Chapter 22 - Electric fields Flashcards
Where does an electric field act?
Around any charged body. Greater the charge, stronger the field strength
What do electrical conductors contain a lot of?
Free electrons
What are free electrons?
Electrons which move about inside the metal and are not attached to any one atom
What happens when a charged conductor is connected to the earth?
Charge neutralised by electrons transfer between the conductor and the earth
What do electrically insulating materials not contain?
Free electrons
What is earthing?
When electrons move to or from the earth to balance charges on charged objects
What can a gold leaf electroscope be used for?
Used to detect charge
What happens if a charged object is in constact with the metal cap of the electroscope?
Some of the charge of the object transfers to the electrocope. Therefore, the gold leaf and the metal stem which is atttracted to the cap gain the same type of charge and the leaf rises beacause it is repelled by the stem.
What do field lines of an electric field show?
Path a free positive test charge follows when near a body with a much bigger charge.
Draw a field 2 oppositely charged objects would create
Draw the field a point object near an oppositely charged plate would create
Draw the field two oppositely charged plates create
What is the defintion of electric field strength (E)?
What is the unit?
electric field strength at a point in the field is defined as the force per unit charge on a positive test charge placed at that point
What is a test charge?
Charge with a magnitude so small that placing it a point has a negligible effect on the field around that point
What is the equation for electric field strength when a positive test charge is acted on by force F due to the elctric field?
E = F/Q
Decribe the field lines between 2 parallel plates?
- Parallel to each other
- At right angles to the plates
- From the positive plate to the negative plate
What kind of field acts between two parallel plates?
Uniform
What is the equation for electric field strength in a uniform field?
E = V/d
d = seperation
If the charge is moved from the positive plate to the negative plate, what is the equation for work done by the field on Q?
W(Fd) = Q x delta V
because: force,F, on a small charge Q in the field is given by F = QE and if the charge is moved from the positive to the negative plate, the work done, W, by the field on Q is given by W = force (F) x distance moved (d) = QEd. Ed = V therefore, E = V/d
How does electric field strength vary with charge per unit area on oppositely charged parallel plates?
Electric field strength increases as charge per unit area increases
electric field strength depends on concentration of charge on the surface of the plates.
What is the permittivity of free space annd what does it represent?
Eo - 8.85 x 10^-12 farads per metre (Fm^-1)
represents the charge per unit area on a surface in a vacuum that produces an electric field strength of one volt per metre between the plates
Define electric potential at a certain point in an electric field.
Give the units
Work done per unit positive charge on a positive test charge when it is moved from infinity to that position.
Units: volts/ JC^-1
What is the equation linking electric potential, electric potential energy and charge?
V = Ep/Q
What is special about a test charge’s potential energy when moving along an equipotential?
Has constant potential energy as equipotentials are sufaces of constant potential
Lines of force of electric field cross the equipotential lines at right angles
Draw a diagram of field lines and equipotentials around 2 positively charged objects
If a 2 x 10^-6 C test charge is moved from X to Y where the electric potential at X is 1000V and the potential at Y is 400 V, what is the difference in electric potential energy?
Ep at X = 2 x 10^-6 x 1000 = 2.0 x 10^-3J
Ep at Y = 2 x 10^-6 x 400 = 8.0 x 10^-4J
change in electric potential energy = - 1.2 x 10^-3J
What is the potential gradient at any position in an electric field?
Change of potential per unit change of distance in a given direction
How is the potenital gradient effected if the field is non-uniform?
- Varies according to position and direction
- Closer equipotentials are, the greater the potential gradient is at right angles to the equipotentials
How is the potential gradient effected if the field is uniform?
- Equipotentials between the plates are equally spaced lines parallel to the plates.
- Constant
- Equal to V/d
What’s the relation between electric field strength and potential gradient?
Electric field strength is equal to the negative of the potential gradient
E = - delta V/ delta x
What does Coulombs law state?
The force between two point objects in a vacuum is:
F = kQ1Q2/ r^2
where k = 1/4πEo
What is a point charge?
Expression for a charged object in a situation where distances under consideration are much greater than the size of the object.
Using Coulombs law, give an equation for electric field strength (E)?
E= F/q therefore:
E = 1/4πEo x Q/r^2
If 2 forces on a test charge from 2 point charges are in the same direction, what is the resultant electric field strength formula?
E1 + E2
If 2 forces are in opposite directions, what is the equation for resultant electric field strength?
E1-E2
If 2 forces are at right angles to each other, what is the resultant electric field strength formula?
E^2 = E1^2 + E2^2
For a charged sphere, where can you say the charge is?
At the centre of the sphere
At distance r from Q, what is the equation for electric field strength (E) in a radial field?
E = Q/4πEor2
Draw a graph of electric field strength and electric potenital against distance r from Q
What does this graph tell you about the relationships of electric field strength and electric potential with respect to r?
E proportional to 1/r2 - shows inverse-square law relationship
V is proportional to 1/r
What is different between electric potential and gravitationl potential?
Gravitational potential is always negative because the force is always attractive. Electric potential can be either positive or negative depending on whether Q causing the electric field is positive or negative.
How do you calculate change in electric potential from a electric field strength-distance graph?
Area under graph
What is the difference between electric and gravitational fields, in terms of their constant of proportionality?
Both inversely proportional to r²
What is the difference between electric and gravitational fields, in terms of their force equation?
electric - F=Eq
gravitational - F=mg
What is the difference between electric and gravitational fields, in terms of their direction of force?
electric - like charges repel, unlike charges attract
gravitational - all masses attract
What is the difference between electric and gravitational fields, in terms of their relative strength?
electric - strong at close range (responsible for chemical bonding)
gravitational - weak except for massive bodies (responsible for motion of planets)
