Electric Fields

Question 1

What happens to a charged object placed in an electric field?

Answer 1

It will experience a force

Question 2

What assumption can you make about a sphere that is charged

Answer 2

If the charged object is a sphere, and the charge is evenly distributed (it’s spherically symmetrical), you can assume all of its charge is at its centre.

Question 3

What is used to represent an electric field?

Answer 3

Field lines

Question 4

What is Coulomb’s law

Answer 4

F=Qq/4 π Eo r^2

Eo l’epsilon-nought”) is the permittivity of free space,

Qand q are the charges,

r is the distance between Q and q

F - the force of attraction or repulsion between two point charges.

Question 5

What does the F from Coulomb’s law coming out negative mean about the force itself

Answer 5

If the charges are opposite then the force is attractive.
F will be negative.

Question 6

What does the F from Coulomb’s law coming out positive mean about the force itself

Answer 6

If Q and g are like charges then the force is repulsive, and Fwill be positive.

Question 7

What can be said about the charges (Q and q) about the force they exert on each other

Answer 7

The force on Q is always equal and opposite to the force on q.
It’s an inverse square law - The further apart the charges are, the weaker the force between them.

Question 8

What is E in Coulomb’s law and what is it’s value for a vacuum (epsilon nought)

Answer 8

The size of the force F also depends on the permittivity, E, of the material between the two charges.

Given in exam book:
For free space (a vacuum), the permittivity is Eo= 8.85 x 10^-12 C^2N^-1m^-2 (units can also be Fm^-1)

Question 9

Define electric field strength

Answer 9

Electric field strength, E, is defined as the force per unit positive charge - the force that a charge of +1 C would experience if it was placed in the electric field.

Question 10

What is the equation for electric field strength

Answer 10

E=F/Q

F is the force acting on a charge Q which is in the electric field. Here, Qis not causing the electric field.

Question 11

Explain what E is on the equation for electric field strength

Answer 11

E is a vector pointing in the direction that a positive charge would move.
The units of E are newtons per coulomb (NC-1).

Question 12

Why does field strength vary

Answer 12

Field strength depends on where you are in the field.

Question 13

What are the requirements for a body to have a radial field

Answer 13

A point charge - or any body which behaves as if all its charge is concentrated at the centre - has a radial field.

Question 14

What do Q and q represent when looking at a radial field

Answer 14

When the electric field is being generated by a point charge, we call the charge generating the field Q and redefine the charge experiencing the force as q.

(Could be useful for calculations)

Question 15

Equation for Electric field strength of a point charge

Answer 15

E=Q/4 π Eo r^2

In a radial field, E depends on the distance r from the point charge Q

Question 16

What happen to field lines of a positive point charge

Answer 16

For a positive Q, the small positive ‘test’ charge g would be repelled, so the field lines point away from Q.

Question 17

What happen to field lines of a negative point charge

Answer 17

For a negative Q, the small positive charge q would be attracted, so the field lines point towards Q.

Question 18

What is the rule for the direction of field lines

Answer 18

Electric field lines always go from + to -

Question 19

In a radial field what is the relationship between E and r

Answer 19

E is inversely proportional to r^2

Field strength decreases as you go further away from
Q - on a diagram, the field lines get further apart.

Question 20

How can a uniform field be produced

Answer 20

A uniform field can be produced by connecting two parallel plates to the opposite poles of a battery.

Question 21

How is field strength between opposite poles of a battery calculated and does this strength vary

Answer 21

Field strength E is the same at all points between the two plates and is given by:

E=V/d

V is the potential difference between the plates, d is the distance between them.

Here with learn another measurement for E - volts per meter (Vm^-1)

Question 22

How would you draw field lines between opposite poles of a battery

Answer 22

The field lines are parallel to each other.

Areas with the same potential are parallel to the plates, and perpendicular to the field lines.

Question 23

What does the capacitance of a capacitor depend on

Answer 23

The capacitance of a capacitor depends on how easy it is to generate an electric field between its two plates. It also depends on the dimensions of the capacitor.

Question 24

How can capacitance be calculated?

Answer 24

C=EoA/d

Where A is the area of the plates (m^2)
Eo is the permittivity of free space (Fm^-1]
d is the separation of the plates (m).

Question 25

What is capacitance measured in

Answer 25

C is measured in farads (F). This is a large unit though so you’ll often see nano or pico

Question 26

What happens if we don’t assume there is a vacuum between the plates in a capacitor including a relevant equation

Answer 26

If the plates have a material in between them instead of a vacuum, Eo, is replaced with permittivity, E, where:

E=ErEo

Where Er is the relative permittivity (a ratio of the size of the electric field generated in a vacuum, compared to if it was generated in a material).

Question 27

How does the charge of a particle affect the direction it moves comparative to the field lines in a uniform field

Answer 27

If the particle is positively charged then the force is in the same direction as the field lines.
If it’s negatively charged (e.g. an electron), the force is in the opposite direction to the field lines.

Question 28

What force will a charged particle between opposite poles of a battery epicenter

Answer 28

A particle of charge Q will experience a constant force, given by F = EQ, acting parallel to the electric field lines.

Question 29

Explain the path of a charged particle moving through a uniform electric field

Answer 29

1) A particle of charge Q will experience a constant force, given by F = EQ, acting parallel to the electric field lines.
2) If the particle is positively charged then the force is in the same direction as the field lines.
If it’s negatively charged (e.g. an electron), the force is in the opposite direction to the field lines.
3) The work done on the particle by this force (W = Fd) increases its kinetic energy and causes
it to accelerate at a constant rate in the direction of the force (Newton’s second law).
4) If the particle’s velocity has a component at right angles to the field lines, this component will remain unchanged and the velocity in this direction will be uniform. That’s Newton’s first law.
5) The combined effect of constant acceleration and constant velocity at right angles to one another is a curved path

Question 30

What is an electric potential

Answer 30

All points in an electric field have an electric potential, V. This is equal to the work done bringing a unit positive charge from a point infinitely far away to that point in the electric field. This means that at infinity, the electric potential will be zero.

Spec: electric potential at a point as the work done in bringing unit positive charge from infinity to the point; electric potential is zero at infinity

Question 31

In a radial field around a point charge what is electric potential given by?

Answer 31

V=Q/4 π Eo r

V is electric potential (V)
Q is the size of the point charge (C)
and r is the distance from the point charge (m).

Question 32

What does the sign (+ or -) in front of V (electric potential) mean?

Answer 32

The sign of V depends on the charge Q - i.e. V is positive when Q is positive and the force is repulsive (when acting on a unit positive charge), and negative when Q is negative and the force is attractive.

Question 33

Why does the absolute magnitude of V (electric potential vary) change

Answer 33

The absolute magnitude of V is greatest on the surface of the charge, and decreases as the distance from the charge increases.

Question 34

What do the graphs of electric potential against distance look like

Answer 34

See CGP page 148

Question 35

What does the graph of force applied to a unit charge against r (distance from a charge producing the field) look like . What is the area under this graph equal to

Answer 35

See CGP and read paragraph

Area under curve gives the work done

Question 36

What is electric potential a useful def to understand ? -but not the one you should say if asked

Answer 36

Electric potential is the electric potential energy that a unit positive charge (+1 C) would have at a certain point.

Question 37

How do you calculate electric potential energy, why does this make sense?

Answer 37

. Electric potential is the electric potential energy that a unit positive charge (+1 C) would have at a certain point. This means you can find the electric potential energy for any charge at that point in the electric field by multiplying the electric potential by the value of the charge.

Electric potential energy = Vq = Qq/4 π Eo r

where V is electric potential (V)
and q is the size of the charge in the electric field (C).

Question 38

Equation of capacitance of an isolated charged sphere .

Answer 38

.

Question 39

Similarities and differences between gravitational and electric field

Answer 39

.

Question 40

What do you call the shape of an electric field of a point charge, or gravitation field of a mass.

Answer 40

A radial (arranged or having parts arranged like rays around a common center) field