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On the syllabus: GCSE PhysicsA-Level Physics
You rub a balloon on your hair, hold it a few centimetres away — and your hair reaches up toward it without any contact. How can one object exert a force on another across empty space, with nothing in between?
You rub a balloon on your hair, hold it a few centimetres away — and your hair reaches up toward it without any contact. How can one object exert a force on another across empty space, with nothing in between?
The idea of 'action at a distance' troubled Newton himself — the idea that gravity could pull the Moon without touching it seemed almost mystical. Electric charges face the same puzzle. The modern answer is that a charge doesn't act directly on another charge; instead, it creates an invisible structure throughout the surrounding space — an electric field — and that field is what pushes or pulls anything else that enters it. Understanding the field turns a mystery into a precise, measurable quantity.
An electric field is a region of space around a charged object where another charge would experience a force. The field exists whether or not a test charge is present — the source charge creates it. Field strength E at any point is defined as the force per unit positive charge placed at that point. For a single point charge, the field grows stronger as you get closer and weaker as you move away, following an inverse-square relationship with distance.
The formula E = kQ/r² tells you the field strength at any distance r from a point charge Q. Because the force on a test charge q is F = qE, you can find the actual force by multiplying the field by the test charge: F = kQq/r². Field lines represent the direction a positive test charge would move: they point away from positive source charges and toward negative ones. Where field lines are dense, the field is strong; where they spread apart, the field is weak. In uniform fields (like between two parallel plates), the lines are parallel and equally spaced — field strength is constant. The superposition principle states that the total field at any point is the vector sum of the fields from all individual charges.