PHY C17: Electric Field Strength (AS)

This post covers:

• Electric Field Strength
• Uniform Fields & when they occur
• Formula derivation

Let’s go!

What is electric field strength?
Force per unit charge (acting on a small positive test charge).

It is often represented by E (annoyingly, since it shares this symbol with energy, but they are NOT THE SAME).

E = F/Q

If that confused you, revisit gravitational field strength. While gravitational field strength is gravitational force per unit mass, electric field strength is force per unit charge.

In general, field strength is a force per unit property which is affected by that force.

For now, we will focus on 1 type of electric field: UNIFORM FIELDS.

What is a uniform electric field?
An electric field where the electric field strength is the SAME at all points in the field.

This means that the electric force acting on a charge does NOT change as it travels within a uniform field.

This differs from a radial field, where the field strength obeys the inverse-square law (travelling away from the centre, the force gets weaker). In a uniform field, as long as a charge is within the field, there will be a constant force.

Generally, uniform fields are generated between 2 PARALLEL CHARGED PLATES, like so:

This can be done with 2 conductive plates connected to a power source, creating a potential difference between them. Thus, an electric field is created, with field lines pointing from a HIGH (+) potential to a LOW (-) potential.

This is usually simplified to be from a positively-charged plate to negatively-charged plate.

However, there will also be an electric field between 2 like charges with different magnitudes:

Or between a charged plate and an EARTHED plate: The earthed plate has a potential of 0, so the field lines travel from the higher + potential to the 0 potential.

Formula for Electric Field Strength
In my A2 coverage of this chapter, we’ll look at a more complete formula for electric field strength (hint: it’s almost identical to gravitational field strength), as applied to radial fields. For now, we will simplify things:

For uniform fields, we can describe electric field strength through the lens of WORK.
You should know that:
W = Fd

Potential difference is the work done per unit charge. If you want a deeper explanation for that, see here.

For now, you should know that:
V = W/q

Rearranging,
W = Vq

Therefore,
W = Fd = Vq

This can be rearranged to:
F/q = V/d

Since E = F/q, electric field strength can also be given as:

E = V/d

It might be a little confusing just memorising a formula, so I will explain this logically in my A2 coverage of this chapter. In the meantime, refer back to this post on fields and potentials. Think about how forces are related to the potential energy at different positions in the field – and how you can derive field strength from that relationship!

This formula is quite powerful on its own, & there are a few things you can solve with it. See the next post for some problem examples!