A charged object experiences a force in an electric field. This is what an electric field is. We say that there is an electric field anywhere where an electric charge experiences a force. An electric field is a field of force.
This is a rather abstract idea. You will be more familiar with the idea of a ‘field of force’ from your experience of magnets. There is a magnetic field around a permanent magnet; another magnet placed nearby will experience a force. You have probably plotted the field lines used to represent the field around a magnet. There is a third type of force field that we are all familiar with, because we live in it. 
You have already met this force in Chapter 17, the gravitational field. There are many similarities between electric fields and gravitational fields, there are also key differences. 
To summarise:
- electric fields – act on objects with electric charge
- magnetic fields – act on magnetic materials, magnets and moving charges (including electric currents)
- gravitational fields – act on objects with mass.
Later, we will see that the electric force and the magnetic force are closely linked. They are generally considered as a single entity, the electromagnetic force.

Representing electric fields 

We can draw electric fields in much the same way that we can draw magnetic fields or gravitational fields, by showing field lines (sometimes called lines of force). The three most important field shapes are shown in Figure 21.6.
As with magnetic fields, this representation tells us two things about the field: its direction (from the direction of the lines), and how strong it is (from the separation of the lines). The arrows go from positive to negative; they tell us the direction of the force on a positive charge in the field.
A uniform field has the same strength at all points. Example: the electric field between oppositely charged parallel plates.
A radial field spreads outwards in all directions. Example: the electric field around a point charge or a charged sphere.

Figure 21.6: Field lines are drawn to represent an electric field. They show the direction of the force on a positive charge placed at a point in the field. a A uniform electric field is produced between two oppositely charged plates. b A radial electric field surrounds a charged sphere. c The electric field between a charged sphere and an earthed plate.

We can draw electric fields for other arrangements. Note the symbol for an earth, which is assumed to be uncharged (in other words, at zero volts).

Questions

 

1) Which of the three field diagrams in Figure 21.7 represents:
a: two positive charges repelling each other?
b: two negative charges?
c: two opposite charges?

Figure 21.7: Electric fields between charges. For Question 1.

2) Many molecules are described as polar; that is, they have regions that are positively or negatively charged, though they are neutral overall. Draw a diagram to show how sausage-shaped polar molecules like those shown in Figure 21.8 might realign themselves in a solid.

3) Figure 21.9 shows the electric field pattern between a thundercloud and a building. State and explain where the electric field strength is greatest.