7.3 Electric fields Flashcards
What is an electric field?
A region where charged objects will experience a non-contact force.
What is the unit for electric charge?
Coulombs (C)
What happens when a charged object is placed in an electric field?
It experiences a force.
In electric field questions, what can he assumed about a charged object that is a sphere?
All of its charge is at its centre.
How can electric fields be represented?
Using field lines.
State Coulomb’s law.
The magnitude of the force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
What type of law is Coulomb’s law?
- Inverse square law
- Since F ∝ 1/r²
What is the significance of the ε in Coulomb’s law?
- This is the permittivity of the material the charges are in
- This affects the size of the force between the charges
- If the system is in air, it can be considered the same as in a vacuum
What is electric field strength?
The force per unit positive charge exerted at a certain point in an electric field.
What is the unit for electric field strength?
N/C
What does a field line diagram look like?
What happens to the field lines as you get further away from a point charge?
They get further apart.
Describe the graph for E against r for an electric field around a point charge.
A
How can a uniform electric field be produced?
Connecting two parallel plates to opposite poles of a battery.
What can a uniform electric field be used for? How?
- Determining whether a particle is charged.
- If a particle curves in the same direction as the field lines, it is positively charged
- If a particle curves in the opposite direction as the field lines, it is negatively charged
How to find out whether a
particle is charged, and whether its charge is negative or positive ?
- This is done by firing the particle
at right angles to the field and observing its path - A charged particle will experience a constant
electric force either in or opposite to the direction of the field (depending on its charge), this
causes the particle to accelerate and so it follows a parabolic shape
What is absolute electric potential?
The electric potential energy that a unit positive charge would have at a point in an electric field.
What effects electric potential?
Size of charge creating the electric field and distance from the charge.
When is the magnitude of the electric potential around a point charge the greatest?
On the surface of the charge.
Describe the graph of V against r around a point charge for a repulsive force.
Describe the graph of V against r around a point charge for an attractive force.
How can electric field strength be found from a V-r graph around a point charge?
- Gradient of tangent
* Because E = ΔV / Δr
How can potential difference between two points be found from an E-r graph around a point charge?
- Area under graph between two points
* Because E = ΔV / Δr so ΔV = E x Δr
What is electric potential difference?
The energy needed to move a unit positive charge between two points.
What shape are equipotentials around a point charge?
Spherical
What is the one important difference between electric and gravitational fields?
Gravitational fields are always attractive, whereas electric forces can be attractive or repulsive.
At sub-atomic level, does electrostatic or gravitational force have a greater effect and why?
- Electrostatic
- Because the masses are tiny, so the gravitational force is also tiny
- NOTE: There are other forces that keep the nucleus stable
When can charge transfer between two objects?
When they slide past each other
When two objects slide past each other, how is charge transferred?
Electrons leave one surface and join the other
What are the rules about field lines? (3)
∙ never start or stop in empty space
∙ never cross
∙ density of field lines shows strength of field
How can the strength of the uniform field between parallel plates be made stronger?
∙ increase p.d. across plates
∙ moving plates closer together
Difference between radial field and uniform field
- A uniform field exerts
that same electric force everywhere in the field, as shown by the parallel and equally spaced field
lines - whereas in a radial field the magnitude of electric force depends on the distance between
the two charges
Define electric potential at a point in an electric field
The electric potential at a point in an electric field is the work done per unit charge when a point positive charge is moved from infinity to the point.
