Electric Fields Flashcards
Coulomb’s Law for the electrostatic force between 2 point charges
The electrostatic force between 2 point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation
Electrostatic force between 2 point charges, F, =
1/4πε0 x Q1Q2 / r^2
Where is charge considered to be acting from in a charged sphere
From the centre
Is the electrostatic force between 2 charges of the same type positive or negative
Product Q1Q2 is positive so force is positive.
A positive force means the charges experience repulsion
Is the electrostatic force between 2 opposite charges positive or negative
Product Q1Q2 is negative so force is negative.
A negative force means the charges experience attraction
Why is the permittivity of free space used when calculating the force between 2 charges
Air is treated as a vacuum
Uniform spherical conductor
One where its charge is distributed evenly
Electric field lines around a point charge
Radial
Direction depends on the charge.
Direction of field lines around a positively charged spherical conductor
If positively charged, the field lines are directed AWAY from the centre of the sphere
Direction of field lines around a negatively charged spherical conductor
If negatively charged the field lines are directed towards the centre of the sphere
What are field lines used to represent
Direction and magnitude of an electric field
Which direction are field lines in an electric field directed in
From the positive charge to the negative charge.
Field lines in a uniform electric field
Field lines are equally spaced at all points as electric field strength is constant at all points and the force on a test charge has the same magnitude and direction at all points in the field.
Field lines in a radial electric field
Field lines spread out with distance.
Field lines equally spaced as they exit the surface of the charge
Separation between field lines increases with distance
Magnitude of electric field strength and force on test charge decreases with distance
What does field lines being closer together mean
Stronger field
What do electric field lines between two charges of the same type look like
Field lines are directed away from 2 + charges and towards 2 - charges
They are not connected to show repulsion
What do electric field lines between two charged parallel plates look like
Uniform/ evenly spaced between the plates, moving from positive to negative plate.
Beyond the edges, it is non-uniform and weaker
Electric field strength
The force per unit charge experienced by a small positive test charge placed at that point.
Electric field strength =
Force / Charge
Units for electric field strength
NC^-1
Is electric field strength a vector or scalar
Vector and is always directed AWAY from a positive charge and TOWARDS a negative charge
Magnitude of electric field strength in a uniform field between 2 charged parallel plates (Vm^-1) =
Potential difference between plates / Separation between plates
What does the equation E = V/d show
The greater the voltage between plates, the stronger the field
The greater the separation between plates, the weaker the field
Why can you not use the equation E = V/d for point charges
They have a radial field
Voltage of a plate if one of the parallel plates is earthed
0V
How to find E/F when a point charge moves between 2 parallel plates
Equate the two equations for electric field strength
E = F/Q = V/d
Derivation of work done moving charge between plates
E = F/Q = V/d
Fd = VQ
W = Fd
W = VQ
Which direction will a charged particle move if it remains stationary in a uniform electric field
Parallel to the electric field lines
Which direction will a charged particle move if a charged particle is in motion through a uniform electric field
It will experience a constant electric force and travel in a parabolic trajectory
What does the amount of deflection of the particle depend on
Mass - greater mass = smaller deflection
Charge - greater charge = greater deflection
Speed - greater speed = smaller deflection
What does less deflection look like
Path has a smaller curve
Electric field strength due to a point charge =
F/q = Q / 4πε0r^2
Are electric force and field strength vectors or scalars
Vectors
Direction of electric field strength if the charge is negative
Negative.
Points towards the centre of the charge
How can you find electric force/field strength at a point due to multiple charges
Each field can be combined by vector addition
Similarities between gravitational and electrostatic fields
The field lines around a point mass and negative point charge are identical
The field lines in uniform gravitational and electric fields are identical
Both forces follow inverse square law relationships with distance
Both gravitational field strength and electric field strength follow an inverse square law (1/r2) relationship with distance in a radial field
Both gravitational potential and electric potential both follow an inverse (1/r) relationship with distance
The equipotential surfaces are spherical around a point mass or charge and equally spaced parallel lines in uniform fields
The work done by either field is equal to the product of the mass or charge and change in potential
Differences between gravitational and electrostatic fields
The gravitational force acts on particles with mass whilst the electrostatic force acts on particles with charge
The gravitational force is always attractive whilst the electrostatic force can be attractive or repulsive
The gravitational potential is always negative whilst the electric potential can be either negative or positive
Gravitational fields are relatively weak compared to electric fields as the gravitational constant G is much smaller than the Coulomb constant k
Electric potential of a point charge
Work done per unit charge in taking a small positive test charge from infinity to a defined point
Is electric potential vector or scalar
Scalar but has a +/- sign to indicate the sign of the charge
What does electric potential depend on
Magnitude of the point charge
Distance between the charge and the point
Electric potential when around an isolated positive charge
Has a positive value
What happens to electric potential around an isolated positive charge when a test charge moves closer/further
Increases when test charge moves closer
Decreases when it moves further away
Electric potential when around an isolated negative charge
Has a negative value
What happens to electric potential around an isolated negative charge when a test charge moves closer/further
Decreases when test charge moves closer
Increases when it moves further away
How can you tell if potential decreases or increases with distance from charge
Using the direction of the electric field lines.
Potential always decreases in the same direction as the field lines
Electric potential around point charge =
Q / 4πε0r
How to find the potential at a point caused by multiple charges
Each potential can be combined by addition
Potential gradient of an electric field
The rate of chsnge of electric potential with respect to displacement in the direction of the field
Gradient of V-r graph
Electric field strength at that point
Equation relating V to E
E = - change in V / change in r
Why is there a negative sign in the equation E = -V/r
To indicate the direction of the field strength opposes the direction of increasing potential
What does of V against r
An L shape and is reflected in the x axis
Key features of the graph of V against r
All values of potential are negative for a negative charge
All values of potential are positive for a positive charge
As r increases, V against r follows a 1/r relation for a positive charge and -1/r for negative
The gradient is the field strength
What does the area under a E-r graph represent
The potential difference between those 2 points
What does a E-r graph look like
A L shaped curve
Key features of a E-r graph
All values of field strength are negative for negative charge
All values of field strength are positive for a positive charge
As r increases, E against r follows a 1/r^2 relation
Curve is steeper than V-r graph
Why do 2 points at different distances from a charge have different electric potentials
Electric potential increases with distance from a negative charge and decreases with distance from a positive charge.
Electric potential energy
Q1Q2 / 4πε0r
Work done on a point charge is equal to
The change in electric potential energy
When is work done in an electric field
When a + charge moves against the electric field lines
When - charge moves with the electric field lines
Equipotential lines in a radial field such as around a point charge
Concentric circles around the charge
Progressively further apart with distance
