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Chapter 9 Electric Current
Making a Current:
In this circuit, the direction of the current is from the positive terminal of
the cell, around the circuit to the negative terminal. This a scientific
convention referred to as conventional current.
A wire is made of metal. Inside a metal, there are negatively charged electrons
which are free to move about. These are called conduction or free electrons,
because they are the particles which allow a metal to conduct an
electric current. The atoms of a metal bind tightly together. In a
typical metal such as copper or silver, one electron from each atom
breaks free to become a conduction electron. The atom remains as a
positively charged ion. The metal is neutral as it has no overall charge
because there are equal numbers of free electrons (negative) and
When the cell is connected to the wire, it exerts an electrical force
on the conduction electrons that makes them travel along the length of the wire. The electrons travel away
from the negative terminal and towards the positive terminal because they are negatively charged. This is in
the opposite direction to conventional current.
As soon as a circuit is completed there is a current in every
point of the circuit. This is because the charged electrons are
already present throughout the metal before the cell is connected.
Sometimes a current is a flow of positive charges; for example,
a beam of protons produced in a particle accelerator. The
current is in the same direction as the particles. Sometimes a
current is due to both positive and negative charges; for example,
when charged particles flow through a solution. A solution which
conducts is called an electrolyte and it contains both positive and
negative ions. These move in different directions when the solution is connected to a cell. Any charged
particles which contribute to an electric current are known as charge carriers; these can be electrons,
protons or ions.
Current and Charge:
When charged particles flow past a point in a circuit, there is a current in the circuit. Electrical current is
measured in amperes (A).
Charge is measured in Coulombs (C).
Charge = Current x Time
From this the unit of charge, the coulomb is defined as follows:
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The amount of charge flowing past a point is given by the following relationship:
Q = I t
Electrons are charged particles. They have a tiny negative charge of approximately -1.6 x 10-19 C. this charge
is represented by e. The magnitude of the charge is known as the elementary charge.
Protons are positively charged; with a charge +e. this is equal and opposite to that of an electron. Charge is
always associated with particles having mass.…read more
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The figure shows a length of wire, cross-sectional area A, along which there is current I.
I = Anev
v = mean drift velocity of the electrons
e = elementary charge
A = cross-sectional area
I = current
The conduction electrons are free to move around inside the metal.
When connected to a battery or an external supply, each electron
within the metal experiences an electrical force that causes it to move
towards the positive end of the battery.…read more