7.1 Electric fields Flashcards
What’s an electric field?
A field of space. where a charged particle experiences a force
Hence, electric fields are a type of force field
What’s an electrostatic force?
The force that the charged particles exert on each other within their own electric field
How do we calculate the electric field strength, E?
The electric field strength at a point is defined as:
The force per unit charge acting on a positive test charge at that point
E=F/Q
What’s a positive test charge?
The charge from a positive particle
This means that is going away from the positive particle and towards the negative
This direction is when E = positive
What’s a positive point charge vs negative point charge?
Positive point charge have uniform field lines pointing away from charge
- so pushes the charge away
Negative point charge have uniform field lines pointing towards the charge
- so pushes the charge towards
What are field lines?
They are perpendicular to the charge and point uniformly
They are in 360° of the charge
What are equipotential lines?
Circles perpendicular to the field lines, circling the charge
The further away you go from the charge, the further away the lines are distant, as the force becomes weaker
What’s Coulomb’s law?
The electrostatic force between 2 charges
Coulombs law calculates this force, F(E)
Calculated by the strengths of the 2 charges, which are directly proportional, and the distance away, r, which is inverse square lawed
F(E)=Q(1)Q(2)/4πε(0)r^2
Where ε(0) is the permittivity of free space
ε(0) = 8.85 × 10^−12 Fm^-1
This can be simplified by turning the constants into one, k
F(E) = kQ(1)Q(2)/r^2
k = 8.99x10^9 Nm^2C^−2
What’s the permittivity of free space (ε(0))?
Describes the ability of a vacuum to permit electric field lines.
Used in electromagnetism
‘Permittivity’ = how easy it is for an object to get through something
What’s Electric field strength, E?
When referring to ‘strength’, it talks about from a point mass
Like gravitational fields, with only one charge being used (the point mass’ field strength)
So…
E=Q/4πε(0)r^2
E = kQ/r^2
Where Q = the point mass’ field strength
This shows the electric field strength is not a constant
- it decreases when r increases
- it increases as Q increases
Also works as E=F/Q
What does it mean if Electric field strength is negative?
If the electric field strength (E) is negative, it means the electric field points towards the negative charge.
What does the area under the Electric field strength vs radius represent
The change in Electric Potential, △V
What’s Electric Potential, V?
The amount of work done per unit of charge at that point
Electric potential increases when you move a test charge against the direction of the electric field
(From a point charge)
V=Q/4πε(0)r
V=kQ/r
Very similar to gravitational potential
How does electric field strength, E effect Electric Potential, V?
A stronger electric field means the electric potential changes more rapidly with distance as the test charge moves through it
So this means
E is directly proportional to the gradient of V
E=dV/dr
How do we calculate the electric field strength between 2 parallel plates?
The larger the electric potential, the larger the field stength
The further away the point from the plates, the smaller the electric field strength
so…
E=V/d
d = separation between the 2 plates
