Electric charge and electric fields — Physics, 14–17 years
Electric charge is a property that makes objects push or pull electrically. An electric field describes the force a charge would feel at each place, even before another charge is put there.
A field around charge
Positive and negative charges attract, while charges with the same sign repel. Instead of describing this as an invisible pull acting instantly across space, physics uses an electric field: a map of the force available around a charge. Field direction is the direction a small positive test charge would move.
Why fields were introduced
Forces between separated charges are hard to picture as direct contact, yet their strength changes with distance. The field idea gives each point in space a measurable value, so the source charge can be studied separately from the charge that feels the force. It also makes complicated arrangements easier to predict.
Worked example
A positive charge creates a field of 200 N/C at a point 0.5 m away. Place a small positive charge of 0.003 C there. The force is F = qE = 0.003 × 200 = 0.6 N. Because both charges are positive, the force points away from the source charge. A negative test charge would feel 0.6 N in the opposite direction.
The common mistake
A common mistake is drawing every field arrow away from a charge. That feels simple because an arrow can be read as “what the charge does”. By convention, field direction means the force on a positive test charge: arrows point away from positive charges and towards negative charges.
Where it is used
Inkjet printers steer tiny charged drops with electric fields to form letters. Electrostatic paint sprayers give droplets a charge so they are attracted to a car body, reducing waste. Lightning, photocopiers and capacitors also depend on electric fields, although their designs are different.
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