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Electricity (Pt 1)
NSC1951 - BIOPHYSICAL SCIENCE FOUNDATIONS
Assoc Prof Alfio Parisi
Lecturer in Physics
parisi@usq.edu.au
Ph: 46 31 2226
Objectives (Pt 1)
(Cree & Rischmiller, Ch 11)
• Understand concepts of:
• electric charge
• static electricity
• current electricity
• resistance
Electricity in Nursing
•
•
•
•
Equipment and instruments
Safety
Treatment
Electricity plays a major role in the life of
most communities
Electricity
• Atoms consist
of:
• nucleus
• electrons
•
Nucleus
consists of
• Protons
• neutrons
Electricity
• Electrons have
negative (- ve)
charges
• Protons have
positive (+ ve)
charges
• Neutrons have
no charges
Electricity
• Electricity of any kind involves
electrons
• Carry a negative electrical charge
• Electrons can be removed from
various elements and compounds
with varying degrees of ease
Static & Current Electricity
• Current electricity - the electrons
(charges) are in motion
• Static electricity - the charges are at
rest with a build up of one type of
charge
Static Electricity
•
Static electricity is typically formed by
rubbing (friction)
•
Two objects may become oppositely
charged when surface electrons are
transferred from one to another by rubbing
Static Electricity
•
•
•
Take a comb or plastic pen and rub it on
almost any piece of fabric, it will acquire a
small electric charge
If placed near small pieces of paper, they
will be attracted
(Inquiry into Physics, p297)
Static Electricity
•
•
If electrons are transferred to the comb or
pen from the fabric, the comb or pen have
become negatively charged.
If electrons are transferred away from the
comb or pen to the fabric, the comb will
become positively charged.
Static Electricity
•
•
•
eg walking across a
wool carpet on a
dry day can build
up a charge on a
person
Like charges are
concentrated in one
place on a nonconducting surface
Lightning is a form
of static electricity
(Halliday, Resnick, Walker)
Static Electricity
•
•
•
Higher charges build up in
dry air than in damp air.
The water molecules in
damp air help the charges
leak away from the
charged object
Like charges repel (ie
electrons repel each other
and protons repel each
other)
Opposite charges attract
(ie electrons attract
protons)
(Timberlake, p49)
Static Electricity
•
Uncharged objects will respond to charged
objects due to production of an induced
charge on uncharged objects
(Inquiry into Physics, p302)
Current Electricity
•
•
•
Moving of charges through conductors
Direct current given the symbol DC
• charges moving continually in one
direction
• eg using batteries
Alternating current given the symbol AC
• the direction of charge flow is reversing
or alternating direction continually
• eg using mains power at 50 Hz AC
•
•
•
Charges will move
(flow of electricity)
between two points
if one point has a
higher
concentration of
electrons than
another
There is a difference
in electrical energy
between the two
points
Called electrical
potential difference
or voltage
Current Electricity
Cree & Rischmiller, Fig 11.4)
Current Electricity
•
•
In a battery, a difference in
electrical potential exists
between the two battery
terminals
This difference in electrical
potential is called the
voltage and measured in
units of volts (V)
Current Electricity
•
An electric current needs a
complete pathway of
conductors, called a circuit
(Inquiry into Physics, p310)
Electricity
•
•
•
•
•
•
•
The unit of charge is the coulomb (C)
The charge on an electron is -1.6x10-19 C
The charge on a proton is +1.6x10-19 C
One coulomb is the flow of 6.25x1018
electrons
Electric current is the rate of flow of
electrons
The unit of current is the Ampere (Amp)
One Amp is the flow of one coulomb in one
second
Electricity - Resistance
•
Electrically, materials are classified as :
• Insulators
• Conductors
• Semiconductors
Electricity - Resistance
•
•
Insulators have high resistance
to current
Insulators are:
•
•
•
•
rubber,
glass,
wood,
ceramics
Electricity - Resistance
•
•
•
Conductors have a lower resistance.
Each atom allows 1, 2 or 3 electrons
to drift off and move through the
metal
Conductors are: most metals, eg
copper, silver.
Pure water does not conduct well,
but water with dissolved salts does
conduct
Electricity - Resistance
•
Semiconductors allow electrons to
flow only under certain conditions
eg silicon. For silicon the
conductivity is increased by the
addition of small amounts of
elements such as arsenic or boron
Electricity - Resistance
• Depends on:
• diameter of wire
• temperature of wire
• length of wire
• material wire is made of
• The unit of resistance is the Ohm
• The symbol for the Ohm is 
Electricity –
Electric Circuit
• Typical examples of
symbols used in
circuits are shown
(NSC1951 Instructional Guide)
• Typical circuit with a
battery and resistor
is shown
•
(Cree & Rischmiller, Fig 11.7)
Electricity - Resistance
Ohm’s law:
Voltage = current (Amps) x resistance (Ohms)
Symbol for voltage is V
Symbol current is I
Symbol for resistance is R
So Ohm’s law can be written as:
V=IxR
Electricity - Resistance
•
Given Ohm’s law (V = I x R), a voltage of
•
one volt drives a current of one ampere
through a resistance of one ohm
The energy lost through electricity
flowing through a high resistance is
dissipated as heat and light, eg:
• Heaters
• Light bulbs
• Electrocautery irons
Electricity - Power
•
•
•
•
The flow of electricity through a circuit has
the capacity to do work
The rate at which work is done is called the
power (P)
The unit of power is the Watt (W)
Power = voltage x current
or P = V x I
or P = I2 x R
•
•
Passage of current produces heat. The heat
produced is proportional to current squared
Another unit of power is the horsepower
and 1 horsepower = 746 Watts
Key Concepts
•
•
•
•
•
Electrical quantities and their units
Knowing and using Ohm’s law V=IR
Conductors versus insulators
Static discharge, electrical sparking
Electric power