Electric Fields

Question 1

Define what is an Electric Field

Answer 1

An electric field is a region of space in which charged particles are subject to an electrostatic
force.

Question 2

How can the effect of electrostatic forces can be shown?

Answer 2

These forces can be shown to exist by generating a static charge on an object, for example, rubbing a glass rod with a silk cloth.

Question 3

Describe what happens when generating a static charge on an object using a glass road.

Answer 3

The glass rod will lose electrons to the silk cloth and so become slightly positive.
This rod can then be used to induce dipole charges and attract small neutral objects, such as pieces of paper or water from a tap.
When the positively charged rod is close to a neutral object, the electric field due to the glass rod will attract electrons in the object and shift their positions to be closer to the rod.
This causes the object to be more negative on the side closest to the rod and more positive on the opposite side aka the object remains neutral overall but becomes polar.
The rod and the induced charge on the object will then attract.

Question 4

Protons and electrons are charged particles that can be modelled as…

Answer 4

point charges i.e. their charge exists at a single point in space.

Question 5

Point charges have…

Answer 5

radial fields.

Question 6

Electric field lines point …

Answer 6

outwards from a positive charge or inwards towards a negative charge as the direction of the field represents the direction of the electrostatic force on a positive charge at that point.

Question 7

The strength of the field decreases with…

This is shown by the…

Answer 7

the radial distance or the distance away from the charge.

decreasing density of the field lines, the less tightly packed the field lines are the weaker the electric field is at that point and the weaker the electrostatic force will be on a given charge.

Question 8

examples of objects that can be modelled as point charges are…

This is because…

Answer 8

Spherically charged spheres, e.g. an ion or the metal sphere used in a Van de Graaff generator.

outside the sphere the electric field is radial and decreases in strength with distance away from the surface of the sphere.

Question 9

Field lines generally…

Answer 9

point from positive to negative, are equally spaced as they exit a surface and are perpendicular to the surface.

Question 10

Lines that stretch from a positive charge to a negative charge show…

Answer 10

attraction whereas field lines that do not join up identify repulsion.

Question 11

Define electric field strength

Answer 11

The strength of an electric field, 𝐸 is defined as the force, 𝐹 applied per unit charge 𝑄 on an
object

Question 12

the formula for electric field strength E

Answer 12

𝐸 = 𝐹/𝑄

Question 13

Units of electric field strength

Answer 13

An E field has the unit NC-1 or Vm-1

Question 14

Coulomb’s law states

Answer 14

Coulomb’s law states that any two point charges exert an electrostatic force between them that is proportional to the product of their charges and inversely proportional to the square of the distance between them.

Question 15

the formula for force derived from coulomb’s law is

Answer 15

F=kQq/r^2, where k = 1/(4𝜋𝜀) = 8.99×10^9 Nm^2 C^−2

Question 16

𝜀 is

Answer 16

the permittivity of free space (8.85 x 10-12 Fm-1).

Question 17

Define permittivity.

Answer 17

Permittivity is a constant that defines the ability of a material to become polarized and store charge.

Question 18

Why is the permittivity of free space a constant and not a variable?

Answer 18

The permittivity of free space is the permittivity of a vacuum and as there is no matter in a vacuum to become polarized this is the lowest value possible.

Question 19

In matter generally, the permittivity can be written as a constant:

Answer 19

the relative permittivity, 𝜀𝑟, multiplied by the permittivity of free space.

Question 20

The formula for the electric field of a point charge is

Answer 20

𝐸 = 𝑄/(4𝜋𝑟^2𝜀0)

Question 21

What is a uniform field ?

Answer 21

A uniform field is a field which has no dependence upon position i.e. the field is constant in space.

Question 22

How do u producer a uniform field

Answer 22

A uniform electric field is produced between two parallel oppositely charged plates.

Question 23

How does the force experienced by a particle in a uniform field produced by 2 plates

Answer 23

A particle between such plates is subject to a constant force no matter where it resides between the plates.

Question 24

Why can’t 𝑊 = 𝑉Q = 𝐹d = 𝐸Qd be used to calculate the voltage V in a non uniform field?

Answer 24

Because this implies that V =Ed, which only is the case if the electric field is constant over the distance the charge moves, therefore, we cannot apply this equation to non-uniform fields.

Question 25

Define the electric potential V

Answer 25

The electric potential at a single point can be defined as the work done per unit charge to bring a positive charge from infinity to that point.

Question 26

Electrical energy formula

Answer 26

Work done by a force over a distance is given by 𝑊 = 𝐹d.
However Coulomb’s law states that:

𝐹 =𝑄/4𝜋𝑟^2𝜀0
E𝑒 = F x d
where d = r
thus, 𝐸𝑒 = 𝑄/4𝜋r^2𝜀0

Question 27

As the electric potential is defined as the electrical potential energy per unit charge, what is the formula for V, by using the Coulomb’s law

Answer 27

𝑉𝑉 = 𝐸𝑒/𝑞
Therefore
𝑉 = 𝑄/4𝜋r𝜀0