^2 and y^2
Cancel out everything but factors and x y
Square root both sides
This is the ratio from of x to y (make sure you get it the right way round
Describe the similarities and differences between electric and gravitational fields around a point mass
Similarity: point mass or charge produces a radial field that has spherical symmetry and infinite range
Similarity: Both fields obey the inverse-square law
Difference: Gravitational fields affect all masses, electric fields only affect charged masses
Difference: gravitational fields always produce attractive forces, electric fields can produce attractive or repulsive forces
Difference: the strength of an electric field is generally much higher
Which equations can you use for uniform electric fields?
E=F/q or E=V/d (parallel plates)
How is a uniform electric field produced?
placing two oppositely charged parallel plates parallel to each other
Which way do electric fields lines go in a parallel plate?
Same direction as a positive charge
Same direction as conventional current
From the more positive voltage to the more negative voltage
Draw a simple uniform field
Field lines are straight, parallel and uniform separation
Derive the equation for E=V/d for from W=Fd
Work done W=Fd and electric field strength E = F/q
Replace F so W = Eqd
W/q=Ed but W/q = V (from W=QV)
so V = E/d
Draw a simple graph of variation of electric field strength E against separation d for a uniform field
Constant E
Draw a simple graph showing the variation of potential difference V against distance x for a uniform field
Constant gradient . V/d = E
Units for electric field strength?
Newtons per coulomb
or
Volts per metre
What is an equation which can be used to find final velocity for a charge being accelerated through a uniform electric field starting from rest
QV = 1/2mv^2
What do you do if you’re asked to compare the value of force due to an electric field on a charge when compared to force due to gravity?
Work out each force then
make a ratio of one to the other
What is the name for the path of charged particle sent through a uniform field at right angles?
parabola
Constant horizontal velocity
Constant vertical acceleration
(think suvat)
What does vertical deflection mean?
distance moved up or down due to force from uniform field
Define electric potential?
work done to bring a unit charge from infinity to that point where potential is defined as zero at infinity
Is electric potential scalar or vector?
scalar
What is the more useful equation for electric potential?
V = PE/q = Q/4πε0r
(PE = Qq/4πε0r)
Describe how the equation for capacitance for a metal sphere is calculated
A charged metal sphere behaves like a charged mass Q at the centre of the sphere
So at the surface V = Q/4πε0R
Rearrange for Q/V = 4πε0R
Q/V is capacitance
C=4πε0R
