Electric charge
Charge & Current - OCR A-Level Physics
Key Definition
Electric charge
An elementary property of particles. Measured in coulombs (C).
An elementary property of particles. Measured in coulombs (C).
- A proton carries a chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). of +e. An electron carries a chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). of -e.
- The elementary chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).The magnitude of charge carried by a single electron or proton. Equal to 1.6 × 10⁻¹⁹ C. is $e = 1.6 x 10^-19 C$.
- Net charge of an $object = total charge$ of its protons and electrons.
- A neutral object becomes charged by gaining or losing electrons.
- Objects cannot gain or lose part of an electron.
Key Definition
Charge quantisation
An object's net charge must be a whole-number multiple of the elementary charge, e. Charge cannot exist in fractions of e.
An object's net charge must be a whole-number multiple of the elementary charge, e. Charge cannot exist in fractions of e.
$$Q = ne$$
- Negatively charged: $Q = -ne (object has n extra electrons)$.
- Positively charged: $Q = +ne (object has lost n electrons)$.
Common Mistake
MEDIUM
Wrong: Protons move to create a positive charge on an object.
Right: Only electrons move. A positive charge means electrons have been lost, not that protons have been added.
Right: Only electrons move. A positive charge means electrons have been lost, not that protons have been added.