Electric Field Definition

Field Strength and Distance

The strength of an electric field depends heavily on where you measure it.

• The field is strongest closest to the charged object and rapidly gets weaker as you move further away.
• This follows an inverse square relationship, meaning the field strength is proportional to $\frac{1}{\text{distance}^2}$.
• If you double the distance from the charge, the field strength becomes four times weaker.

Non-Contact Forces

• An electrostatic force is categorised as a non-contact force because the interacting objects do not need to physically touch each other to exert a pull or push.
• When two charged objects are brought near one another, their individual electric fields interact to produce this force of attraction or repulsion.
• For example, if a negatively charged balloon is placed near a positively charged wall, their fields interact to pull the balloon toward the wall via the space between them.

Drawing Electric Fields

To visualise electric fields, physicists draw field lines following strict rules.

• Field lines must always be drawn at right angles (90°) to the surface of the charged object.
• The arrows on the lines point away from positive charges and towards negative charges.
• The direction of an electric field at any point is defined as the direction of the force that a positive test charge would experience if placed there.
• The strength of the field is shown by the density of the lines; lines drawn closer together indicate a stronger field.

There are two main types of field patterns you must know:

• A radial field is created by an isolated point charge or sphere, where straight, equally spaced lines spread outwards or inwards like the spokes of a wheel.
• A uniform field is found between two oppositely charged parallel metal plates, represented by parallel, straight, and equally spaced lines pointing from the positive plate to the negative plate.

Explaining Sparks

A very strong electric field can cause sudden electrical discharges through the air.

• Normally, air is an insulator, but an intensely strong electric field can cause the ionisation of air molecules by stripping electrons away from them.
• Once ionised, the air becomes conductive and allows an electric current to flow easily.
• This sudden flow of charge between a charged object and the Earth (or another conductor) is seen as a visible spark.
• Sparking is much more likely when the distance between a charged object and an earthed conductor is small, as this highly concentrates the electric field.