Electric fields Definitions

What creates an Electric Field

Charge

Electric field strength

Force per unit POSITIVE charge E = F / q

How to draw electric field lines

The number of lines per unit volume represents the strength of the field The lines must touch the charge at 90 degrees Lines must be drawn from positive and towards negative OR from positive to less positive.

Electric field lines between parallel plates

Equal spaced lines from positive to negative

Formulas that can be used when dealing with parallel plates

Electric field between plates, E = V / d V: P.D between plates d: distance between plates Force on a charge between plates, F = EQ Acceleration between plates = F / m = EQ / m = VQ / d m

Coulombs law

The force between two charges is proportional to the product of the charges and inversely proportional to the distance between the charges squared

How do you work out the field strength at a distance from a point charge

E = - k Q / r^2

Force acting between two protons in a nucleus

Use F = k Qq / r^2

Q = q = 1.6 x 10^-19 r = 10^-15 m

How would you determine the overall electric field strength MIDWAY between two charges which have the same sign

Determine the electric field strength at the distance from each charge separately using E = k Q / r^2 and then subtract the answers (both fields will be in opposite directions)

How would you determine the overall electric field strength MIDWAY between two charges which have the opposite sign

Determine the electric field strength at the distance from each charge separately using E = k Q / r^2 and then add the answers (both fields will be in the same direction)

What would happen to a positively charged particle if placed in a field?

It would accelerate in the direction of the field (along a field line)

How would one determine the velocity gained by a charge as it moves between a potential difference

VQ = 1/2 m v^2 Electric potential energy is converted to Kinetic energy

Define the e V

Define a MeV

The energy gained by an electron as it is accelerated through a p.d of 1V

The energy gained by an electron as it is accelerated through a p.d of 1MV

Define the kWh

The energy dissipated by a 1kW device in 1 hour

A charge gains a K.E of E when accelerated between parallel plates of p.d, V and separation, d. What happens to the K.E of the same charge if the separation was doubled?

Nothing. The energy gained is independent upon the distance. W.D = F d = E Q d = V/d x Q d = VQ

Similarities between E - fields and g - fields

The both cause action (force) at a distance. They both produce fields that touch the origin at 90 degrees. They both obey the inverse square law. Both produce radial fields where the field lines touch the equipotential surfaces at 90 degrees.

Differences between E - fields and g - fields

One is force per unit mass and the other force per unit + charge (one created by mass the other charge). G - fields do not depend upon the medium but electric fields do.

Electric fields can be attractive or repulsive

A charge moving at 90 degrees to the direction of the field (i.e. moving between plates - parallel to the plates).

How would the charge move? What formulas could you apply

Use SUVAT in the direction of the electric field. Remember: u = 0, a = F / m = EQ / m

= V Q/ d m, s is NOT the separation of the plates but how far the charge has moved vertically. Use d = v / t perpendicular to the field. In some cases the horizontal velocity is determined using VQ = 1/2 m v^2

Explain how Millikan's oil drop experiment was used to determine the charge of an electron. What did Millikan realise about charge?

With no electric field between the plates the charge moved at terminal velocity.

The mass can be obtained by equating mg to viscous drag. Stokes law 6𝜋ηrv

An electric field can be applied to leviatate the charged oil drop. Here EQ = mg.

The charge ends up being a multiple of e. Charg is quantised.

Definition of electric potential

Work done per unit charge in moving a point positive charge from infinity to that point

V = - kQ / r

What is an equipotential surface surrounding a charge?

A line on which the electrical potential energy is always the same. If a point charge is moved to any point along the same potential line (regardless of the path taken) no work is done

What is the relationship between electric field strength and electric potential

They touch at 90 degrees

Electric field strength = potential gradient

= - ∆ V / ∆r Potential gradient is the change in potential per unit distance moved

What can be obtained from a graph of electric potential (y - axis) against distance ( x - axis)

The gradient = the electric field strength since E = = - ∆ V / ∆r

What can be obtained from a graph of electric field strength (y - axis) Vs distance from a charge (x - axis)

The area of the graph represents the change in electric potential between the two distances in question since ∆ V = E ∆r

Definition of electric potential energy

The work done in moving a point positive charge from infinity to that point

Formula linking work to potential difference and derivation (this is on the specification).

Imagine moving a charge between two equiptential surfaces with a potential difference of ∆V between them across a distance ∆r. Work done in doing so = F ∆r. = EQ ∆r Since E =∆V / d = ∆V Q

so F d = Q V

How to determine the potential at a distance from charges

Use V = kQ / r or -kQ/r (if the charge is negative). Add up or subtract the individual potentials to get the total potential.

Note: Potential is a scalar so a vector addition will not be necessary. We simply add or subtract

How to determine the total electric field at a distance from charges

Determine the electric field due to each charge. Draw the direction of the field. You will need to do a vector addition after to determine the resultant. This may involve splitting each vector into perpendicular components and then combining them using pythag and trig to determine the magnitude and direction,