6.2 Electic Fields Flashcards
what is an electric field?
an electric field is the region around a body in which other charged particles will feel a force due to the electric charge of the body
what do electric field lines show?
the direction a small positive charge would move
what is the equation for electric field strength?
E = F / Q
force per unit charge
what is the field shape of a point charge?
a point charge or any body which behaves as if all its charge is concentrated at the centre - has a radial field
what is Coulomb’s law? (in words)
the electrical force between two point charges is directly proportional to the product of their charges an inversely proportional to the square of their separation
what is the equation for Coulomb’s law?
F = Qq / 4πɛor^2
where Q and q = respective charges
ɛo = permittivity of free space
r = the separation between centre of charges
what sign will an attractive force have?
negative, -
what sign will an repulsive force have?
positive, +
what is the electric field strength strength equation for a point charge?
E = Q / 4πɛor^2
how can you work out the resultant electric field strength when you have multiple electric fields?
VECTOR sum of all the electric fields
what is a similaritiy between gravitational fields and electric fields?
both follow an inverse square law
what are some differences between gravitational fields and electric fields?
- g is defines as the force per unit mass, E is defined as the force per unit charge
- gravitational field lines always act radially inwards whilst electric field lines for a radial source act radially outwards for a positive charge and inwards for a negative charge
- gravitational force is always attractive whilst electrostatic force can be repulsive or attractive
- objects cannot be shielding from gravitational fields however objects can be shielded from electric fields
- the medium between the masses makes no difference on the gravitational force however the medium between the charges does effect the size of the electrostatic force
what is the equation for a uniform electric field?
E = V / d (like a capacitor)
where V = voltage between the plates
d = distance between them
how do charged particles move through uniform electric fields? similar to what?
like projectiles
why do charged particles in electric fields move like projectiles?
because of the combined effect of constant acceleration/force and constant velocity at right angles to one another is a curved path
(the component of the particle’s velocity at right angles to the field will remain constant)
what is capacitance dependent on?
permittivity of the dielectric, area of overlap (between plates) and plate separation
what is the other equation for capacitance?
C = ɛo x A / d
where ɛo = permitivity of free space (if dielectric is air)
A = area of the plate overlap
d = separation of the plates
what if the plates of the capacitor have a material between them or a different dielectric, what equation should you use for the permittivity?
ɛ = ɛr x ɛo
new permitivity = relative permitivity x permitivity of free space
what is the definition for electric potential?
electric potential is the work done per unit POSITIVE charge to move that that from infinity (where the potential is zero) to a point in an electric field, measured in volts
what is the definition for electric potential energy?
electric potential energy of a body of charge q, is the work done to move that charge from infinity to a point in an electric field
what is the equation for electric potential energy?
E = Vq or E = Qq / 4πɛor
where charge q is at a distance r from Q (point charge)
what is the equation for electric potential?
V = Q / 4πɛor, measured in volts
because V = E / q, look at electric potential equation
what does the graph of electric potential against distance look like for a point positive charge moving closer to a positive source charge?
exponential downwards, because you have to put in work against the field to bring it closer, there is a repulsive force so there is an increase in electric potential as you get closer to the source charge
what does the graph of electric potential against distance look like for a point positive charge moving closer to a negative source charge?
exponential upwards starting from negative axis, because electric potential will decrease as seperation decreases, electric potential gets bigger further away because its now an attractive force not a repulsive one
what is the equation for the capacitance of an isolated charged sphere? how can you get to it?
C = 4πɛor
(because it is a charged sphere you can assume all of its charge is at its centre and treat it like a point charge)
because V = Q /C and V = Q / 4πɛor hence C = 4πɛor
what does the force-distance graph for a point or spherical charge being moved in a repulsive radial (two like charges) electric field look like? and what does the area underneath equate to?
an exponential downwards, force decreases with distance, the area underneath is equal to the work done or the electric potential energy
