Transcript Electricity - drrossymathandscience

The Energy Challenge –
Electricity
Forms of Energy
• Energy comes in many forms from many
different sources. Some examples are:
– Electrical energy - movement of electrons
between atoms
– Thermal energy - movement of particles within a
substance
– Radiation energy - energy of electromagnetic
waves
– Chemical energy – energy stored in chemical
bonds
Forms of Energy
– Wind energy – movement of air
– Hydraulic energy – movement of water
Transformation of Energy
• Energy is constantly converted from one form
to another
• Wind, hydraulic, radiation (solar) and geothermal
energy can all be converted to electrical energy.
• Electrical energy from a circuit can be converted into
the radiation (light) energy of a light bulb or the
thermal energy of a furnace
Law of Conservation of Energy
• During these transformations, energy can be
neither created nor destroyed. The total
amount of energy always remains constant
Electrical Charge
• All matter contains charges:
• Positive charges from its protons
• Negative charges from its electrons
• A substance is said to be charged if its positive
and negative charges are not balanced
• A substance has a negative charge if it has gained
electrons and has more electrons than protons
• A substance has a positive charge if it has lost electrons
and has more protons than electrons
+++++++++
--------
+++++
--------
Electrical Charge
• Substances that have a like charge repel each
other
+++++++
------
+++++++
------
• Substances that have an opposite charge
attract each other
+++++++
------
+++++
---------
Electrical Charge
• When two electrically neutral (balanced + and
– charges) rub against each other, the friction
can cause electrons to move from one to the
other
• When a charged object touches an electrically
neutral object, it can transfer its charge
• In some cases, a charged object can even
transfer its charge just by being near a neutral
object
Static Electricity
• Electrostatic series
Tendency
Substance
Tend to gain electrons
Rubber
Ebonite
Polyethylene (vinyl)
Cotton
Silk
Wool
Glass
Acetate
Tend to give up electrons
Cat’s Fur
Static Electricity
When you rub these
together…
Then you bring these
together…
Ebonite & Rubber
Wool & Glass
Rubber & Glass
Silk & Vinyl
Acetate & Ebonite
Silk & Acetate
Cotton & Glass
Cat’s Fur & Silk
Cotton & Cat’s Fur
Ebonite & Silk
Cotton & Wool
Wool & Ebonite
Vinyl & Cotton
Rubber & Glass
Vinyl & Glass
This is the force you get
(attract or repel)
Electrical Functions
• In an electric circuit, different parts perform a
variety of functions
• The main functions are:
• Power supply – provides energy to make current flow
(ex. batteries, electrical outlets)
Electrical Functions
• In an electric circuit, different parts perform a
variety of functions
• The main functions are:
• Power supply – provides energy to make current flow
(ex. batteries, electrical outlets)
• Conduction – carry the electrons through the circuit
(wires)
• Insulation – cover wires to prevent shocks (plastic)
Electrical Functions
• Protection – devices that stop current flow in case of
malfunction (fuses or circuit breakers)
Electrical Functions
• Protection – devices that stop current flow in case of
malfunction (fuses or circuit breakers)
• Control – opens and closes to complete the circuit
and control the flow of current through the circuit
(switch)
Electrical Functions
• Protection – devices that stop current flow in case of
malfunction (fuses or circuit breakers)
• Control – opens and closes to complete the circuit
and control the flow of current through the circuit
(switch)
• Transformation of energy – converts electrical energy
to another form of energy (most devices and
appliances)
Electrical Circuits
• When electrical charges are placed in a circuit
made of conductive materials, electricity can
flow in a loop
• Because electrons all have the same charge
(-), they repel each other and push each other
along from one atom to the next
• This movement is called electric current
Types of Current
• Alternating Current
• Electrons move back and forth many times per second
• http://www.absorblearning.com/media/attachment.act
ion?quick=oy&att=1787
• Direct Current
• Electrons continuously move in the same direction
Flow of Electricity
• http://ippex.pppl.gov/interactive/electricity/
moving.html
• http://faraday.physics.utoronto.ca/PVB/Harris
on/Flash/LightSwitch/LightSwitch.html
Electrical Circuits
• There are two kinds of electrical circuits:
• Series Circuits
• Parallel Circuits
Series Circuits
• All parts of the circuit are connected one after
the other, so that the current can only follow
one path
Parallel Circuits
• There is at least one branch in the circuit, so
the current may follow different paths
Current Intensity (I)
• The number of electrons passing a point in a
circuit per second can be measured
• This is called current intensity (represented by
the letter I)
• Current intensity is measured in units called Amperes,
or amps, represented by the letter A
• It is measured with a device called an ammeter, which
must always be connected in series
Current Intensity
Voltage or Potential Difference (V)
• Voltage, represented by the symbol V, is a
measurement of the amount of energy that
can be supplied along a circuit
• Voltage is measured in units called Volts, which are also
represented by the letter V
• Voltage is measured using a voltmeter, which must
always be connected in parallel
Current and Voltage
• http://www.bbc.co.uk/schools/ks3bitesize/sci
ence/energy_electricity_forces/electric_curre
nt_voltage/activity.shtml
Connecting a Circuit
Drawing a Circuit Diagram
Component
Drawing
Wire
Power Supply/Battery
+ Closed Switch
Resistor
Open Switch
Ammeter
Voltmeter
Light Bulb
A
V
Draw the following:
• A circuit with a power supply, 3 light bulbs and
a switch connected in series
• A circuit with a power supply and 3 light bulbs
connected in parallel.
• Place a switch on it so all 3 lights will turn off if it is opened.
• Place a second switch on it that will only turn off one light if it is
opened.
• Place an ammeter (labelled A1) to measure current flowing
through the whole circuit
• Place ammeters (labelled A2, A3, A4) to measure current flowing
through each light bulb
Draw the following:
• A circuit with a power supply, 2 lights in series
and 1 light in parallel with the other two.
•
•
•
•
Place a switch so that one light goes off.
Place another switch so that two lights go off.
Place another switch so that all lights go off.
Place a voltmeter (V1) to measure the voltage drop at
the power supply
• Place voltmeters (V2, V3, V4) to measure voltage drop
over each light bulb
Resistance (R)
• In an electrical circuit, the force that slows
down the flow of electrons is called
RESISTANCE
• Resistance can be calculated using this
formula:
R= V
I
• Resistance is measured in Ohms (Ω)
Resistance (R)
• http://mste.illinois.edu/users/Murphy/Resista
nce/default.html
Conductance (G)
• Conductance is the opposite of resistance, so
it measures how well a substance or circuit
allows electricity to flow through
• Conductance is calculated using the formula
G= I
V
• Conductance is measured in Siemens (S)
Factors affecting electrical
conductivity
(short, fat, cold, copper)
• Four factors determine how well a conductor
will conduct electricity
– Length
• A shorter wire conducts better than a longer wire
– Diameter
• A thicker wire conducts better than a thinner wire
Factors affecting electrical
conductivity
(short, fat, cold, copper)
– Temperature
• A colder wire will conduct better than a warmer wire
– Material
• Certain materials are better conductors than others
• Copper is usually the best
My Formula Sheet
(these will always be given to me)
R= V
I
G=I
V
My Life-Saver
(I need to remember this)
When measuring:
The symbol is:
And you measure in:
Voltage
V
Volts (V)
Current Intensity
I
Amps (A)
Resistance
R
Ohms (Ω)
Conductance
G
Siemens (S)
Electrical Power (P)
• The amount of work (how much energy it can
transform in a period of time) is called
electrical power
• Power (P) is calculated using this formula:
P = IV
• Power is measured in Watts (W) or Kilowatts
(kW)
• W ÷ 1000 = kW
My Formula Sheet
(these will always be given to me)
R= V
I
P = IV
G=I
V
My Life-Saver
(I need to remember this)
When measuring:
The symbol is:
And you measure in:
Voltage
V
Volts (V)
Current Intensity
I
Amps (A)
Resistance
R
Ohms (Ω)
Conductance
G
Siemens (S)
Power
P
Watts (W)
Relationship between Power and
Energy
• Energy can be measured using two different
units:
• kilowatt-hours (kWh)
• Joules (J)
Calculating Energy in kWh
• Convert the watts into kilowatts
• Multiply by the total number of HOURS
• Multiply by the cost per kilowatt-hour if the total cost is
requested
Calculating Energy in Joules
• Use one of the two formulas below:
• E = VIt
• E = Pt
• In both, t stands for time (must be measured
in seconds)
My Formula Sheet
(these will always be given to me)
R= V
I
G=I
V
P = IV
E = VIt
E = Pt
My Life-Saver
(I need to remember this)
When measuring:
The symbol is:
And you measure in:
Voltage
V
Volts (V)
Current Intensity
I
Amps (A)
Resistance
R
Ohms (Ω)
Conductance
G
Siemens (S)
Power
P
Watts (W)
Energy
E
Joules (J)
Or
Kilowatt-hours (kWh)
Time
t
seconds (s)
Or
hours (h)
Energy Efficiency
• During any transformation of energy, only a certain
amount of the total energy is actually used. The rest
is changed into another form or “lost” outside of the
system.
• Ex. A light bulb only uses 5% of the electrical energy it
consumes to produce light. The rest is lost as heat
energy.
• Energy efficiency (%) is calculated using the following
formula:
Quantity of energy used (usable or useful) x 100
Quantity of energy consumed
Magnetism
• Discovered in the ancient Greek region of
Magnesia, a substance named magnetite was
the first known magnet
• Magnets have certain properties:
• They can attract or repel other magnets
• They can attract ferromagnetic substances
Magnets
• Magnets behave as they do because they have
permanently aligned “magnetic domains”
Ferromagnetic Substances
• Ferromagnetic substances have certain
properties:
• They are attracted to magnets
• They can become temporarily magnetized, because
they also have magnetic domains
• Three substances are considered ferromagnetic: iron,
nickel and cobalt
Non-Magnetic Substances
• Non-magnetic substances are not attracted to
magnets because they have no magnetic
domains
Magnetic Fields
• Magnetic fields always exist around magnets
• These fields have a shape and a direction that
can be represented by lines of force
• By convention, these lines of force always go from
north to south poles
Forces of Attraction and Repulsion
• All magnets have two poles: a north pole and
a south pole
• When magnets are brought near one another,
they affect one another’s magnetic fields:
• Like (same) poles will repel each other
• Opposite poles will attract
Forces of Attraction and Repulsion
Magnetic Fields
Magnetic Fields
Magnetic Fields
Electromagnetism
• The movement of electricity also generates
magnetic fields
• The shape and direction of the field depends
upon the direction of the flow of current
• In a straight wire, we use the right hand rule to
determine both shape and direction
• Another kind of electromagnet is a SOLENOID in which
conductive wire is wrapped around a core
Right Hand Rule
• Point your thumb in the direction of the flow
of conventional current through the wire
(+ to -)
• Your fingers holding the wire show you the
shape and direction of the magnetic field
Electromagnets
• Electromagnets have a major advantage that
they can be turned on and off
• They also have many practical uses
•
•
•
•
MRI machines in medicine
Burglar alarms in homes
Metal detectors in airports
Microphones, speakers, headphones, doorbells…