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 experience a force due to the electric charge of the body
How can you create an electric field by yourself
By rubbing a glass rod with a silk cloth. Friction transfers electrons from the glass to the silk, making the cloth negative and the rod positive. The rod is then surrounded by an electric field - it will attract small pieces of a paper or water.
what do electric field lines show?
the direction of the force on a small positive charge (point charge)
what is the equation for electric field strength?
E = F / Q
force per unit +ve charge
Define electric field strength
The electric field strength of an electric field at a point in space is defined as the force experienced per unit positive charge at that point
How can you test the presence of an electric field using basic equipments
Using a thin strip of gold foil attached to the bottom of an insulator. The gold foil is given a constant +ve charge by momentarily touching it to the charged ball. The charged foil experiences an electrostatic force when close to the able. This force is smaller the further away the foil is from the charged ball.
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
F = k Qq/r^2
K = 1/4pie x epsilon
F = Qq/4pie x epsilon x r^2
what is the equation for Coulomb’s law?
F = Qq / r^2
K =1/4πɛo
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 = F / q = Qq/ 4πɛor^2q
E = Q / 4πɛor^2
what is a similaritiy between gravitational fields and electric fields?
both follow an inverse square law
A graph of E again 1/r^2 will produce
A straight line through the origin
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 attractive so the direction of field always act radially onwards towards the point of mass of object
For electric field: +ve point charges produce a repulsive force where the direction of field is away from the object
-ve point charges produce an attractive field where the direction of field is towards the object
- 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 other equation for a uniform electric field in a capacitor?
E = V / d (like a capacitor)
where V = voltage between the plates
d = distance between them
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 capacitance dependent on?
permittivity of the dielectric, area of overlap (between plates) and plate separation
Explain the experiment that works out the permitivity of any insulator places between the plates
The charged stored on a capacitor can be measured directly by discharging it into a coulombmeter
Using the flying lead, the capacitor is charged to a p.d. of V. The charge Q is measured by tapping the flying lead to the plate of the coulombmeter
Then draw a graph of Q against V
Gradient will give you capacitance
Then work out the permitivity using equation
C = Epsilon x A / d
Derive the equation E = V / d
When a charge accelerates from the +ve plate to the -ve plate it gains energy. Now between the two plates there is a p.d.
W = Fd
Or
VQ = (EQ)d V = Ed E = V/d
An electron is fired from a +ve capacitor plate towards the -ve plate along the direction of the electric field, with a velocity of 1.0 x 10^7 m/s. The p.d. across the plates is 600 V and their separation is 3.0 cm. Calculate the maximum distance the electron will travel
s = 1.4 x 10^-2 m
the electron will travel about 1.4 cm from the +ve plate before it turns back
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 there is no acceleration in the horizontal content, hence the horizontal velocity of the particle remains constant
The time spent in the field is given by the equation
t = L/v
There is however, acceleration in vertical component of the particle
Which gives the equation
a = F/m = EQ/m
Initial certainly velocity u = 0
Vv = u + at = 0 + EQ/m x L/v = EQL/mv
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 = Q/V =4πɛorV/V = 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
A football of diameter 22cm is covered with aluminium foil and suspended from a nylon string. A high-tension supply is set to 5.0kV. It’s +ve electrode is used to give a +ve charge to the ball. Calculate the charge stored on the ball
6.1 x 10^-8C
In terms of work explain when a +ve charged is pushed against another +ve charge
The charges experience an electrostatic force of attraction so they repel each other. So you have you do work to decrease the seperation between the charges. All the work done is stored as electrical potential energy. This stored energy is recoverable — all you need to do is let go
Define electric potential difference
It is the work done per unit charge between two points around the particle of charge Q
