Fields (2): Electric Fields Flashcards
What is a difference between charge and mass?
Charge can attract or repel (have a +ve or -ve value)
Mass can only attract
What happens when an uncharged perspex rod is rubbed with an uncharged dry cloth?
Electrons are transferred from the rod to the cloth, therefore, the rod is +vely charged and the cloth -vely charged
What is the definition of an electrical conductor?
A material that contains lots of free electrons
What do you first have to do to an electrical conductor to charge it?
Isolate it form the earth - otherwise electrons would flow between itself and the earth, discharging it
What is the definition of an electrical insulator?
A material that does not contain free electrons (electrons are bound to the atoms)
What is the definition of charge?
A fundamental property that causes objects to interact with electric fields
What is the definition of an electric field?
A region of space around a charged object where other charged objects that are placed in it will feel a force
What is the definition of a field line/line of force?
The direction that a charged particle will follow if placed in a field strong enough to move it
What happens when a small charged is placed near a large charge?
The small charge and large charge exert equal and opposite forces on one another without being directly in contact. However, since the force is too small on the large charge to move it noticeably, only the small mass gets moved (away or towards depending on whether charges are like or not)
Like charges …?
repel
Opposite charges …?
attract
What are some rules about drawing electric fields in diagrams?
- Lines end at infinity
- Arrows show the force on a positive test charge
- Closer lines = stronger force
How do you draw field lines between two parallel plates of opposite charges?
Uniform field between plates:
- Equally spaced
- +ve to -ve
- On the edges the field starts to curve
What is the definition of Electric Field Strength, E?
The force per unit charge on a positive test charge placed at that point in the field
What is the equation for the electric field strength acting on a positive test charge at a point in the field?
E = F/Q
What are the units for electric field strength?
NC^-1
What is the equation for the electric field strength of a uniform field between two parallel plates?
E = V/d
V - the voltage difference between the two plates
d - the separation between the two plates
How do you prove the equation E = V/d
E = F/Q
W = Fd
F = W/d
V = E/Q
E = VQ
since energy = work done
F = VQ/d
E = VQ/dQ
= V/d
What is the definition of Electric Potential, V?
The work done per unit positive charge on a positive test charge when it is moved from infinity to that position in the field
What is the equation for the work done between 2 points in a field?
W = Q(V2 - V1)
V2 - final potential
V1 - Initial potential
What is the definition of Electric Potential Difference?
The energy required to move a unit charge between two points in a field
What is the definition of an Equipotential?
Lines showing points in a field where the potential is the same
Is work done when moving along an equipotential?
No
What is the definition of a potential gradient?
The change in electric potential per unit step of distance in a given direction
How does relative potential change as you move away from the surface of a negative plate?
Directly proportional to distance / potential increases when moved away from. negative charge
What is the relationship between electric field strength and potential gradient?
- E = (delta)V/ (delta)d
Does electric potential have a fixed sign?
No
What does the sign of electric potential depend on?
Whether the charge is positive or negative
If a charge is positive what sign is potential and, therefore, what type of force is there?
potential = +ve
force = -ve/repulsive
If a charge is negative what sign is potential and, therefore, what type of force is there?
potential = -ve
force = +ve/repulsive
How do you prove that Work done = Q(delta)V
W = fd
F = EQ
E = (delta)V/d
subbing in
W = EQ x (delta)V/E
Work done = Q(delta)V
Describe the graph for potential against r for a positive charge?
- V is inversely related to r in an exponential relationship
- V begins positive and becomes less positive as moved away
Describe the graph for potential against r for a negative charge?
- V is positively related to r in a decreasing relationship
- V begins negative and becomes more positive as moved away
What does the gradient of a potential against r graph give you?
The electric field strength at that point
What does Coulombs Law state?
The magnitude of the force between 2 point charges in a vacuum is directly proportional to the product of their charges and inversely proportional to the square of the distance between the charges
Are electrostatic charges smaller or larger than gravitational forces? Why?
Larger - as charges of particles are much greater than masses of particles
What is the equation for the forces between two point charges?
F = kQ1Q2/r^2
F = 1/4(pi)(epsilon)0 x Q1Q2/r^2
What is epsilon 0?
The permittivity of free space
What does r stand for in the equation F = kQ1Q2/r^2?
The separation of the charges from their centres
What is the equation for the electric field strength in a radial field?
E = 1/4(pi)(epsilon)0 x Q/r^2
What is the equation for the electric potential in a radial field?
E = 1/4(pi)(epsilon)0 x Q/r
What are the units for electric potential?
JC^-1
