Voltage in Electrical
Systems
Notes (p71)
Electrical systems comprise:
4 basic components:
• Voltage sources (batteries and generators)
• Control elements (switches)
• Electrical appliances or loads (lights, heaters,
•
computers, motors etc)
Conductors (copper wire)
Types of electricity
Direct current (DC)
Alternating current (AC)
• Electric charge in wires moves one way only
• DC is produced by batteries, or generators
• Electric charge moves back and forth in the wire
•
many times per second
AC is produced by alternators
Current is the movement of electric charge
and is measured in amperes or amps
Voltage is measured in volts
DC Supplies I
Batteries are a source of chemical energy which
is converted into electrical energy when the
battery is placed in a circuit.
• Dry cells
• contain a paste electrolyte
• D, AAA, AA
• voltages can be 1.5V, 3V, 6V, 9V
• composition can be carbon-zinc, Ni-Cad, Li-Ion
• Wet cells
• contain a liquid electrolyte
• Auto batteries (lead-acid)
• Cell voltages can be 2V
Recharging?
Recharging capability
•
•
Pumping weak current into the battery in the opposite
direction from a battery charger
Wet cells and some dry cells can be recharged from a
“dead” state
Terminology
•
Primary cells
• Difficult to recharge
• Secondary cells
• Easily recharged
How are DC Voltage sources
connected?
Connection options
•
•
•
•
Batteries have terminals or electrodes
Terminals are positive (anode) and negative (cathode)
When connected into a circuit electrons flow from the
negative electrode to the positive electrode.
Series connection of batteries
•
the positive terminal of one battery is connected to the negative
terminal of another battery, providing a net voltage that is the sum
of both batteries.
What makes up a DC circuit?
The following components:
• Source
• Conductors
• Control
• Load
>battery
>wires
>switch
>lamp
This circuit can be shown in schematic
form with symbols representing the
components above, as on next …
What makes up a DC circuit?
Voltage - a “forcelike” quantity
(prime mover)
In all electrical systems, voltage acts like a force,
moving electricity, or negative charges, through the
circuit.
The atomic structure underpins the electricity model
in that each atom comprises:
•
Nucleus with
•
Surrounding the nucleus are
• Protons (+ve),
• Neutrons (neutral), and
• Most of the atomic mass
• Electrons (-ve)
Electrical forces create a voltage difference
whenever the positive and negative charges are
separated.
How can a voltage difference be
created?
Chemical – dry & wet cell batteries
Magnetic/mechanical – a coil of wire
moving in a magnetic field produces a force
on electrical charges that separates the
positive and negative charges.
Light – photoelectric effect in which
incident light falling on some substances
causes the separation of electrons from
their atoms resulting in a movement of
charges (or current).
• Example – photocell
How are voltages measured?
The difference in voltage between any 2
points in a circuit is measured with a
voltmeter in units of volts, millivolts,
kilovolts…
A voltmeter can be of many types, such as:
• Analog
• Digital
• Oscilloscope