The Basics
Introduction
In this part we introduce the terms voltage, current, resistance, capacitance and inductance and
explain how devices can be constructed to exhibit each of these properties. We also explore the
relationships between current, voltage and resistance and define the direction of current flow
within an electrical circuit.
The Voltage
The voltageV is the driving force in an electrical
circuit. It acts on the free electrons contained within metallic wires and forces these charges to
move around the circuit. Mathematically it is calculated using
V = W/q
where W is the work done in moving the charge
q. Its meaning can be understood by way of an
analogy. When water is pumped from a reservoir to some holding tank above, work is done to
fill the vessel and the water stores the work done as potential energy. The change in the potential
energy (or potential difference) in taking the water from the receiver to the tank is analogous to
the voltage in the electrical circuit and can be considered as the driving force to move the charge.
One of the most common sources of voltage comes from a battery, which is typically composed
of two electrodes separated by a chemical medium.
If designed appropriately the chemical medium can undergo reaction to establish a positive and
negative charge on the electrodes. This separation of charge creates a potential difference which
can be harnessed by connecting the two electrodes to an electrical circuit.
The Current
The current (i) is related to the movement of
electrons through a circuit, typically induced by an applied voltage. The current is reported in
units of amperes (amps) and is the rate of flow of charge:
i = q/t
The direction of electron flow in a circuit will be towards the positive terminal of the voltage
source since this will be the attractive pole for the negatively charged electrons.
The Resistor
The resistance of a wire arises from the collisions of the moving electrons with
atoms within the material. These collisions result in the loss of some of the energy of the
electrons and is dissipated as heat in the wire. The resistance for a wire or other material of
length
l, area A and resistivity p can be calculated using the following expression
R = pl/A
The unit of resistance is the Ohm and is related to the current and voltage by Ohm's law:
V = iR
The Capacitor
In its simplest form the capacitor is a device composed of two parallel plates separated
by an insulating material - termed a dielectric. This device is unlike the resistor above as it is
capable of storing charge. In its neutral form both plates contain the same number of free
electrons
however when the capacitor is connected to a voltage source via an electrical circuit it is possible
to move the free electrons from one plate to another. The net result is that one plate becomes
positively charged and the other negatively charged.
The amount of charge that may be stored by the capacitor per unit of voltage is the capacitance
C. The unit of capacitance is the farad (F) and is related to the charge and voltage in the
following way
C = q/V
The inductor
The inductor is another device which is capable of storing charge and in its simplest form is a
coil of wire 
When a current is passed through the wire the flow of charge around the loops creates a strong
electromagentic field. The magnitude of this field is related to the total current flowing, an
increase in the current increases the magnitude of the field, correspondingly a decrease in the
current decreases the field. When an oscillating current is passed through the inductor this
creates a changing electromagnetic field and this inturn creates an induced voltage. This can be
represented mathematically as
Vinduced =
L(di/dt)
where (L) is the inductance and takes the units
of
henry's (H).
In the following sections we see how to construct and analyse electrical circuits composed of the
above components.
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