Electric Fields Flashcards
Define an Electric Field.
Is a region in which a charged particle experiences a force
What is Electric Charge measured in?
Coulombs
Oppositely charged particles [………] each other.
attract
Like charges [……….] each other
repel
What can you assume if the charged object is a uniformly charged sphere?
All the charge is at the centre, it behaves like a point charge.
How can Electric and Gravitational Fields be represented?
By drawing field lines
What does Coulombs Law give us?
Gives us the force of attraction or repulsion between two point charges in a vacuum.
What is Coulombs Law?
Can also merge the q1 q2 and r*2 to the other side to give one equation
How do you calculate Coulombs Constant?
Show the equation for Coulombs Law but with Coulombs Constant instead.
What do the symbols stand for in this equation?
F = Force of attraction or repulsion between two point charges in a vacuum
ε0= the permittivity of free space (= 8.85 x 10*-12 Fm*-1)
Q1 & Q2 = Are the charges
r = the distance between Q1 and Q2
What is Epsilon naught?
ε0= the permittivity of free space
What is the value for Epsilon Naught?
8.85 x 10*-12 Fm*-1
What is Epsilon Naught measured in?
Farads per metre (Fm*-1)
What will F be if the charges are opposite? Including Coulombs Law values.
The Force is attractive
- In Coulombs Law F will be NEGATIVE
What will F be if the charges are alike? Including Coulombs Law values.
- The Force is REPULSIVE
- F will be POSITIVE
Coulombs Law is an […………]
Inverse Square Law
Why is Coulombs Law an Inverse Square Law?
The further apart the charges, the weaker the force between them.
What happens if the point charges are between air? Coulombs Law.
Air can be treated as a vacuum so you can use the Epsilon Naught constant
In Coulombs Law, what do you do if the point charges are not in a vacuum?
Epsilon Naught is replaced by the permittivity value.
How do you use an electric balance to measure the force between two charges?
1 - Fix a charged sphere to a mass balance and zero the balance
2 - Clamp another charged sphere carrying the same charge directly above the first sphere
3 - The spheres will repel each other, causing the lower sphere to push down the scales, so the scales will register a mass
4 - Use W = mg to calculate the force
5 - If you vary the distance, r, you found that F is proportional to 1/r*2
What is the definition of Electric Field Strength?
Force per unit Charge
What is the Electric Field Strength Equation?
E = F/Q
E = Electric Field Strength (NC*-1 newtons per coulomb)
F = Force (N)
Q = Charge (C)
A Radial Field is…
Inversely Proportional to r*2
How can you identify if a object has a Radial Field?
If it has a point charge
OR
or any body that behaves as if all its charge is concentrated at the centre.
Is an Electric Field a Scalar or a Vector?
Vector
In what direction does an Electric Field flow?
Points in the direction that a positive charge would move
What kind of field must it be if it has the same Electric Field Strength throughout?
Uniform field
In a Radial Field, what does the Electric Field Strength depend on?
Depends on the distance r from the point charge Q
What is the Electric Field Strength equation including Coulomb’s Constant? k
E = kQ/r2
E = Electric Field Strength (NC*-1)
k = Coulomb’s Constant
Q = Point charge (C)
r = distance from the point charge (m)
What is the value of Coulomb’s Constant (k)?
8.99 x 109 N m2/C2
What is the Electric Field Equation without Coulomb’s Constant?
E = Electric Field Strength (NC*-1)
e0 = Epsilon Naught (8.85 × 10^-12 Fm*-1)
Q = Point charge (C)
r = distance from the point charge (m)
What is the graph for E = 1/r*2 ?
Shows that the field strength decreases as you get further away from Q.
In a Uniform Field what is the same everywhere?
Electric Field Strength
How can you produce a Uniform Field?
By connecting two parallel plates to the opposite poles of a battery.
- One plate should be positive and the other should be negative
What are the field lines like in a Uniform Field?
The field lines are parallel so they’re always the same distance apart.
This means the field strength is the same at all points within the field.
What does the field strength between two parallel plates depend upon?
Depends upon…
- The Potential Difference, between the plates, V.
- Distance, d, between them.
Define Electric Potential
Potential energy that a unit positive charge ( + 1C) would have at that point
What does the Electric Potential depend upon?
Depends on…
- HOW FAR it is from the CHARGE creating the Electric field.
- Size of the CHARGE
What is the Electric Potential Equation, in a Radial Field?
V = Electric Potential (V)
Q = Size of the charge (Q)
r = distance from the charge (m)
What does the sign of V depend on?
Depends on Q
E.G - V is POSITIVE when Q is POSITIVE, therefore the force is repulsive.
V is NEGATIVE when Q is NEGATIVE, therefore the Forces ATTRACT.
When is an Electric Potential at its greatest possible value? What happens as V gets further away from this point?
V is greatest directly next to the charge, V decreases as the distance from the charge increases.
Show the V over r graph for a Repulsive force and describe what you see.
You see that…
- V is initially positive and gets closer to zero as r increases
Show the V over r graph for an Attractive force and describe what you see.
You should see…
- V is initially negative and gets closer to 0 as r increases.
What does the gradient of a V over r graph give us?
Gives the Field Strength at that point.
E = (delta) V / (delta) r
How can you use a E over r graph to find the change in Electric Potential (change in V)?
Take the area underneath two points on the graph of E against r
What do Equipotentials show?
Show all points of Equal Potential in a Field
What is different about equipotentials for Radial Fields & Parallel Plates?
- Equipotentials in a Radial Field are the same distance from the centre of the charge, spheres.
- For Parallel Plates the Equipotentials are flat planes.
What happens to the Energy Transferred to a charge travelling along an Equipotential?
No work is done! Because you are travelling ALONG it!
