Everything Science Grade 10

CHAPTER 16. ELECTROSTATICS 16.4

When two identical conducting spheres on insulating stands are allowed to touch they

share the charge evenly between them. If the initial charge on the first sphere isQ 1 and

the initial charge on the second sphere isQ 2 , then the final charge on the two spheres after

they have been brought into contact is:

Q=Q^1 + 2 Q^2

Quantisation of charge ESAEX

The basic unit of charge, called the elementary charge,e, is the amount of charge carried

by one electron.

Unit of charge ESAEY

The charge on a single electron isqe= 1, 6 × 10 −^19 C. All other charges in the universe

consist of an integer multiple of this charge. This is known as charge quantisation.

Q=nqe

Charge is measured in units calledcoulombs (C). A coulomb of charge is a very large

charge. In electrostatics we therefore often work with charge in micro-coulombs ( 1 μC=

1 × 10 −^6 C) and nanocoulombs ( 1 nC= 1× 10 −^9 C).

FACT

In 1909 Robert Mil-

likan and Harvey

Fletcher measured

the charge on an

electron. This exper-

iment is now known

as Millikan’s oil drop

experiment. Millikan

and Fletcher sprayed

oil droplets into the

space between two

charged plates and

used what they knew

about forces and in

particular the electric

force to determine

the charge on an

electron.

Example 3: Charge quantisation

QUESTION

An object has an excess charge of− 1 , 92 × 10 −^17 C. How many excess electrons

Physics: Electricity and Magnetism 267