chapter 17 electric field Flashcards
what is an electric field
a region in which a charged particle experiences a force
in a uniform field what is the equation linking electric field strength, potential difference and separation of charges
E = V / D E = electric field strength, V = potential difference, D = separation of charges
in a uniform field what is the equation linking electric field strength, electric force and charge
E = F / Q E = electric field strength, F = electric force, q = charge
what is the angle between equipotentials and field lines
Equipotentials are always perpendicular to the field lines.
what is the equation for electric field strength where the field lines curve, and what is the minus for
E = -dV / dr where r is the distance along the field line
the minus shows the direction of the electric field is in the opposite direction to the increase in V
what is Coulomb’s law used for
to calculate the electric force on a charge, as well as to calculate the force of repulsion or attractiveness between 2 point charges.
what are the requirements for modelling a charged object as a point charge
is the charged object is a sphere with evenly distributed charge, so it will act as if all of its charge is at its centre so that it can be modelled as a point charge
what is the inverse square law between 2 point charges
1 / r^2, the further apart the charges, the weaker the forces between them
what can be said about the forces on 2 interacting point charges
the force on one point charge is the equal and opposite of the force on the other one
if F(electric) is positive in the coulomb’s law equation, what can be said about the 2 point charge’s interactions?
if the charges on the point charge’s are the same, there will be repulsive forces and so F will be positive
if F(electric) is negative in the coulomb’s law equation, what can be said about the 2 point charge’s interactions?
if the charges on the point charge’s are opposing each other, there will be attractive forces between them and so F will be negative
define electric field strength
force per unit charge
what is coulomb’s law
the force between 2 charges q and Q is directly proportional to each of the charges, and inversely proportional to the square of the r, the distance between the centres of the 2 spheres
when is the coulomb’s equation negative and when is it positive
negative for attractive forces, positive for repulsive forces
what is the value of electric potential V for a charge sphere where there’s a radial distance of infinity
as r goes to infinity, electric potential goes to 0
define electric potential energy
the work done to move a charge q from infinity to a distance r away from a point charge Q
how does a point charge q gain potential energy in an attractive field
the charge q gains energy as r increases, because you need to do work overcome the attractive forces between the 2 attracted point charges so that you can move them further away
how does a point charge q gain potential energy in an repulsive field
the charge q gains energy as r decreases because you have to do work against the repulsion to bring q closer to Q
what is the equation to calculate the charge on an oil drop in Millikan’s experiment
q = mgd / v q = charge m = mass in kg d is distance in metres v = voltage
what are the 2 equations that are equal when an oil drop is suspended between 2 charged plates
F = qE, w = mg, qE = mg
define equipotential surface
a surface where the electric potential is constant
what was the milikan’s oil experiment used to determine
the charge on an electron e
what is the process in the milikan’s oil drop experiment, and what were the results
oil drops fall through a hole in the top plate, the positively charged plate, where they were ionised between 2 oppositely charged plates
the pd was then adjusted until a drop was suspended so that the uniform gravitational and electrical fields were equal, mg = qe
from this equation we can work out the charge on an electron with the different pd results
All of the values for q were multiples of 1.6 * 10^-19C.
this gave us the value of charge for electron as -1.6*10^-19C
evidence for discreteness of charge on
electron
milikan’s results showed that the charge on an electron was 1.6 * 10^-19C, and as all his results were whole number multiples of this charge exists in packets of 1.6 * 10^-19C
what does the area under an electric force, F-electric graph between 2 values on the x-axis give
change in electrical potential energy, E
how do you work out change in electric potential energy, E from an electric force, F-electric, graph
find the area under the electric force graph between 2 values on the x-axis to find the change in potential energy
what does the gradient of an electric potential, V, give
the electric field strength, E-electric
how do you work out electric field strength, E-electric, from a graph of electric potential, V-electric
the gradient of the electric potential graph gives us the electric field strength
what does the area under an electric field strength, E-electric, graph between 2 points give
The area under an electric field strength graph gives the electric potential, V-electric
how do you work out electric potential, v-electric from a graph of electric field strength, E-electric
find the area under the graph of electric field strength between 2 values on the x-axis to give electric potential
what does the gradient of an electrical potential energy, E, graph give
the electric force, F-electric
how do you work out electric force from a graph of electrical potential energy
the gradient of an electric potential energy gives the electric force
how do you work out electric force for the oil drop in Millikan’s experiment
F-electric = q V / d
what path do charged particles in a magnetic field take
deflected in a circular path
An infinite distance from Q, what potential energy does a charged particle q have?
Zero potential energy
Describe the relationship between potential energy and distance in a repulsive field? What sign are the charges of q and Q?
Q and q are both positive (or both -ve). You have to do work against the repulsion to bring q closer to Q. The charge q gains potential energy as r decreases
Describe the relationship between potential energy and distance in a attractive field? What sign are the charges of q and Q?
Q is negative and q is positive. The charge q gains potential energy as r increases (epe becomes less negative)
Charged particles in a magnetic field are deflected in a [ ] path
circular
Describe the motion of charged particles in a magnetic field
By Fleming’s left hand rule the force on a moving charge in a magnetic field is always perpendicular to its direction of travel
Different charged particles will have paths with different [ ] - the higher the ratio of m go q, the [ ] the radius of the path
Radii
Larger
Why does a current carrying wire experience a force in a magnetic field?
The negatively charge electrons are affected by magnetic fields
What sign is electric potential for repulsive and attractive forces?
Repulsive force = positive potential
Attractive force = negative potential
Why is there a negative sign in the relationship between electric field strength and the rate of change of electrical potential with distance?
To increase the charge’s potential, and potential energy, you have to do work against the force - they ‘act’ in the opposite direction
Explain the effect of moving a unit charge between two distances R (bigger) and r. Use force, graphs and energy in your answer
You change the charge’s electric potential energy when you move it between two distances. In order to do this, you have to apply a force and do work.
The force applied is equal to the electric force. For a point charge q, you can plot the electric force against distance from the charge Q which is producing the electric field.
This is an inverse square law and the area under the curve between R and r gives the change in electric potential energy
similarities between electric and magnetic
have a force per unit:
force per unit charge for electic
force per unit mass for grav
follow the inverse square law
differences between electric and magnetic
electric can be attractive or repulsive, grav can only be attractive
objects can be shielde3d from electric fields, can’t be shielded from grav
electric field strength depends on medium, weak over air etc
grav field strength independent of medium
