C22 - Electric Fields Flashcards
What’s an electric field?
A region around a charged particle which will experience a force.
(Can be attractive or repulsive)
In which direction does an electric field act?
From positive to negative.
If charges aren’t shown, arrows/field lines will be from positive to negative (so will point to negative).
What’s electric field strength?
Force experienced per unit positive charge at that point.
It’s a vector quantity
E = F/Q (SI units are N/C)
What do electric field lines show?
Lines of force of electric fields.
- arrows show direction of the field
- electric field lines are always at right angles to the conductor surface
- equally spaced, parallels lines represent a uniform field
- closer field lines represent greater electric field strength
What’s Coulomb’s law?
Any 2 point charges exert an electrostatic (electrical) force on each other that is directly proportional to the product of their charges, and inversely proportional to the square of the distance between them.
F = Qq / 4π ε0 r²
How do the electric field lines appear between 2 parallel plate?
Uniform - arrows/direction of force from positive to negative
How do the electric field lines appear between 2 point charges/spheres?
Radial. Not uniform.
Force is from positive to negative/lines bend towards the negative.
How can electric field strength (at distance r) from the centre is a sphere be found?
E = F/q
And F = Qq / 4π ε0 r²
Therefore E = Q / 4π ε0 r²
How is the property that creates the field different between gravitational and electric fields?
G.F: field created by a mass
E.F: field created by a charged particle
How is the type of field produced different between gravitational and electric fields?
G.F: they’re always attractive (direction of field always towards the object)
E.F: attractive and repulsive
Positive point charges produce a repulsive field (field away from object).
Negative point charges produce an attractive field (field towards the object)
How does the field strength (definitions) differ between gravitational and electric fields?
G.F: force per unit mass
g = F/m
E.F: force per unit positive charge
E = F/Q
How does the force between particles differ between gravitational and electric fields?
G.F:
force ∝ product of masses
force ∝ 1/separation ²
E.F:
force ∝ product of charges
force ∝ 1/separation ²
How does the force and field strength equations differ between gravitational and electric fields?
G.F:
F = -GMm/r ²
g = -GM/r ²
E.F:
F = Qq/4π ε0 r²
E = Q / 4π ε0 r²
What type of field forms with gravitational and electric fields?
G.F:
Point masses produce a radial field
E.F:
Point charges produce a radial field
What are the 2 (simple) formulae for calculating electric field strength, E?
E = F/Q
Force per unit positive charge
E = V/d
Potential difference per metre
This can only be used in a uniform field
What does the capacitance of a parallel plate capacitor depend on? (3)
The separation, d, between plates
The area, A, of overlap between the plates
The insulator (dielectric) used between the plates
For plates in a vacuum, how is capacitance related to area and separation?
C ∝ A/d
What happens if an electron is fired between plates, parallel to the electric field?
Being negative, it experiences attraction towards the positive plate so will either accelerate towards the positive (if fired in the same direction) or, if the positive plate is behind the electron, will slow down and change direction.
What happens if an electron is fired between plates, perpendicular to the electric field?
It acts as a projectile motion and is deflected towards the positive plate.
(Horizontal and vertical components considered independently)
How can electrons be accelerated?
Having a -ve charge, an electron between plates will travel away from the -ve plate and towards the +ve plate.
The electron experienced a constant electrostatic force because of the uniform electric field so it as a constant acceleration.
An electron travelling in the direction of the electric field, from +ve to -ve plate, will experience deceleration.
What happens to the horizontal and vertical motion of a charged particle moving at right angles to an electric field?
Horizontal:
- There’s no acceleration therefore Vh remains constant.
- Time spent in the field = t = L/v
Vertical:
- Vertical acceleration of the particle = a = F/m = EQ/m
- Initial velocity = 0
- The final component of velocity as the particle exits the field is given by v = u + at = 0 + EQ/m * L/v = EQL/mv
What is the area under a force - distance/separation graph represent?
Work done
What’s total work done equal to (electric field)?
It’s the same as electric potential energy, E.
What’s electric potential, V?
At a point, it’s the work done per unit charge in bringing a positive charge from infinity to that point.
V = E/q
(Like potential difference)
What’s a capacitor?
A device that stores charge.
How is capacitance calculated?
C = Q/V
What is capacitance?
Charge stored per unit potential difference across a capacitor.
An isolated metal sphere is charged using a high-voltage supply.
Discuss the factors that affect the charge stored on the surface of the sphere:
(2)
V=Q / 4πε0 r
Therefore charge, Q is proportional to the (surface) potential, V and the radius r of the sphere.
Explain the effect on the charge stored by a capacitor when a thick sheet of plastic inserted in the space between capacitor plates:
(2)
The permittivity of the material between the plates increases, hence the charge stored is greater.
Define electric potential at a point in space around a charged object:
Work done per unit positive charge in bringing a charge from infinity to the point in space.
Define electric field strength at a point in space:
Force per unit ‘positive’ charge
