6.2 Electric fields Flashcards
What is an electric field?
An electric field is the reigon around a body in which other charges bodies will feel a force due to the electric charge on the body.
What are electric field lines?
They show the shape of the field and the direction of the field line at a point is the direction in which a small positive charge would move when placed on that spot.
what is the electric field strength?
The electric field strength is defined as the force per unit positive charge. Units are newtons per coulmb (NC-1). It is the force that a charge of +1C would experiece if placed at that point in the field.
What are electric fields caused by?
Electric charge
What force does an electric field exert on a charge object?
An electric field exerts an electrical force on any charged object.
Which way do electric field lines act in regards to it’s source?
Electric field lines can axct towards or away from the charged sourse as the electric feild can be attractive or repusive in nature.
Define electric field strength?
The force acting on unit charge placed at that point in the field.
What sort of quantity is electric field strength?
A vector quantity
What is the equation for electric field strength?
E= F/Q
E=F/C
What does each letter represent?
E= electric field strength
F= force
Q= size of positive charge in coulmbs
What are the units for electric field strength?
NC^-1
What does the direction of the electric field lines show?
It shows the direction that unit positive charge would move if placed at that point in the fields.
What do close electric field lines indicate?
A strong electric field.
What is a neutral point between 2 charged objects?
The neutral point is the pace between 2 charged objects where the feilds exactly cancel and the net force is zero.
What is Coulmb’s law?
Coulmb’s law states that the electrical force, F between two point charges is directly proportional to the product of their charges and inversley proportional to the square of their separation, F=Qq/4πε0r^2
What is permittivity of free space?
ε0, is. the physical constant related to the size of the firce between electric charges in free space (a vacuum). It has a value of 8.85x10^-12C^2m^-2N^-1/
What did the french physicist Charles-Augustin de Coulmb discover with his investigations relating to the behavior of charged bodies in the vicinity of one another.
- The magnitude of the force between 2 charged bodies is directly proportional ti the product of their charges.
- The magnitude of the force is inversely proportional to the square of their separation.
What equation provides an explanation od Coulmb’s law?
F=kQq/r^2
In F=kQq/r^2, what does k stand for?
k is a constant of proportionality. The physicist James Cleak Maxwell was able to relate te value of k to a physical constant known as the permitttivity of free space ε0.
What is the relationship between k and ε0?
k=1/(4πε0)
How can you write coulmbs law from F=kqQ/r^2 and k=1/4πε0?
F=Qq/4πε0r^2
When there is more than one force on a charge, how do you find the resultant force?
The resultant force is equal to the vector sum of all the forces.
Using F=qQ/4πε0r^2 abd E=F/q, what new equation can be used to find the electric field strength for a point charge?
E=Q/4πε0r^2
What does the graph of electric field strength ,E, agaist distance from the point charge, r look like?
What does this graph show us?
Electric field strength is an inverse square law with distance, written as E∝1/r^2
How do you find the resultant field of 2 or moe charges?
Find the vector sum of the electric fields of individual charged.
What is the similarity between an electric field and gravitational field and how their strength is defined?
- Feild strength in an electric field is defined as the force per unit positive charge.
- Feild strength of gravity is defined as force per unnit mass.
What is the similarity between an electric field and gravitational field and the equations that related force and field strength?
- In electric fields, both force ans field strength follow an inverse square law: F∝q/r^2 and E∝Q/^r
- In gravitational fields the force and field strength follow an inverse square law F∝Mm/r^2 and g∝M/r^2.
What is the similarity between an electric field and gravitational field and the lines that represent the field?
- Electric field for a radial source act radially outwards from a positive charge and inwards for a negative charge.
- Gravitational field lines always act radially inwards.
What is the similarity between an electric field and gravitational field and if they are attractive or repulsive forces?
- The electrostatic force can be attractive or repulsive.
- The gravitational force is always attractive- it attracts masses towards one another, so both F and g have a negative sign in this equation.
What is permittivity?
ε is a constant releated to the size of the capacitance of a capacitor. For a parallel plate capacitor of plate induction d and area, A, C=εA/d. Permittivity has the units C^2m^-2N^-1.
What is relative permittivity?
εr of a material is the factor by which the capacitance of a capacitor with that material between its plates is inreased relative to the capacitance of a capacitor with air or vacuum between its plates.Relative permittivity is a pure umber and has no units, and is related to the permittivoty of free space, ε0 by εr=C/C0= εA/d/ε0A/d= ε/ε0
What happens the the field lines of an electric field as we move further away from the source?
The field lines diverge as the firld strength follows an inverse square relationship.
What happens to the field strength as you move further away from the an electric source?
The strength of the field decreases
Is the field stength uniform with a radial field?
No.
THe field strangth is not uniform within a radial field.
What sort of electric field will 2 conducting plates that have been charged by connection to the terminals of a battery create?
They will create a uniform field between them.
Which way do field lines showing an electric field point?
They will point from the positive charge to a negative charge.
How could you set up a uniform electric field?
Put 2 parallel plates a small distance apart from eachother and are connected to a battery, so that the potential difference across the plates is V. The plates carry equal and opposite charges, so there is a uniform field between them except just at the edges.
What is required to move a small positive charge to a positive plate in a uniform electric field?
It will require a force, beacuse the electric field acts downwards.
This force is constant because the electric fied is constant and does not vary with distance.
What is the work done on a charge against the electric field equal to?
Work= force x distance
W=Fxd
How can you derrive the equation E=V/d
- W=Fxd
- V=W/q, W=Vq
- E=F/Q, F=EQ
- Vq=EQ x d
- V=Exd
- E=V/d
What are the units for electric field strength?
NC^-1
What equation shiws the relationship between electric field strength, potential difference between the plates and the separation of the plates?
E=V/d
What will happen to a charged particle when placed in an electric field?
It will cause the particle to accelerate.
What are the 2 stituation when placing a charged particle in an electric field, causing it to accelerate?
- Placing a stationary charged particle within a uniform electric field.
- Injecting a moving charged particle into a uniform electric field, initially at right angles to the direction of the field.
How do you work out the gain in kinetic energy on a charged particle places in a one-dimensional electric field?
The charged particle will either accelerate to one of the charged plates depending on the charge on the particle. We can applythe equation F=ma to describe it’s motion and also use the transfer of energy is equal to work done, to calculate the gain in kinetic energy.
Why will charged particles move in a parabollic motion When thy enter sn electric field at a right angle to the direction of the field?
A charged particle hass a component of motion in the direction perpendicualr to the field as well as a component of the motion parallel to the field. Since the components of motion are independant from each other, there will be a constant velecity in the direction parallel to the field but am acceleration in the direction in the field- giving a parabolic motion.
How can we decribe the motion of a charged particle travelling parallel to the plates?
We can use the equations of linear motion to describe the motion of the charge parallel to the plates, as well as F=ma to a=decribe it’s acceleration in the direction perpendicular to the plates.
What equation gives the capacitance of a capacitor with the permittivity of the material between the plates, the area of the overlap of the plates and the separation of the plates?
C=ε0A/d
How do you work out permittivity?
When a material of relative permittivity εr is used, the permittivity is found by multiplying the relative permittivity of the material by the permittivity of free space, ε=εrε0
What is electric potential?
Electric potential is the work done per unit positive charge to move that charge from infinity to a point in an electric field. The unit is JC^-1 or V. One volt is 1 joule per coulmb.
What is electric potential energy?
The electric potential energy of a body of charge is the work done to move that charge from infinity to a point in an electric field.
To move a charge in a direction opposite to the electric field requires work done. What is this work done equal to?
The work done/ is equal to the stored electric potential energy.
At infinity, what will the electric potential be?
0
For point Q, what equation gived the magnitude of the electric potential?
V=Q/4πε0r
V=Q/4πε0r. What will happen as a small positive charge,q gets closer to a positive charge, Q?
q will need to do work against the field, due to the repulsive force exerted by Q.
What does the graph of electric potential against distance look like for a repulsive field? Why does it look like this?
In a repulsive feild, potential will inecrase as the sparation decreases. This is because more and more energy is requires to bring a unit of positive charge closer to a like charge since the force of repusion increases the closer it gets.
What does the graph of electric potential against distance look like for an attractive field? Why does it look like this?
Electric potential will become more negtive as the separation decreases beacuse as a positive charge moves towards a negative charge they will be attactive, ‘negative work’ is done.
Describe the electric field on the sphere?
The charge is uniformly distrubuted on it’s outer surface and so outside the sphere the charge can be treated as a point charge at the centre of the sphere. The electric field is radial and all points on the sphere have the same value of potential, V.
What do you get is you substitiute C=Q/V into V=Q/4πε0r?
C=Q/(Q/4πε0r) or C=4πε0r
What equations give the electric potential energy?
- E=Vq
- E=Qq/4πε0r
For a charge being moved in a repulsive radial electric field, how do you find the work done to move that charge a cerain distance?
As work done= force x distance, the area beheath a force-distence graph is equal to the work done.
