Electricity – Part 1
Peter Coulahan
Learning Outcomes
Having completed all of the learning activities associated with this
lecture, you should be able to:
Electricity – Part 1
Dr Peter Coulahan
University College Dublin Safety Officer
Explain the basic principles of electricity in the context of its
relevance to the safety of persons at work
Explain the conditions / factors that must be present for a
person to suffer an electric shock
Explain the key terminology associated with electrical safety
Describe the physiological effects of electric shock and the
associated risks
Detail the arrangements that can be put into place to mitigate
the risks of electric shock and electrical fire.
What is electricity?
Terminology – voltage, current, resistance,
earth, Ohms Law, electrical circuits, short
circuit, direct current (DC), alternating current
(AC), static electricity, electrical arcing
Electricity is at its simplest the flow of electrons
through a material
Like charges repel each other whilst opposite
charges attract – if you try and force electrons
together they will repel each other and flow to
a positively charged area or to an area with no
charge, i.e. earth
Electricity
Part 1
Part 1
An introduction to electricity
Part 2
Practical electrical safety measures
What is electricity?
Structure of a
carbon atom
Electron / Electricity Flow
6 protons
+ 6 neutrons
Electron
Proton
Neutron
Carbon atom
1
Electricity – Part 1
Peter Coulahan
Electron / Electricity Flow
Voltage
The movement of negatively charged electrons
to a positively charged area or to an area with
no charge (i.e. earth) is the basic principle
Voltage is a measure of the force which
motivates electrons / electricity to flow
behind electricity
For electricity to flow between two points there
must be a ‘voltage difference’ between those
two points
Electrons will always take the path of
least
Voltage is measured in volts
Resistance is a measure of a materials ability
to either impede or allow the flow of electricity
Conductors allow electricity to flow easily
– they exhibit low resistance
Insulators do not allow electricity to flow easily
– they exhibit a high resistance
Resistance is measured in Ohms (Ω)
An electrical circuit is a closed path through
resistance to earth
Current
Current is a measure of the amount of
Resistance
electricity flowing in a circuit
It is measured in amps or milliamps
The term amperage is sometimes used
interchangeably with current
Ohm’s Law
There is a fixed relationship between voltage,
Electrical Circuit
current and resistance
This is known as Ohm’s Law
which electrical current can flow
An electrical circuit can be disrupted by
Voltage = Current x Resistance
breaking the pathway at any point along its
Current = Voltage / Resistance
route
Resistance = Voltage / Current
A short circuit occurs when electricity is lost
from the closed pathway
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Electricity – Part 1
Peter Coulahan
Alternating Current (AC) –
Domestic Supply
Direct Current (DC)
Electron flow
+
Battery
RMS
Average
230 Volts
Voltage
Electric
lamp
(glowing)
Time
-
Electron flow
One Cycle
1/50 sec (50 Hz)
The flow of electricity in the same direction through
a circuit at a constant and steady voltage.
Alternating Current (AC)
Three Phase Alternating Current
In an alternating current electricity is supplied
at a constantly changing voltage and direction
When referring to AC voltages we refer to the
Root Mean Square of the supply, i.e. the
average voltage of the supply
The number of cycles per second is known as
the frequency of supply
Three phases
Voltage
Time
Static Electricity
When two materials are brought into contact
with each other electrons can be transferred
from one to another giving the latter a negative
charge
If a static charge is allowed to build up on a
material then when a conductor is brought
close to the affected area a spark can jump
from one to the other – static discharge
Electrical Arcing
An electrical arc is a discharge of electricity
between two or more conductors across a gap,
e.g. as when electrical current ‘jumps’ from a
high voltage power line across an empty space
to a metal digger located below
It differs from static discharge in that no
contact between materials is required
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Electricity – Part 1
Peter Coulahan
Summary
What is electricity?
Voltage
Current
Resistance
Ohms Law
Electrical circuits
Earth
Direct current (DC)
Alternating current
(AC)
Static electricity
Electrical arcing
Electricity
Part 1
An introduction to electricity
Part 2
Practical electrical safety measures
4