22 - Electric Fields Flashcards
Define electric field strength at a point in space.
The force experienced per unit positive charge at that point.
What is an electric field caused by?
Charged objects
What sort of objects experience force in an electric field?
Charged objects
Is electric field strength a vector?
YES
What is the direction of an electric field?
The direction that a positive charge would move when placed in the field.
POSITIVE TO NEGATIVE
What does the arrow on an electric field line represent?
The direction of the electric field
What are electric field lines always perpendicular to?
The surface of a conductor.
How is the strength of an electric field represented by field lines?
Their density
What is Coulomb’s Law?
In words
Any two point charges exert an electrostatic force on each other that is directly proportional to the product of their charges and inversely proportional to the square of the distances between them.
What equation defines Coulomb’s law?
F = kQq / r²
What is ε₀?
The permitivity of free space?
What do we treat a uniformly charged sphere as?
A point charge
How do we calculate the electric field strength of a point charge using Coulomb’s law?
E = kQ / r²
What does the electric field strength of a point charge NOT depend on?
The charge of the point charge in the field.
What is the main difference between electric and gravitational fields?
Gravitational fields are always attractive whereas electric fields can be either attractive or repulsive.
What sort of field does a point charge produce?
A radial field.
What is the equation for electric field strength between two parallel plates?
E = V / d
What are the two units for electric field strength?
NC⁻¹
Vm⁻¹
What two factors does the capacitance of a parallel plate capacitor depend on?
The surface area of the plates, A
The distance between the plates, d
C ∝ A / d
What is the constant of proportionality in the relationship, C ∝ A / d?
ε₀ (For air)
What do we use instead of the permitivity of free space when a dielectric other than air is used in a parallel plate capacitor?
ε
ε = ε₀ε₁ (Where ε₁ is the permitivity of the dielectric)
How would you suspend a droplet of a charged liquid in an electric field?
Make the force of the electric field equal to the weight of the droplet.
If an object between two parallel plates experiences a constant force due to the electric field, describe its acceleration?
Constant as well
What must we do when calculating anything from a particle travelling HORIZONTALLY in a uniform electric field?
Consider its vertical and horizontal components separately.
How do charges store energy?
Two like charges repel each other.
By forcing them together we are storing potential energy as we need to do work.
This energy is released when we release the charges.
Where is the energy represented on a force distance diagram?
The area under the curve
How do we calculate the work done on a charge?
W = Fx
Define electric potential energy at a point
The work done per unit charge in bringing a point charge from infinity to that point.
Why does the equation for electric potential energy only have a single r in the denominator?
Because FORCE has r² in its denominator and work is force x distance so we get rid of one of the rs.
What is the unit for electric potential?
JC⁻¹
Volts
What is electric potential difference?
The difference in electric potential energy between two points.
The work done per unit charge to move a point charge between two points in an electric field.
How do we calculate electric potential difference?
Calculate electric potential at both points and subtract them.
What is the symbol for electric potential?
V
NOT UNIT
What is the capacitance of an isolated sphere, radius R?
C = 4πε₀R
What is an isolated sphere?
A sphere that is far away from other objects.
What can an isolated sphere act as?
A capacitor.
What is the difference between energy and electric potential regarding electric fields?
Energy is not per unit charge.
Potential is per unit charge.
