11.8P–11.10PElectric fields
Physics only — not on the Combined Science specification
A charged object creates an electric field around itself. The electric field is strongest close to the charged object. The further away from the charged object, the weaker the field.
A second charged object placed in the field experiences a force. The force gets stronger as the distance between the objects decreases.
You should be able to draw the electric field pattern for an isolated charged sphere. Four conventions have to be right:
• the lines are radial — straight, and pointing directly towards or away from the centre • the arrows point away from a positive charge and towards a negative charge • the lines are evenly spaced all around the sphere • the lines are closer together near the sphere, where the field is strongest, and further apart away from it.
You should also be able to explain the concept of an electric field. A field is a region in which a charged object experiences a force. Drawing it as lines is a way of recording two things at once: the direction of the force on a positive charge placed there, and — through the spacing — how strong that force would be.
You should be able to explain how the concept of an electric field helps to explain the non-contact force between charged objects, as well as other electrostatic phenomena such as sparking.
The non-contact force: rather than saying one charge somehow reaches across empty space to push another, the field lets you say that the first charge fills the space around it with a field, and the second charge experiences a force because of the field where it actually is. The force is then explained locally, and the fact that it gets stronger as the objects get closer follows directly from the field being stronger nearer the charge.
Sparking: as charge builds up on an object, the potential difference between it and a nearby earthed object rises, and so does the field strength in the air between them. If the field becomes strong enough, it pulls electrons off the atoms in the air. The air is no longer an insulator — it now contains charged particles that can move — so charge flows through it suddenly, and that flow is the spark you see and hear.
Common mistake
Field lines must never cross, and they must be drawn with arrows. A pattern without arrows does not say whether the sphere is positive or negative, which is usually half the mark.
- Electric field
- the region around a charged object in which a second charged object experiences a force; the field is strongest close to the charged object and weaker further away
Before you practise
- An electric field is the region where an electric charge experiences a force.
- Field lines point in the direction of the force on a POSITIVE charge, so away from a positive charge and towards a negative one.
- The closer the lines, the stronger the field.
- Around an isolated point charge the field is radial; between parallel plates it is uniform, with evenly spaced parallel lines.
- The field is strongest close to the charge, and gets weaker with distance.
- The field is how a non-contact force acts: one charge creates a field, and the other feels a force in it.
- Sparking happens where the field is strong enough to ionise the air.