Electric Fields Flashcards
how do we accelerate electrons in x-rays
by putting them in the appropriate electric field
how do we visualise the forces particles feel when in an electric field
by drawing electric field lines
what is a fundamental law about all field lines
- the closer the field lines are
- the stronger the field
- so the stronger the force exerted
what things can only be affected in an electric field
charged particles
what is the equation for calculating the force a charged particle feels
F = EQ
what are each of the variables in that equation
- F = force (N)
- E = electric field strength (Vm-1)
- Q = coulombs of charge
how would you use that equation to calculate the acceleration of a charged particle within an electric field
- F = ma
- ma = EQ
- a = EQ / m
what force will an electron feel when it is in an x-ray machines electric field which has a strength of 4.5x10^5 Vm-1
- F = EQ
- Q of an electron = -1.6x10-19
- (4.5x10^5)x(-1.6x10-19) = -7.2x10-14 N
why do we use a negative sign for the charge of an electron
- because the force is trying to accelerate the electron to the more positive end of the field
- this is in the opposite direction to the conventional field
- which goes from positive to negative (like field lines)
how would you calculate the velocity of the electron if it accelerates from rest for a distance of 10cm
- you need a kinematics equation with all the variables you need
- without time, its s = (v^2 - u^2) / 2a (v^2 = u^2 + 2as)
- as u = 0, youre left with v = the root of 2as
- as we know a = EQ / m
- root 2as = root (2EQs / m)
- inputting the numbers gives you a negative velocity
- as the electron is moving in the opposite direction to the field
what is electric potential
- the energy a charged object posses
- by virtue of its position within an electric field
- per unit charge
what transfer of energy occurred with the electron in the x-ray machine
- a transfer of its electric potential
- to its kinetic energy stores
what is the equation for electric potential
V = E(p) / Q
what similarity does this equation have to the equation for electric field strength
- F = QE goes to E = F / Q
- which tells us that electric field strength is the force felt by a particle within an electric field per unit charge
what equation can you use to work out the kinetic energy a particle gains by moving through an electric field
E(k) = QV
what is the equation similar to
- the energy supplied to a component within a circuit with a given charge and pd
- E = QV
what is the mass of an electron
9.11x10-31 kg
what is a uniform electric field
- an electric field between two plates with opposite charges
- in which the field lines connecting them are parallel and equidistant
what is the definition of the strength of a uniform electric field
a measure of how rapidly it changes the potential over a distance
what is the equation for the electric field strength of a uniform field
E = V / d
what are equipotentials
positions within a field with no potential difference between them
how would you draw the equipotentials in a uniform field
- vertical lines parallel to the plates
- equidistant from each other
why would you draw them like this
- because the pd across the plates changes the closer to get to one or the other
- so to maintain the same distance between both plates
- you would need to be parallel to them
what is the general rule for drawing equipotential field lines
- they will always be perpendicular to the field lines
- as field lines are indicating the directions that pd across the charged bodies would change
- so going perpendicular to that would give you the opposite effect
