Field Concept Application

This gives a useful chain for explaining static-electricity phenomena:

source charge → electric field → force on another charge → movement or discharge

The field is not itself a force. It exists around the source charge, whether or not another charged object is present. A second charge placed in the field experiences the electrostatic force.

Attraction and repulsion in a field

The effect of the force depends on the signs of the interacting charges:

• charges with the same sign repel, so each is forced away from the other;
• charges with opposite signs attract, so each is forced towards the other.

For example, a negative particle in the field of a positive object is attracted towards it. Two negative particles repel one another.

The force also depends on position in the field. Charged objects exert stronger attractive or repulsive forces when they are closer together and weaker forces when they are farther apart. This is why nearby charges generally produce the most noticeable static effects.

Why charged objects can attract neutral objects

A neutral object contains equal amounts of positive and negative charge overall, but those charges do not always remain distributed evenly. The electric field of a nearby charged object can cause electrons within the neutral object to be redistributed. This is induction.

Suppose a negatively charged balloon is brought near a neutral wall. Its field repels some of the wall's electrons away from the surface. The surface nearest the balloon is then relatively positive, while negative charge is displaced slightly farther away. The attraction to the nearer positive region is stronger than the repulsion from the more distant negative region, so there is a resultant force towards the wall. The balloon can therefore stick even though the wall remains neutral overall.

The same reasoning explains why a charged comb can pick up small neutral pieces of paper: its field redistributes charge in the paper, creating a resultant attraction.

Using repulsion in an insecticide sprayer

An electrostatic insecticide sprayer gives the emerging droplets the same type of charge—for example, positive charge. Each charged droplet creates a field, so neighbouring droplets experience repulsive forces and move apart.

The repulsion stops the droplets gathering into larger drops. The insecticide remains a fine mist, spreads over a large area and can cover crops while using less chemical. The charged droplets can also experience attraction towards oppositely charged regions, helping direct the spray. Similar electrostatic forces are used in car spray-painting.

A complete explanation must go beyond saying that the droplets are charged: their charge produces electric fields, the fields exert repulsive forces on other like-charged droplets, and those forces spread the mist.

Fields, shocks and sparks

When static charge builds up, it creates an electric field around the charged object. That field can exert forces on electrons in nearby materials. If charge moves suddenly through the air or through a person, there is an electrical discharge, which may be felt as a shock or seen as a spark. Lightning is a much larger example of a sudden electrical discharge between differently charged regions.

A spark can transfer enough energy to ignite a nearby flammable gas, vapour or liquid. Static charge is therefore particularly hazardous while vehicles are being fuelled.

During aircraft refuelling, fast-flowing fuel interacting with the pipe can produce a build-up of static charge. If this led to a spark near the flammable fuel, it could cause a fire or explosion.

Preventing a dangerous field from developing

Earthing gives electrons a conducting route between an object and the Earth. If an object has excess electrons, electrons can move from it to Earth. If it has an electron deficit, electrons can move from Earth onto it. In either case, its overall charge is reduced.

During aircraft refuelling, a conducting copper bonding line connects the aircraft or fuel tank to Earth. Charge is carried away rather than being allowed to accumulate, reducing the electric field produced by the build-up and making a spark less likely.

Electric fields therefore explain both useful and hazardous effects of static electricity. In a sprayer, forces in the fields are deliberately used to move droplets. During refuelling, unwanted charge is removed so that its field does not lead to a dangerous discharge.