Transcript electric current - Fort Thomas Independent Schools

ELECTRIC CURRENT
Chapter 13.2
Voltage, Current and Resistance
The flow of electricity depends upon…
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+
Voltage
Current
Resistance
voltage
-
resistance
Current
(electron flow)
http://cache.eb.com/eb/image?id=100742&rendTypeId=4
Voltage
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Force that causes charges to move (the push
or the electric field)
Voltage exists if there is a potential difference
in charges (negatively charged vs. positively
charged)
SI unit is Volt (V)
Does the size of the cell matter?
D cells
1.5 volts
AA cells
1.5 volts
6 volts? 9 volts?
120 Volts

110-120 V
0V
To the ground
(the path of
least
resistance and
easiest flow!)
There must be a
voltage difference
between the two
terminals for current
to flow.
Voltage Analogy (the push that causes
charges to move)
0V
Balanced charges
1.5 V
0V
0V
9V
120 V
0V
ball
(electron)
Angle of slide
(voltage)
0V
Batteries
1.5 V
High voltage is at the
positive terminal
0V
Zero voltage is at the
negative terminal
http://www.qrg.northwestern.edu/projects/vss/docs/Pow
er/2-how-do-batteries-work.html
Batteries
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http://www.explainthatstuff.com/batteries.html
Electric Current
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The flow of charged particles.
Charges move from a position of higher to
lower electric PE (or higher voltage to lower
voltage).
SI unit is ampere (A or amps)
Resistance
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Resistance restricts the flow of electric
charges.
The cause: internal friction as moving
charged particles collide.
Unit for resistance is ohm
Factors influencing the
resistance of a wire
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length
temperature
cross-sectional area
conductivity of the material
Increasing Resistance
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Which is more resistant?
Long or short wire?
long
Thin or thick wire?
thin
Hot or cold wire?
hot
Less conductive or more conductive wire?
Less conductive
Circuits
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Series circuit
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only one path for electric current to flow
bulbs dim if more bulbs are added in series
other bulbs go out if one bulb burns out
Parallel circuit
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two or more paths for electric current to flow
bulbs maintain same brightness if more bulbs are
added in parallel
other bulbs stay lit if one bulb goes out
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Series circuit
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Parallel circuit
http://iss.cet.edu/electricity/pages/a17.xml
Match statements with the correct circuit
Series circuit
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1
2
1.
3
3.
4
5
4.
6
6.
2.
5.
vs.
Parallel circuit
other bulbs go out if one bulb burns out
bulbs maintain same brightness if more bulbs are
added
two or more paths for electric current to flow
bulbs dim if more bulbs are added
other bulbs stay lit if one bulb goes out
only one path for electric current to flow
Series or Parallel?
3
1
2
series
parallel
parallel
parallel
4
6
5
7
series
parallel
series
Hewitt and electric circuits
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Series circuits __________ resistance and
decrease in _________ and __________ as
more light bulbs are added.
Adding more light bulbs to parallel circuits
causes the resistance of the circuit to
__________, the voltage across each bulb to
___________ the ________, and the current
flowing through the entire circuit to
____________.
Draw these circuits using schematic
symbols in your spiral notebook.
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Series, 1.5 V dry cell, three bulbs, one closed
switch
Parallel, 1.5 V dry cell, three bulbs in parallel,
one open switch controlling only the second
bulb.
What’s wrong with this picture?
In a parallel circuit, each device will
receive the amount of electric
current needed to operate. This can
lead to wires overheating, which can
cause fires.
http://www.fire-extinguisher101.com/article0705-electrical-fire-safety.html
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http://www.animatedsoftware.com/environm/n
ukequiz/nukequiz_one/nuke_parts/reactor_pa
rts.swf
http://www.eas.asu.edu/~holbert/flash.html
Fuses and Circuit Breakers
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For overloaded current flow, the path will be broken
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fuse burns out
breaker springs open.
Each parallel circuit in your home has a fuse or
circuit breaker rated at a certain amount of amperes
(15A, 20A, 30A, and so on)
As more electrical devices are added to the parallel
circuit, the resistance of the circuit decreases. This
means that more electric current can flow.
Circuit Breaker and Fuse Box
http://www.smarthome.com/7103.html
http://jimspearsfusebox.tripod.com/help/oldfusebox.jpg
Exit Question: Draw these two circuits
using schematic symbols.
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Series, three 1.5 V dry cells, two bulbs, one
open switch
Parallel, one 6 V battery, three bulbs, one
closed switch controlling one bulb, another
open switch controlling all three at once.
Series or parallel?
http://www.school-for-champions.com/science/dc_circuits.htm
http://www.inclusive.net/exemplars/exemplar_b_4_1.shtml
http://www.fortunecity.com/greenfield/eagles/180/current.html
http://www.pz.harvard.edu/ucp/curriculum/circuits/s5_lessonplan.htm
http://mrskingsbioweb.com/IntegratedScience.htm
Circuits: Basic Questions
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Series circuits
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How many paths can an electron follow from one end of the
terminal to the other?
Do the bulbs get brighter, dimmer or stay the same if more
bulbs are added in series?
What happens to the other bulbs if one bulb goes out?
Parallel circuits
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How many paths can an electron follow from one end of the
terminal to the other?
Do the bulbs get brighter, dimmer or stay the same if more
bulbs are added in series parallel?
What happens to the other bulbs if one bulb goes out?
Ohm’s Law
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Resistance = Voltage/Current
R=V/I
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R is resistance, unit is ohm
V is voltage, unit is volt (V)
I is current, unit is ampere (A)
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Do practice problems 1-4 on page 443.
Using Ohm’s Law
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Use to calculate electrical resistance,
amperage or voltage.
Ohm’s Law predicts the degree of electric
shock one can experience
Do practice problem #1 on page 443.
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Adding load in parallel causes a decrease in
resistance (more current is available, voltage
stays the same)
Adding load causes an increase in resistance
and decreases voltage across each device.
How batteries produce voltage
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A chemical reaction facilitated by an electrolyte causes electrons
to move internally from one terminal of the battery (one type of
metal) to the other (a different type of metal). This difference in
charges produces voltage.
Positive end (cathode) loses electrons
Negative end (anode) gains electrons
The reaction starts only after a conducting wire connects both
terminals. Unconnected batteries still react, but very slowly. This
means they have a shelf life.
The speed of the reaction determines how many electrons will be
transferred.
Batteries wear out when the reaction slows or is impeded and
cannot keep up with the demand of the electric device (power!)
What’s A Coulomb?
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A coulomb is a measure of charge consisting
of a large group of electrons (6.24 x 1018
electrons)
One ampere contains one coulomb of charge
passing by each second.
1 volt is one joule of energy per coulomb
A 100 W bulb in a 120 V circuit needs
A 100 W bulb needs 100 joules of energy per
second. (power = work or energy/time)
Electric Shock
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5 milliamps
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0.001 amps is equal to 1 milliamp!
70
milliamps
Electric Shock
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For you to receive an electric shock…
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There must be a voltage difference between one part of
your body and another.
The current will pass along the path of least electrical
resistance.
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Average wet skin… 1000 ohms
Dry skin… 100,000 ohms
Range 100 to 500,000 ohms
Wet skin reduces your resistance to the flow of
current.
DANGER!
How to Prevent Electric Shock
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Increase the resistance to the flow of charged
particles (electric current).
Cut off the pathway for electric current.
Re-route the pathway for electric current
(lightning rods and grounding wires)
Ground Fault Current Interrupter
The GFCI senses
changes in current and
will trip if the leakage
current reaches
0.005 A (5 milliamps).
Lightning Rods
Lightning rods leak off charges to prevent build up of charges.
Lightning rod systems act as a conductor, directing the electrical
energy away from the house to the ground.