20.3 Electric Circuits
QuickTime™ and a
decompressor
are needed to see this picture.
http://www.legoeducation.us/sharedimages/content/Large/L_Circuit.gif
What we have learned so far…
Electric charges
Static electricity
Coulomb’s Law
Electric current
Insulators and
Conductors
Resistance and
Voltage
Ohm’s Law
7
QuickTime™ and a
decompressor
are needed to see this picture.
What we are going to learn…
How do we keep track of the flow of electrons?
How do series and parallel circuits differ?
How do you calculate electric power and electrical
energy?
What devices make electricity safe to use?
QuickTime™ and a
decompressor
are needed to see this picture.
4
Circuit Diagrams
Remember that, in order for electricity to power our TVs
and computers, electrons have to be moving through
them.
An electric circuit is a complete path through which
charge can flow.
Wires in a house form a
complex network of circuits,
and each wire must allow
QuickTime™ and a
electrons to flow through it
MPEG-4 Video decompressor
are needed to see this picture.
in order to be useful.
3
Circuit Diagrams
The idea of electric circuits is very similar to roads.
In order for them to be useful, they have to allow
cars to travel to different places.
Also, dead ends do not allow for a lot of traffic flow.
Disconnected wires are
like dead ends, they
don’t allow electrons to
QuickTime™ and a
flow past them.
decompressor
are needed to see this picture.
4
Circuit Diagrams
If you want to know possible routes to get to your friends
house, you would look at a road map (or use some
computerized road map that would tell you where to
go).
QuickTime™ and a
decompressor
needed to see this picture.
Electricians have toareknow
where the electrons can
flow.
In order to do that, they use
QuickTime™ and a
decompressor
are
needed
to see this picture.
something like a road map
- a circuit diagram.
3
Circuit Diagrams
A circuit diagram uses symbols to represent parts of a
circuit, including a source of electrical energy and
devices that are run by the electrical energy.
It essentially maps out where the electrons can flow and
QuickTime™ and a
decompressor
what they might encounter.
are needed to see this picture.
2
Circuit Diagrams
A circuit diagram includes many symbols:
Anything that uses electricity is seen as a resistor.
This symbol could represent a light bulb or a
computer.
QuickTime™ and a
decompressor
are needed to see this picture.
A battery is represented by the following symbol.
Note that current “flows” from the positive terminal to
the negative terminal.
+
3
Circuit Diagrams
Switches are important! (Like light switches)
A switch is like a draw bridge. When the bridge is up, no
cars can travel.
QuickTime™ and a
decompressor
are needed to see this picture.
2
QuickTime™ and a
decompressor
are needed to see this picture.
Circuit Diagrams
When the switch is up, no electrons can flow, so no
electricity.
When the switch is down, the circuit is complete and
then electrons can flow. This is then called a “closed
circuit.”
2
Types of Circuits
There are basically two types of circuits: series and
parallel.
Once again, each of them act like roads for electrons.
To understand them, we can think about cars on a road.
QuickTime™ and a
MPEG-4 Video decompressor
are needed to see this picture.
3
Series Circuits
In a series circuit, charge has only one path through
which it can flow.
So the electrons only have one road to drive on.
If anything happens to just one section of the path, all of
the electrons will feel the effects!
So, if one section of the path
is blocked, all the electrons
will be blocked from
QuickTime™ and a
traveling through the path.
decompressor
are needed to see this picture.
4
Series Circuits
And if electrons can’t travel, there is no electric current.
When a light bulb burns out, it essentially blocks the
path.
So, for two light bulbs “in series,” if one of them burns
out, then the other one will not work either.
3
Series Circuits
If you add more light bulbs to the series circuit, what
does that do to the electrons?
Adding a light bulb is like adding something difficult for
electrons to travel through.
2
Series Circuits
The more resistance you add, the more difficult of a time
the electrons have to travel through the whole path.
As a result of more resistance, the electrons have to go
slower (less current), and each bulb shines less
brightly.
2
Parallel Circuits
Imagine what would happen if circuits in your
home were wired in series…
If a light bulb burned out, the television would turn
off!
2
Parallel Circuits
To avoid this problem, circuits in the home are
mostly wired in parallel.
A parallel circuit is an electric circuit with two or
more paths through which charges can flow.
2
Parallel Circuits
This is like having two roads to drive on.
If one of the paths gets blocked, the electrons can still
drive on the other unblocked road.
So if one element stops functioning in a parallel circuit,
the rest of the
elements still can
operate.
QuickTime™ and a
decompressor
are needed to see this picture.
3
Parallel Circuits
Also, the more parallel paths you have, the less
resistance electrons feel.
More parallel paths mean there are more roads to for the
electrons to take, which means an easier time for the
electrons to travel.
2
Power and Energy Calculations
The purpose of electricity is to power other appliances.
Recall that power is the rate of doing work.
The rate at which electrical energy is converted to
another form of energy is electric power.
The SI unit for power is the “watt” (W).
1 W = 1 joule / second
We usually measure power in
kilowatts (kW) for
QuickTime™ and a
decompressor
convenience.
are needed to see this picture.
6
Power and Energy Calculations
Electric power can be calculated by multiplying voltage
by current.
Electric Power:
P (watts) = I (amps) X V (volts)
Every time you turn on a 75-watt light bulb, you use
electric power.
4
Calculating Power
An electric oven is connected to a 240-volt line,
and it uses 34 amps of current. What is the
power used by the oven?
Finding Electrical Energy
To find the electrical energy used by an appliance,
multiply power by time.
E=Pxt
Most electric power companies usually determine the
cost of your electric bill using kilowatt-hours as a unit
of energy.
QuickTime™ and a
MPEG-4 Video decompressor
are needed to see this picture.
3
Electrical Safety
Electricity can cause serious damage to your health, so
it is important to know how to avoid electric shock.
Inspectors check all new houses to make sure electrical
wiring is installed safely.
QuickTime™ and a
decompressor
are needed to see this picture.
2
Electrical Safety
All wires must be able to carry the maximum expected
current.
The amount of current depends on the number of
devices that are in the circuit. The more devices
turned on, the more
current you get.
2
Electrical Safety
If the current exceeds the circuit’s safety limit, the wire
may overheat and start a fire.
In order to prevent this, we use fuses and circuit
breakers.
A fuse is a device that is
inserted into a circuit. The
fuse has a wire that is
designed to melt if too
much current passes
QuickTime™ and a
decompressor
through it.
are needed to see this picture.
3
Electrical Safety
This is called “blowing a fuse,” and this event breaks the
circuit so that electricity doesn’t flow.
The fuse has to be replaced before the circuit can be
used again.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
2
Electrical Safety
Circuit breakers are like fuses, and in most houses,
they replace the need to have a fuse.
A circuit breaker is a switch that opens when current in a
circuit is too high.
The circuit breaker must be reset before the circuit can
be used again.
QuickTime™ and a
decompressor
are needed to see this picture.
3
Personal Safety
Touching any open wire is dangerous, so wires
are usually put inside the walls of the building.
Also, we use three-prong plugs to prevent
QuickTime™ and a
shocks.
decompressor
are needed
to see this
picture.
The third prong
connects
to ground,
so if a short
circuit develops, the charges flow through the
grounding wire instead of your body.
The transfer of excess charge through a
conductor to Earth is called grounding.
4
Short Circuits
In a short circuit, current finds a short path through the
circuit with less resistance than the full path.
This means that current will flow excessively, which will
result in overheating, fire or explosions.
If you are close to an object that is “shorting out,” there
is a chance for a very nasty shock.
QuickTime™ and a
decompressor
are needed to see this picture.
3
Effect of Current on the Body
1 mA = slight tingling sensation
5 mA = slight shock
5-30 mA = Painful shock; loss of muscular control
50-150 mA = Extreme pain; severe muscular
contractions. Breathing stops.
1-4.3 A = Nerve damage; heart stops, death is likely.
10 A = Severe burns; heart stops, death is probable.