3.4 - Electric Fields

Electric fields and field lines

• An electric field is created around any electrically charged object.
• It's the region around the object where another charge feels a force.
• The closer to the object, the stronger the field.

Field lines

Electric fields are represented using field lines. Field lines:

• Point from positive to negative (the direction of the force on a positive charge)
• Meet the surface of the charged object at a right angle
• Have arrows showing the field direction
• Show field strength by their spacing - the closer the lines, the stronger the field

Field patterns to recognise

• An isolated point charge
• A charged conducting sphere (which has the same field pattern as a point charge)

Force on a charge from interacting fields

• When a charged object is placed in the electric field of another charged object, it feels a force.
• The electric fields of the two objects interact, producing an electrostatic force on each object.

Attraction and repulsion

• Opposite charges attract - The field lines between them point in the same direction and join up, pulling the objects together.
• Like charges repel - The field lines point in opposite directions and push against each other, forcing the objects apart.

Size of the force

• The size of the force depends on the strength of the field at that point.
• Increasing the distance between the charged objects weakens the field, making the force smaller.

The uniform field between parallel plates

• The electric field between two oppositely charged parallel plates is uniform: its strength and direction are the same everywhere between the plates.
• A uniform field is drawn as parallel, evenly spaced field lines running from the positive plate to the negative plate.
• A charged particle placed in a uniform field feels a constant force everywhere between the plates.

Field patterns between two point charges

Two opposite charges

• Field lines run from the positive charge to the negative charge, curving across the gap.
• Lines leave and enter the charges at right angles to their surfaces.

Two like charges

• Field lines bend away from each other and do not join up across the gap.
• Midway between two equal like charges, the fields cancel out and the resultant field is zero. This point is called the neutral point.