Electric Fields Flashcards
What is an electric field?
the region around a body in which other charged bodies will feel a force due to the electric charge of the body
What do electric field lines show?
the direction of the field at a point is the direction in which a small positive charge would move when placed at that point
How can the strength of a field be shown on its field lines?
in a strong electric field , the lines are drawn close together, whereas in a weaker field they are drawn further apart
Define Electric field strength
force per unit positive charge (NC^-1)
Equation for electric field strength
E = F/Q
E- electric field strength (NC^-1)
F- size of force (N)
Q- size of positive charge (C)
*F is the force acting on the charge Q, which is within the electric field.Here,Q IS NOT causing the field
State Coulomb’s law
the electrical force between two point charges is proportional to the product of their charges and inversely proportional to the square of their seperation
State Coulomb’s law in equation form
F= Qq/4 x π xεo x r²
(for the force between two point charges)
εo- permitivitty of free space (8.85x10^-12)
Q and q are the charges
r is the distance between Q and q
Keeping Coulombs law in mind, if the charges are opposite what will the force be?
the force will be attracitve but F will be negative
Keeping Coulombs law in mind, if the charges are like charges what will the force be?
the force will be repulsive and F will be positive
State the equation to find the electric field strength for a point charge
E = Q/ 4πεor²
State 2 similarities and differences between the gravitational field of a point mass and the electric field of a point charge
-they both have an inverse square law with distance
g=GM/r² , E = Q/4 x π x Eo x r²
- g.f.s is the force per unit mass and e.f.s is the force per unit charge
- electric field lines for a radial source act radially outwards from a positive charge and inwards for a negative charge whereas gravitational field lines always act radially inwards
- electrostatic force can be attractive or repulsive whereas gravitational force is always attractive
What do electric, magnetic and gravitational fields have in common?
they are regions where a force can be exerted on an object at a distance
what is a neutral point?
the place between two charged objects where the fields exactly cancel and the net force is zero
State the equation to find the electric field strength of a uniform field
E= V/d
V- potential difference between the plates (V)
d- distance between plates (m)
State the equation to find the capacitance of a parallel capacitor
C = EA / d
C- capacitance ( F)
E- permitivity of the material between the plates
A- area of the overlap of the plates
d- the seperation of the plates
* if the material between the capacitor plates is air then you use : C= EoA / d
How do you find the permitivity E, if you are given the relative permitivity Er, of the material?
E = Er x Eo
E- permitivity
Er- relative permitivity
Eo - permitivity of free space ( 8.85x10¯¹² C²m¯²N¯¹)
What would occur if you placed a stationary charged particle within a uniform electric field?
the charged particle would accelerate either from the +ve plate to the -ve plate or vice versa depending on its charge
What would occur if you were to inject a moving charged particle into a uniform electric field , initially at right angles to the field
the particle’s vertical velocity component will remain unchanged and the velocity in this direction will be uniform but there will be an acceleration in the direction of the field which results in a parabolic path
Define electric potential
the work done in moving a unit of positive charge from infinity to a point in the field
*the electric potential at infinity is zero
State the equation for electric potential
V= Q/4πEor
State the equation to find the capacitance of an isolated sphere and how you derive it
C= 4πEoR
Here , R is the radius of the sphere
Demonstrate how a force-distance graph for a point/spherical charge looks like and state what the area under the graph is
work done = force x distance therefore area under the force-distance graph is numerically equal to the work done in moving the charge from one point in the field to another
State the equation for electric potential energy
E=VQ
or
E = Qq/4πEor
where v is the electric potential
q is the size of charge in electric field
Sketch the elctric field near two parallel plates
there is a uniform electric field between the plates except at the edges
State an example where a uniform field can be created
two parallel conducting plates that have been charged by connection to the terminals of a battery
the uniformity of the field is shown by the constant seperation of the field lines
*remember the plates carry equal and opp charges so there is a uniform electric field between them except at the edges
What does a capacitor consist of?
two parallel conducting plates which are seperated by a gap of size d - the gap can be filled with air or an insulating material called a dielectric
Describe the similarities and differences between a radial electric field and a uniform electric field
similarities : field lines show the direction in which a positive unit charge would move if placed at that point in the field
difference : for a radial field , field lines diverge as we move further away from the source and field strength decreases with distance
for a uniform field the field lines are parallel and the field strength is the same at every point
What is a point charge
a charge which exists at a single point in space e.g. an electron or proton
