Electric Fields Flashcards
What does a charged object create?
An electric field around itself
What can an electric field be defined as?
A region where an electric charge experiences a force
What happens if other charges enter an electric field and what does the direction depend on?
They will experience an electric force, attracting or repelling them from the object. The direction of this force depends on whether the charges are the same or opposite
What is the force experienced by other charges entering an electric field?
Attractive or repulsive
What do opposite charges do?
Attract each other
Who do like charged do?
Repel each other
Describe what an electric field looks like
Arrows point towards the negative charge / away from the positive charge
For a point outside a spherical conductor, what may the charge of the sphere be considered to be?
A point charge at its centre
What is a uniform spherical conductor?
One where its charge is distributed evenly
What are the electric field lines around a spherical conductor identical to?
Those around a point charge
What is an example of a spherical conductor?
A charged sphere
What are the field lines for a spherical conductor and what does their direction depend on?
Radial and their direction depends on the charge of the sphere
If the spherical conductor is positively charged, where are the field lines directed?
Away from the centre of the sphere
If the spherical conductor is negatively charged, where are the field lines directed?
Towards the centre of the sphere
What is the direction of electric fields represented by?
Electric field lines
Where are electric fields directed?
From positive to negative (away from positive charge and towards negative charge)
How does a radial field spread?
Uniformly to or from the charge in all directions, e.g. the field around a point charge or sphere
What are the electric field lines around a point charge?
Directly radially inwards or outwards
For a point charge, if the charge is positive the fields linear are …
Radially outwards
For a point charge, if the charge is negative the fields linear are …
Radially inwards
What does a uniform electric field have?
The same electric field strength throughout the field, e.g. the field between oppositely charged parallel plates
How is a uniform electric field represented?
By equally spaced field lines
What does a non-uniform electric field have?
Varying electric field strength throughout
How is the strength of an electric field determined ?
By the spacing of the field lines
How is a stronger field represented?
By field lines closer together
How is a weaker field represented?
By field lines further apart
What is a radial field considered and what does this mean?
A non-uniform field, so the electric field strength E is different depending on how far you are from a charged particle
What is an electric field?
A region of space in which an electric charge ‘feels’ a force
What is the electric field strength at a point defined as?
The electrostatic force per unit positive charge acting on the charge at that point
What is the equation for electric field strength (NC-1)?
Electrostatic force on the charge (N) / charge (Q)
Why is it important to use a positive test charge (in the definition of electric field strength)?
Because it determines the direction of the electric field
What is a test charge?
A charge with a magnitude so small that placing it at a point has a negligible affect on the field around it
Is electric field strength a scalar or vector quantity?
Vector quantity
What is the magnitude of the electric field strength in a uniform field between two charged parallel plates defined as (Vm-1)?
Potential difference between the plates (V) / Separation between the plates (m)
What does the equation E= V/d show?
The greater the voltage between the plates, the stronger the field
The greater the separation between the plates, the weaker the field
Why can the equation E= V/d not be used for the E around a point charge?
Because the field is radial
How do you derive E= V/d?
When 2 points in an electric field have a different potential, there is a potential difference between them. To move a charge across the potential difference, work needs to be done. Two parallel plates with a potential difference deltaV across them creates a uniform electric field
DeltaV = W/Q
W = DeltaV x Q
W = F x d
F x d = DeltaV x Q
F / Q = DeltaV / d
Since E = F / Q, E = DeltaV / d
What do all charged particles produce?
An electric field around them
What is the electrostatic force between two charges defined by?
Coulomb’s law
What does Coulomb’s law state?
The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation
What is the equation relating to Coulomb’s law?
Force = (Charge 1 x charge 2) / (4 x pi x E0 x distance^2)
What is the inverse square law?
1 / r^2 relation
What does the inverse square law mean?
When the separation of two charges doubles, the electrostatic force between them reduces by (1/2)^2 = 1/4
What is E0?
The permittivity of free space (and refers to charges in a vacuum)
What is ε?
A measure of the resistance offered by a material in creating an electric field within in
What is the product Qq of like and opposite charges?
Like: positive
Opposite: negative
What is the sign of the force of like and opposite charges?
Like: positive
Opposite: negative
What is the difference between like and opposite charges?
Opposite attract
What does the electric field strength describe?
How strong or weak an electric field is at that point
What type of field does a point charge produce?
A radial
What does a charged sphere act like?
A point charge
What is the equation for: electric field strength E at a distance r due to a point charge Q (in free space)?
E= Q/4 x pi x E0 x r^2
Q - point charge producing the radial electric field
r - distance from the center of the charge
What does the equation E= Q/4 x pi x E0 x r^2 show?
Electric field strength in a radial field is not constant
As the distance from the charge r increases, E decreases by a factor of 1/r^2
Is electric field strength a vector or scalar quanitity?
Vector quantity
What sign is the electric field strength for positive and negative charges?
Positive: positive, points away from the center of the charge
Negative: negative, points towards the center of the charge
What is the equation E= Q/4 x pi x E0 x r^2 analogous to?
The gravitational field strength around a point mass
What is permittivity?
A measure of how easy it is to generate an electric field in a certain material
What is the relative permittivity εr sometimes known as?
The dielectric constant
What is relative permittivity defined as?
The ratio of the permittivity of a material to the permittivity of free space
What is the equation for relative permittivity
Er = E / E0
Er - relativity permittivity
E - permittivity of a material
E0 - permittivity of free space
Why does relative permittivity have no units?
Because its a ratio of two values with the same unit
What is a dielectric?
A substance that is a poor conductor of electricity but a good supporter of electric fields
What happens when the polar molecules in a dielectric align with the applied electric field from the plates?
They each produce their own electric field and this electric field opposes the electric field from the plates
What does a larger opposing electric field from the polar molecules in the dielectric mean?
A larger permittivity - the permittivity is how well the polar molecules in a dielectric align with an applied electric field
As the opposing electric field reduces the overall electric field, what does this do to the potential difference and capacitance?
The potential difference between the plates decreases, therefore the capacitance of the plates increases
What is the equation for capacitance?
C = A x E0 x Er / d
A - cross sectional area of the plates
d - separation of the plates
Er - relative permittivity of the dielectric between the plates
Capacitor plates are generally square, so if it has a length of L on all sides what will the cross sectional area A be?
L^2?
What causes a charged particle in an electric field to move?
It will experience a force on it causing it to move
What happens if a charged particle remains still in a uniform electric field?
It will move parallel to the electric field lines (along or against the field lines depending on its charge)
What happens if a charged particle in motion travels initially perpendicular through a uniform electric field (e.g. between two charged parallel plates)?
It will experience a constant electric force and travel in a parabolic trajectory
What does the direction of parabolic trajectory depend on?
The charge of the particle
Positive charge will be deflected towards the negative plate and negative charge will be deflected towards the positive plate
In a uniform electric field, does the force on the particle change?
No, it stays the same at all points and is always in the same direction
How does an uncharged particle travel through a uniform electric field (parallel plates)?
It experiences no force in an electric field and therefore travels straight through the plates undeflected
What does the amount of deflection depend on?
Mass, charge and speed
How does mass affect deflection?
The greater the mass, the smaller the deflection and vice versa
How does charge affect deflection?
The greater the magnitude of the charge of the particle, the greater the deflection and vice versa
How does speed affect deflection?
The greater the speed of the particle, the smaller the deflection and vice versa
