Electric fields Flashcards
What is coulombs law? (Look at equation book- first one in electric fields)
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
What does a charged particle produce?
An electric field around it which exerts a force on another charged object
What are the units for electric field strength?
NC^-1 (newtons per coulomb)
and
Vm^-1 (volts per metre)
How is the direction of an electric field defined?
The direction of the force on a positive charge
Is electric field strength a vector or scalar?
Vector
How can a diagram show a uniform field?
When the field lines are equally spaced
How do field lines look like on a positive point charge?
equally spaced, radially, outwards
How do field lines look like on a negative point charge?
equally spaced, radially, inwards
How is potential gradient related to electric field strength?
potential gradient is equal to the magnitude of the field strength but is in the opposite direction
How can we find the potential difference between 2 points from a graph of electric field strength?
The area under the graph
What does the area under an electric field strength against r graph give?
The change of potential
What will happen to a moving charged particle when it enters a field?
It will change direction
What happens to charged particles in an electric field?
They experience a force
How can a charged particle move in the same direction in an electric field?
When it is parallel to the field
How does the field strength change as you move away from a point charge and how do we know?
It gets weaker because the field lines spread out
What is absolute electric potential
The potential energy per unit charge of a positive point charge at that point in the field
When is absolute electric potential energy at a maximum?
At the surface of a charge
Define electric potential
The work done per unit positive charge in moving it from infinity to that point
define potential difference
the work done against an electric field in moving unit charge from one point to a second point at a higher potential
What are lines of equipotential?
The lines connecting points of equal potential
Are electric forces contact or non-contact forces?
non-contact forces
What is the range of electric forces?
infinite
What are capacitors?
Components of electrical circuits that temporarily store charge
What is the structure of a capacitor?
2 parallel conducting plates separated by a thin insulating layer (dielectric) which are all encased in a metal or plastic
Define Capacitance
The ability of a capacitor to store charge per unit potential difference
What is the unit of capacitance?
The Farad
What is 1 Farad equal to?
1 CV^-1
What is the permittivity of a material?
The resistance of the material to an electric field passing through it
What are dielectric materials?
insulating materials where the molecules that make up the material can be polarised inside an electric field. Electric charges cant move through the field but the polar molecules align themselves within the field
What component in a circuit does not follow both decay and charging rules?
The current which only follows discharge
What is the time constant?
The time taken for the capacitor to drop to 37% of its initial value
or the time taken to charge a capacitor to 63% of its initial value
How can you decrease the time in which the capacitor charges?
Keep resistance low
What assumptions are made in coulombs law?
Air is a vacuum
The charge acts the centre of the sphere
How do the magnitudes of electrostatic forces and gravitational forces between subatomic particles compare?
The electrostatic force is significantly greater than the gravitational force because the masses are incredibly small whereas they have large charges
What is electric field strength?
The force per unit charge experienced by an object in an electric field
How do you calculate the work done by a moving charged particle between parallel plates?
Work done=QV
What will a charged particle travelling at right angles to the field experience? What shape will it follow?
A constant electric force
Parabolic shape
When is electric potential energy zero?
At infinity
When is electric potential positive?
when charge is positive and the force is repulsive
When is electric potential negative?
when the charge is negative and the force is attractive
How do you find electric field strength from a graph of potential difference against radius?
Gradient of the tangent
Define electric potential difference?
The energy needed to move a unit charge between two points
What happens when a charge moves along an equipotential surface?
No work is done
How can you find the electric energy stored by a capacitor on a graph of charge against p.d?
The area underneath
How does a capacitor work?
When it is connected to a power supply a current starts to flow and a negative charge build up on the plate connected to the negative terminal. On the opposite plate electrons are repelled by the negative charge building up on the initial plate therefore electrons move to the positive terminal and an equal and opposite charge is forced on each plate creating a p.d.
What happens when the charge across the plates of a capacitor increases?
The p.d increases but the electron flow decreases due to the force of electrostatic repulsion also increasing
How do the current, charge and p.d across the capacitor decrease?
Exponentionally
Which value has the same equation for charging and discharging of a capacitor?
Current uses discharging each time
How do you find the time constant from a graph of lnQ against t?
-1/ gradient
