Download 2.1 electricity and magnetism

Chapter 2; Lesson 2.1
T.O.C:
Charge Needs a Continuous
Path to Flow
Electric Charge Flows in a Loop
• *Remember* Electric current is electric
charge that flows from one place to
another
• Charge does NOT flow continuously
through material unless the material
forms a closed path, or loop.
• Circuit(w.w)- closed path thru which a
continuous charge can flow.
– Path provided by low-resistant material
(usually wire)
– Designed for specific jobs (like light a bulb)
Parts of a Circuit
• Typically have:
1. Voltage Source- Battery or power plant
2. Conductor- forms connection from voltage
source to electrical device & back (usually
wire)
3. Switch- designed to break the closed path
of charge. When switch is open, it produces
a gap so the charge cannot flow.
4. Electrical Device- any part of circuit that
changes electrical energy in another form
of energy.
– Resistor(w.w)- electrical device that slows flow
of charge in a circuit. This allows energy to be
converted into light or heat. Ex: light bulb is a
resistor. Converts energy into light.
Identify Circuit Parts
Open & Closed Circuits
• Current in circuit is similar to water
flowing thru hose. However, if you cut
hose in half the water keeps flowing out.
Not so if you cut a wire in half.
• Most cords on appliances have 2 wires.
One leads to device from source, the other
goes from device back to source to
complete the circuit.
• Switches work by opening/closing the
circuit.
• Most circuits have standard symbols used
to represent parts of a circuit.
Paperclip/nails/switch Lab!
Current Follows the Path of
Least Resistance
• Although current follows a closed path,
the path does not have to be made of
wire.
• All materials w/ low resistance are good
conductors. This includes you.
– This is why you should never use electronics
w/ bare wires!
– Water is a good conductor, especially when
mixed w/ salt from a person’s skin. This is
why you don’t use electronics near sinks/tubs
full of water!
Dangers in Electricity
Short Circuit
• Short circuit(w.w)- an unintended path connecting one part of a
circuit with another.
• The current follows a path a closed path, but not the right one.
1. Functioning circuit- charge flows thru one wire to device and
back thru other wire to outlet.
2. Short circuit- cord is damaged& 2 wires inside have formed a
connection. w/out resistance from lamp there more current in
the wire. This causes wires to overheat & start a fire.
*Create Illustration
Grounding Circuits
• Recall; when lightning strikes a rod, charge flows thru the least
resistant path. This is why it hits the lightning rod instead of a
house or person.
• The 3rd prong (the ground) on some plug-ins acts in a similar
fashion as the lightning rod.
• A circuit that connects stray current safely to the ground is a
grounded circuit.
• Charge usually flow thru 1 prong to device & back thru 2nd prong
to outlet. If there is a short circuit; charge might flow dangerously
to the outside of the shell of the appliance.
• If there is a ground wire; the current will flow along 3rd wire &
safely into ground, along buried rod or cold water pipe.
• Imagine: you are sitting at home in the
living room. The circuit for the room is
currently supplying charge to your t.v.
& several lights. You turn the A/C on
& suddenly the circuit wires are
providing more current than before.
The lights dim. What has happened?
Safety Devices Control Current
• Imagine: you are sitting at home in the
living room. The circuit for the room is
currently supplying charge to your t.v.
& several lights. You turn the A/C on
& suddenly the circuit wires are
providing more current than before.
The lights dim. What has happened?
• Too much current is dangerous! There
are safety devices built into circuits to
prevent dangerous situations from
occurring.
How Fuses Work
• Fuses automatically shut off current before
circuits overheat.
• Fuses open circuits when there is too much
current flowing thru.
• Found in cars, some appliances, and older
houses
• Consist of thin strip of metal inserted into
the circuit. The charge then also flows thru
the fuse. Too much charge melts the metal
strip causing the circuit to open.
• Fuses are measured in amps, because fuses
monitor electric current.
Build your own Fuse Lab!
Other Safety Devices
• Most modern homes use Circuit Breakers. (breaker
boxes)
• They do not have to be replaced every time they
open a circuit like fuses.
• How they work: when too much current is moving
thru the circuit; a piece of wire in the breaker begins
to heat up & expand which presses against a switch
which flips off & opens the circuit stopping the flow
of charge.
Ground Fault Control Interrupter
• (GFCI)- Sometimes a little current leaks out of an
outlet or an appliance. Often it is so small you don’t
notice it. However, if you have wet hands, even a
small amount of current is dangerous.
• GFCI- has tiny circuit inside which monitors
incoming & outgoing current. If it detects a change
in the current it will flip a switch to open the circuit
Circuit Breaker
2.1 Review Q’s 16 points
1. Describe the 4 parts of a circuit &
explain what each one does. (8 pts)
2. Explain why short circuits are
dangerous. (2 pts)
3. What is the purpose of a ground wire?
(2 pts)
4. How do switches control the flow of
electric current? (2 pts)
5. What do fuses & circuit breakers have
in common? (2 pts)