What is Coulomb’s law?
- States that the force between two point charged Q1 and Q2 separate by distance r
- F=(Q1xQ2) / 4PiEor^2
What is epsilon zero?
- Refers to the permittivity of free space ( a vacuum and the permittivity of air is very close to this )
- 8.85 x 10^-12 Fm^-1
What does Coulomb’s law mean?
- The force between two masses is always attractive
- Coulomb’s law may also be used to calculate the electrostatic force between two charged spheres carrying c charges Q1 and Q2 and r is the separation of the centres of the two spheres
- It is an inverse square law and the further apart the charges the weaker the force beaten them
When is the force between two charges repulsive?
When the two charges are of the same sign
When is the force between two charges attractive?
When the two charges are of opposite sign
Where is there an electric field?
Around a charged object
What happens when a charged object is placed in an electric field?
It will experience a force
What can you assize if the charged object is a sphere?
All of its charge is at its centre
What is electric field strength defined as?
The force per unit positive charge
E= F/Q
Why do two point charges exert a force on each other?
A charge produces an electric field around it, which exerts a force on another charged object
What is electric field strength? What is its direction defined as?
- E is a vector quantity because it has both direction and magnitude
- The direction of the electric field is defined as the direction of the force on the positive charge
What is the electric field strength in a radial field like?
- In a radial field the electric field strength, E, depends on the distance r from the point change Q
E=(1/4piE0)(Q/r^2) - It’s inverse square law
- Field strength decreases as you go further away from Q (the field lines get further apart)
How can an uniform field be produced?
- By connecting two parallel plates to the opposite poles of a battery
- The field strength E is the same at all points between the two plates
- Field lines start at the positive charge and need at a negative charge
How much work is done moving the charge from negative side to positive side?
- Wd = f x d but if the force to move the charge must be equal in magnitude to the force on the charge, due to the electric field, E x Q so Wd=EQd
- Wd = VQ (energy gained by the charge moving through a pd V so, EQd=VQ so E=V/d
How do you calculate the magnitude of the electric field between two parallel plates?
E = V/d (in a uniform field) E= -DeltaV / Deltar (generally)
What is the electric field strength equal to?
The electric field strength is equal in magnitude to the potential gradient but it is opposite in direction
What does the graph of E against r show?
Area under graph is deltaV (change in potential)
What can uniform electric fields be used for?
- They can be used to determine whether a particle is charged or not
- The path of the charged particle moving through an electric field will bend
- The direction depends on whether the charge is positive or negative
Why does a charged particle change direction?
- Any charged article experiences a force in a an electric field
- So when a moving charged particle enters an electric field, it will change direction
When will a charged particle keep moving in the same direction in an electric field?
Only when a charged particle is moving parallel to an electric field does it keep moving in the same direction
How can an electron beam tube be used to deflect electrons?
- Electrons are accelerated by an ‘electron gun’
- They travel across the fluorescent screen, which shows electron path
- The electrons are deflected by applying a pd vertically across
- When a pd is applied such that the top plate is positive, the electrons are deflected upwards
Why is the path of an electron a parabola?
- The electron are travelling in a a vacuum so their velocity in the direction PQ remains unchanged
- While the electrons are in an electric field, they experience a constant acceleration upwards due to the electric filed
What happens as an electron enters an electric field?
- A charged particle that enters an electric field at right angles to the field feels a constant force parallel to the electric field lines
- If the particle is positively charged then the force acts on it in the same direction as the field lines. If it is negatively charged, the force is in the opposite direction to the field lines
- This causes the particle to accelerate at right angles to the particles original motion and so it follows a curved path (parable)
- In a 3D situation, the motion is the same (a parabola) as there are no significant forces acting on the charged particle
What is absolute electric potential?
- Absolute potential is potential energy per unit charge
- All points in an electric field have an absolute electric potential, V and this the electric potential energy that a unit positive charges would have at that point
- The absolute electric potential depends on how fair it is from the charge creating the electric field and the size of that charge
