Electric Fields Flashcards
What are the differences between the electrostatic force and the gravitational force?
- The gravitational force is felt by objects with mass but the electric force is felt by objects with charge.
- Around the mass there is a gravitational field whereas around the charge there is an electric force
- There is only one type of mass, but there are 2 types of charges : positive and negative
- The gravitational force is very weak compared to the electrostatic force
What are the similarities between the electrostatic and gravitational force?
- The gravitational and electrostatic force are both attractive, but the electrostatic force can be repulsive also
- Both forces are inversely proportional to distance squared
Electric force and electric field strength
The Electric force is a vector and is equal to F=qE where q is charge and E is electric field strength.
In a radial field, the electric force is kqq/r^2.
Units N.
The electric field strength is a vector and it is defined as the electric force per coulomb positive charge. E=F/q. In a radial field it is E=kq/r^2
Units NC^-1.
Drawing electric field lines
When drawing electric field lines, the arrows show the direction of the positive charge, so they are drawn pointing from positive towards negative.
The spacing of electric field lines show the strength of the electric field - the closer together the lines, the stronger the field.
Uniform electric field
The electric field strength for a uniform field is given by E=V/d.
In a uniform field, the electric field strength is the same everywhere, no matter the distance.
Because the electric field is constant everywhere in a uniform field, this also means that the electric force is also constant everywhere is a uniform field.
Motion of charge in a uniform electric field
The path tends to be parabolic.
In questions determine the direction, and determine if the charge is positive or negative.
The charge will accelerate in a parabolic trajectory towards the side of the field is it attracted to.
Define electric potential and show its equation
This is the work done per unit positive charge, to move the charge from infinity, the to point we are interested in.
V=kQ/r
Potential difference
This is the energy transferred when 1c of charge passes from one point to another point.
W=QV
V=W/Q
Why is the electric potential positive for a positive charge but negative for a negative charge.
We can take the potential at infinity to be 0.
Remember, the electric potential is the work done per unit POSITIVE charge, to move it from infinity to our point of interest.
When we have a positive charge, there is a force of repulsion and we have to do work against this force of repulsion so electric potential is positive is positive as we DO WORK.
When we have negative charge, there is a force of attraction so we do negative work, so the electric potential is negative.
Equipotential surfaces and lines
An equipotential surface is one where all the points have the same electric potential.
We draw equipotential lines perpendicular to the electric field lines.
In a radial field, the equipotential lines are closer together where the field is strongest.
No energy is transferred when we move a charged particle along a line on constant potential (equipotential).
What is the electric potential gradient?
This is the rate of change of potential with respect to distance. So, it is the gradient of a electric potential, V, and a distance graph.
Electric potential is proportional to electric field strength.
It gives the electric field strength - the gradient of a V-r graph.
How can we see electric potential on a graph?
The area under an Electric field strength ( y-axis) and distance (x-axis) gives the electric potential.
E=kQ/r^2 x r = kQ/r
kQ/r gives the electric potential
Electric field of plates when one plate is earthed
To determine the direction of the electric field between two plates when one plate is earthed, follow these steps:
Understand the Setup:
One plate has a positive charge.
The other plate is earthed, meaning it becomes neutral or gains negative charge to balance the field.
Electric Field Direction:
The electric field always points from the positive plate to the negative plate.
Since the earthed plate acts as the negative plate, draw arrows for the electric field from the positively charged plate toward the earthed plate.
What is Coulombs Law?
The force between 2 point charges is proportional to the product of the charges but inversely proportional to the square of the distance between the charges.
