7. Electric and Magnetic Fields Flashcards
What is a force field?
An area in which an object experiences a non-contact force.
How can force fields be represented?
- Vectors.
- Field lines.
What is an electric field?
A force field in which a charged particle experiences a force.
What is the electric field strength?
The force per unit charge experienced by an object in an electric field.
Given by the formula E = F/Q
What is the difference between the electric field strength of uniform fields and radial fields?
In uniform fields the value of the electric field strength is constant.
In a radial field, it varies.
What does coulombs law state?
F = (Q₁Q₂) / 4πε₀r²
The magnitude of the force between two point charges is:
- directly proportional to the product of their charges.
- inversely proportional to the square of the distance between them.
what is ε₀ and r in:
F = (Q₁Q₂) / 4πε₀r²
ε₀ is the permittivity of free space.
r is the distance between charges (from their centres).
What happens if F is negative in coulombs law?
F = (Q₁Q₂) / 4πε₀r²
The force is attractive if F is negative.
The fore is repulsive if F is positive.
When do you use this equation?
E = Q/4πε₀r²
To find the electric field strength of a point charge forming a radial field.
What is the absolute electric potential (V)?
The potential energy per unit charge of a positive point charge at a point in the field.
How does the absolute magnitude of electric potential change in a radial field?
- It is greatest at the surface of the charge.
- as the distance from the charge increases, its potential decreases.
- Its electric potential at infinity is 0.
What happens to the potential and polarity if the sign of the charge is positive/negative?
If a charge is positive…
- its potential is positive.
- its charge is repulsive.
If a charge is negative…
- its potential is negative.
- its charge is attractive.
What does V-r look like for a repulsive charge?
What does V-r look like for an attractive charge?
(V = electric potential) (r = distance from centre of charge)
Repulsive:
- As V decreases, r increases at a growing rate.
- V is initially positive.
Attractive:
- As V increases, r increases at a growing rate.
- V is initially negative.
When do you use this equation?
E = V/d
To find the electric field strength in a uniform electric field.
How can you form a uniform electric field?
Using parallel plates with a potential difference across them.
What is electric potential difference?
The energy needed to move a unit charge between two points.
What do field lines show?
in an electric field
The direction of the force acting on a positive charge.
What does the distance between field lines represent?
The magnitude of force.
What do equipotential lines show?
The points where the potential in a field are the same.
- When a charge moves along an equipotential line, no work is done.
What is capacitance?
The charge stored by a capacitor per unit potential difference.
What is the area under a graph of charge against potential difference for a capacitor?
The electrical energy stored by a capacitor (W).
Describe the graph of charge against potential difference for a capacitor?
A straight line through the origin.
as potential difference is directly proportional to charge
When charging a capacitor, what do the graph of:
I-t
V-t
Q-t
…look like?
I-t
decreasing slope
(are under = charge)
V-t
Increasing slope
Q-t
Increaing slope
(gradient =current)
describe what happens when you charge a capacitor to explain the graphs of I-t, V-t and Q-t?
Once a capacitor is connected to a power supply…
- current starts to flow and negative charge builds up on the plate connected to the negative terminal.
- On the opposite plate, electrons are repelled by the negative charge building upon the initial plate.
- Therefore electrons move from the positive terminal and an equal but opposite charge is formed on each plate.
- This creates a potential difference across the plate.
- As the charge across the plate increases, the potential difference increases but the electron flow decreases.
- This is due to the electrostatic repulsion also increasing.
- Therefore the current decreases — eventually to zero.