Electric fields: The electric field is strongest close to the charged object.

The idea

Field strength is not the same everywhere: it is greatest right at the surface of the charged object and falls away with distance. On a field line diagram this shows up as spacing, because close to the sphere the radial lines are crowded together, and further out the same number of lines has spread over a much larger area, so they lie further apart. Closely spaced lines mean a strong field and widely spaced lines a weak one. The practical consequence is that a second charge placed near the object feels a large force while the same charge further away feels a much smaller one. It is also why sparks start at the nearest point, where the air sits in the strongest field and breaks down first.

What earns the marks

• The field being strongest at or nearest to the surface of the charged object.
• Closely spaced field lines representing a strong field.
• A larger force on a charge placed close to the object.
• Linking the strongest field to where the air breaks down and sparking begins.

Common mistakes and how to put them right

• Drawing radial field lines evenly spaced at all distances, so the diagram shows a uniform field.

• Saying the field is strongest halfway between two objects with no justification.

• Confusing field strength with the size of the charge on the object.

• Stating that the field is strongest where the lines lie furthest apart.

Exam tips

• Use line spacing to justify any statement about field strength taken from a diagram.
• If asked where a spark will start, answer at the point where the field is strongest.
• Do not call the field bigger; use stronger, and refer to field strength.