chapter 17 electric field Flashcards
what is an electric field
a region in which a charged particle experiences a force
in a uniform field what is the equation linking electric field strength, potential difference and separation of charges
E = V / D
E = electric field strength, V = potential difference,
D = separation of charges
in a uniform field what is the equation linking electric field strength, electric force and charge
E = F / Q
E = electric field strength, F = electric force,
q = charge
what is the angle between equipotentials and field lines
Equipotentials are always perpendicular to the field lines.
what is the equation for electric field strength where the field lines curve, and what is the minus for
E = -dV / dr where r is the distance along the field line
the minus shows the direction of the electric field is in the opposite direction to the increase in V
what is Coulomb’s law used for
to calculate the electric force on a charge, as well as to calculate the force of repulsion or attractiveness between 2 point charges.
what are the requirements for modelling a charged object as a point charge
is the charged object is a sphere with evenly distributed charge, so it will act as if all of its charge is at its centre so that it can be modelled as a point charge
what is the inverse square law between 2 point charges
1 / r^2, the further apart the charges, the weaker the forces between them
what can be said about the forces on 2 interacting point charges
the force on one point charge is the equal and opposite of the force on the other one
if F(electric) is positive in the coulomb’s law equation, what can be said about the 2 point charge’s interactions?
if the charges on the point charge’s are the same, there will be repulsive forces and so F will be positive
if F(electric) is negative in the coulomb’s law equation, what can be said about the 2 point charge’s interactions?
if the charges on the point charge’s are opposing each other, there will be attractive forces between them and so F will be negative
define electric field strength
force per unit charge
what is coulomb’s law
the force between 2 charges q and Q is directly proportional to each of the charges, and inversely proportional to the square of the r, the distance between the centres of the 2 spheres
when is the coulomb’s equation negative and when is it positive
negative for attractive forces, positive for repulsive forces
what is the value of electric potential V for a charge sphere where there’s a radial distance of infinity
as r goes to infinity, electric potential goes to 0
define electric potential energy
the work done to move a charge q from infinity to a distance r away from a point charge Q
how does a point charge q gain potential energy in an attractive field
the charge q gains energy as r increases, because you need to do work overcome the attractive forces between the 2 attracted point charges so that you can move them further away
how does a point charge q gain potential energy in an repulsive field
the charge q gains energy as r decreases because you have to do work against the repulsion to bring q closer to Q
what is the equation to calculate the charge on an oil drop in Millikan’s experiment
q = mgd / v
q = charge
m = mass in kg
d is distance in metres
v = voltage
what are the 2 equations that are equal when an oil drop is suspended between 2 charged plates
F = qE, w = mg,
qE = mg
define equipotential surface
a surface where the electric potential is constant
what was the milikan’s oil experiment used to determine
the charge on an electron e
what is the process in the milikan’s oil drop experiment, and what were the results
oil drops fall through a hole in the top plate, the positively charged plate, where they were ionised between 2 oppositely charged plates
the pd was then adjusted until a drop was suspended so that the uniform gravitational and electrical fields were equal, mg = qe
from this equation we can work out the charge on an electron with the different pd results
All of the values for q were multiples of 1.6 * 10^-19C.
this gave us the value of charge for electron as -1.6*10^-19C
evidence for discreteness of charge on
electron
milikan’s results showed that the charge on an electron was 1.6 * 10^-19C, and as all his results were whole number multiples of this charge exists in packets of 1.6 * 10^-19C
