ELECTRIC CHARGE Electric charge (Q or q) indicates if a body has an excess or a deficiency of electrons.
It’s a scalar quantity with an SI unit of coulomb (C). Q is the symbol used when dealing with total charge on a body. q is the symbol for charge on a individual particle.
1 coulomb is the amount of charge that passes any point in a circuit when a current of 1 ampere (A) flows for 1 second.1 1 C 1 A s
1 coulomb is the amount of charge on about 6.25 1018 electrons. That’s 6 250 000 000 000 000 000 electrons or 61/4 billion billion electrons!
The charge on 1 electron e 1.6 1019 C 0.000 000 000 000 000 000 16 C.
Atomic nature of electric charge
The atom is made from three particles: protons, neutrons and electrons.2 Protons and neutrons are located in the centre of the atom (forming the nucleus); electrons orbit the nucleus.
In a neutral atom the number of electrons equals the number of protons.
A body becomes positively charged when it loses electrons. It becomes negative when it gains electrons. It never loses or gains protons.
Fig 15.1: Illustration of an atom
Forces between charges Like charges repel each other; unlike charges attract each other..3
1The ampere is the unit of electric current; it will be defined in Chapter 20. 2 In Chapter 23 we will see that protons and neutrons can be further subdivided into quarks.The fact that protons and
neutrons are not fundamental has no bearing on the behaviour in the observable world. 3The explanation of, and processes involved in, electrostatic forces are both complicated and fascinating. They are well
beyond the scope of Leaving Cert. Physics but if you’re interested you can Google ‘quantum electrodynamics’ or ‘Feynman diagrams’ for detailed explanations. Don’t expect to understand too much of the explanations at this stage—there’s plenty of college graduates still struggling with them!
