Electric Fields Flashcards
Define a field
A field is a region in which a body can experience a non-contact force.
What are the rules for drawing electric field lines?
When drawing electric fields: The lines should never cross.
The spacing of the lines represents the strength of the field.
An arrow should be drawn to show the direction of force that a positive charge would feel
What is a field line (electric)?
The path a free positive test charge follows.
Describe a field between two oppositely charged plates
Field lines run parallel, at right angles to the plates.
Field lines are evenly spaced.
At the edges of the field the lines are slightly curved.
Inside the plates is a uniform field.
What kind of a field is it between two oppositely charged plates?
Uniform field
What is a point charge?
An expression for a charged object in a situation where distances under consideration are much greater than the size of the object.
Sketch the electric field around a point charge?
Radial field with arrows pointing away. See diagram
Sketch the field around a negative point charge
Radial field with arrows pointing towards the centre of the charge.
See diagram
Sketch the field pattern around two positive charges
See diagram
Sketch the field pattern around a positive and negative charge
See diagram
Define electric field strength at a point in the field
The force per unit positive charge.
E = F/Q
Sketch a graph of electric field strength against separation?
What can be found from this graph?
See diagram
Area = Electrical potential
Define Coulombs law
The force of attraction or repulsion between two charges is proportional to the product of the charges and inversely proportional to the separation squared.
Sketch a graph of force against separation?
What can be found from this graph
See diagram
Area = work done = Electrical potential energy
If two forces are in the same direction, what is the resultant electric force?
F1 + F2
If two forces are in opposing direction, what is the resultant electric force?
F1 - F2
If two forces are at right angles to each other, what is the resultant electric force?
Resolve using pythagorus.
(F12 + F22)0.5
State the factors that affect the electric field strength between two parallel plates
- Potentail difference between the plates (directly proportional)
- Plate seperation (inversley proportional)
Describe the motion of a positive charged particle moving parallel to a uniform electric field.
Uniform acceleration due to uniform field.
Describe the motion of a positive charged particle moving perpendicular to a uniform electric field.
Motion perpendicular to the field - constant velocity.
Motion parallel to the field - constant acceleration
This causes the particle to move in a parabolic curve.
Define electric potential difference
The total work done to bring a positvely charged particle from infinity to a separation r
State the equation for electrical potential difference.
Ep = kQq / r
Sketch a graph for electrical potential difference against seperation for like charges.
What can be found from this graph?
See diagram - like charges repel so work is done to bring them together.
There is 0 electrical potential difference at infinity.
Gradient = force
Sketch a graph for electrical potential difference against seperation for opposite charges.
What can be found from this graph?
see diagram - Opposite charges attract so work is done moving them apart.
There is 0 electrical potential difference at infinity.
Gradient = Force
Define electric potential at a point in an electric field
Work done per unit positive charge on a positive test charge when it is moved from infinity to that point.
V = W/Q
What can be found from a graph of electric potential against separation?
Gradient = Electric field strength
Sketch a graph of potential vs seperation when moving a positive charge towards another positve charge.
See top half of the diagram (positive y axis only).
Positive charges repel so work is done moving them closer together. As they move closer potential and potential energy would increase.
Potential at infinity is 0.
Sketch a graph of potential vs seperation when moving a negative charge towards a positve charge.
See bottom half of the diagram (negative y axis only).
Opposite charges attract so work is done moving them further apart. As they move further apart potential and potential energy would increase.
Potential at infinity is 0.
What do lines of equipotentail show?
Lines along which electric potential is constant.
What do closer lines of equipotential show?
Closer equipotentials means greater potential gradient at right angles to them.
More work is needed to move a charged particle a set distance
How is potential gradient (spacing of lines of equipotential) effected if the field is uniform?
Equipotentials between the plates are equally spaced parallel to the plates.
Potential gradient is constant.
Describe the lines of equipotential for a uniform field between two parallel plates
Lines of equipotential are perpendiculer to the electric field and are evenly spaced.
See diagram
Describe the lines of equipotential for a radial field from a point charge
Lines of equipotential are circular rings around the charge.
The spacing increases as distance from the charge increases.
See diagram
Describe the lines of equipotential between two positive charges
See diagram
Describe the lines of equipotential a positive and negative charge
See diagram
State some similarities between E fields and g
- Both have infinite range
- Both follow the inverse square law with seperation
State some differences between E fields and g
- E depends on charges but g depends on mass
- E can cause attraction or repulsion. g can only attract.
