Electric Fields

Question 1

What is Coulomb’s law?

Answer 1

• States that the force between two point charged Q1 and Q2 separate by distance r
• F=(Q1xQ2) / 4PiEor^2

Question 2

What is epsilon zero?

Answer 2

• 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

Question 3

What does Coulomb’s law mean?

Answer 3

• 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

Question 4

When is the force between two charges repulsive?

Answer 4

When the two charges are of the same sign

Question 5

When is the force between two charges attractive?

Answer 5

When the two charges are of opposite sign

Question 6

Where is there an electric field?

Answer 6

Around a charged object

Question 7

What happens when a charged object is placed in an electric field?

Answer 7

It will experience a force

Question 8

What can you assize if the charged object is a sphere?

Answer 8

All of its charge is at its centre

Question 9

What is electric field strength defined as?

Answer 9

The force per unit positive charge

E= F/Q

Question 10

Why do two point charges exert a force on each other?

Answer 10

A charge produces an electric field around it, which exerts a force on another charged object

Question 11

What is electric field strength? What is its direction defined as?

Answer 11

1. E is a vector quantity because it has both direction and magnitude
2. The direction of the electric field is defined as the direction of the force on the positive charge

Question 12

What is the electric field strength in a radial field like?

Answer 12

1. 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)
2. It’s inverse square law
3. Field strength decreases as you go further away from Q (the field lines get further apart)

Question 13

How can an uniform field be produced?

Answer 13

• 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

Question 14

How much work is done moving the charge from negative side to positive side?

Answer 14

1. 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
2. Wd = VQ (energy gained by the charge moving through a pd V so, EQd=VQ so E=V/d

Question 15

How do you calculate the magnitude of the electric field between two parallel plates?

Answer 15

E = V/d (in a uniform field) 
E= -DeltaV / Deltar (generally)

Question 16

What is the electric field strength equal to?

Answer 16

The electric field strength is equal in magnitude to the potential gradient but it is opposite in direction

Question 17

What does the graph of E against r show?

Answer 17

Area under graph is deltaV (change in potential)

Question 18

What can uniform electric fields be used for?

Answer 18

• 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

Question 19

Why does a charged particle change direction?

Answer 19

1. Any charged article experiences a force in a an electric field
2. So when a moving charged particle enters an electric field, it will change direction

Question 20

When will a charged particle keep moving in the same direction in an electric field?

Answer 20

Only when a charged particle is moving parallel to an electric field does it keep moving in the same direction

Question 21

How can an electron beam tube be used to deflect electrons?

Answer 21

1. Electrons are accelerated by an ‘electron gun’
2. They travel across the fluorescent screen, which shows electron path
3. The electrons are deflected by applying a pd vertically across
4. When a pd is applied such that the top plate is positive, the electrons are deflected upwards

Question 22

Why is the path of an electron a parabola?

Answer 22

1. The electron are travelling in a a vacuum so their velocity in the direction PQ remains unchanged
2. While the electrons are in an electric field, they experience a constant acceleration upwards due to the electric filed

Question 23

What happens as an electron enters an electric field?

Answer 23

1. A charged particle that enters an electric field at right angles to the field feels a constant force parallel to the electric field lines
2. 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
3. This causes the particle to accelerate at right angles to the particles original motion and so it follows a curved path (parable)
4. In a 3D situation, the motion is the same (a parabola) as there are no significant forces acting on the charged particle

Question 24

What is absolute electric potential?

Answer 24

• 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

Question 25

In a radial field what is absolute electric potentials given by?

Answer 25

V = Q/4piE0r

Question 26

What does the sign of V depend on?

Answer 26

-The charges s V is positive when Q is positive and the force is repulsive and negative when Q is negative and the force is attractive but

Question 27

What is the definition of potential?

Answer 27

The work done by an external agent moving a point unit charge from infinity to that point

Question 28

When is the absolute magnitude of V greater?

Answer 28

• On the surface of the charge
• And decreases as the distance from the charge increases
• V will be zero at an infinite distance from the charge

Question 29

What is electric potential difference?

Answer 29

• Electric potential difference is the energy needed to move a unit charge
• If two points in an electric field have a different potential, then there is an electric potential difference between them
• To move a charge across a potential difference (i.e. from one electric potential to another) you need to use energy
  1. The amount of energy you need (r the work done) depends on the size of the charge you are moving and the potential difference you want to move it across

Question 30

How do you calculate the work done?

Answer 30

DeltaW = QdeltaV

Question 31

What are equipotential lines?

Answer 31

• They show all points of equal potential in a field
  1. Just like in gravitational field, you find equipotentials in electric fields
  2. For a point charge, the equipotentials are spherical surfaces
  3. Between parallel plates, the equipotentials are flat planes
  4. No work is done when you travel along an equipotential, an electric charge can travel along an equipotential without any energy being transferred