ELECTRIC CURRENT

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ELECTRIC CURRENT

• Chapter 13.2
• Voltage, Current and Resistance

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The flow of electricity depends upon

• Voltage
• Current
• Resistance

voltage
-
resistance
Current (electron flow)
http//cache.eb.com/eb/image?id100742rendTypeId
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Voltage

• 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)

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Does the size of the cell matter?
D cells
AA cells
1.5 volts
1.5 volts
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6 volts? 9 volts?
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120 Volts

• There must be a voltage difference between the
  two terminals for current to flow.

110-120 V
0 V
To the ground (the path of least resistance and
easiest flow!)
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Voltage Analogy (the push that causes charges to
move)
Balanced charges
0 V
0 V
1.5 V
0 V
9 V
120 V
0 V
ball (electron)
Angle of slide (voltage)
0 V
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Batteries
High voltage is at the positive terminal
1.5 V
Zero voltage is at the negative terminal
0 V
http//www.qrg.northwestern.edu/projects/vss/docs/
Power/2-how-do-batteries-work.html
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Batteries

• http//www.explainthatstuff.com/batteries.html

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Electric Current

• 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)

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Resistance

• Resistance restricts the flow of electric
  charges.
• The cause internal friction as moving charged
  particles collide.
• Unit for resistance is ohm

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Factors influencing theresistance of a wire

• length
• temperature
• cross-sectional area
• conductivity of the material

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Increasing Resistance

• 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

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Circuits

• Series circuit
• 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
• 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

• Parallel circuit

• http//iss.cet.edu/electricity/pages/a17.xml

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Match statements with the correct circuit

• Series circuit 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

1
2
3
4
5
6
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Series or Parallel?
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1
2
series
parallel
parallel
parallel
4
6
series
5
7
series
parallel
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Hewitt and electric circuits

• 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 ____________.

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Draw these circuits using schematic symbols in
your spiral notebook.

• 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.

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Whats wrong with this picture?
Overloaded!
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-el
ectrical-fire-safety.html
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• http//www.animatedsoftware.com/environm/nukequiz/
  nukequiz_one/nuke_parts/reactor_parts.swf
• http//www.eas.asu.edu/holbert/flash.html

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Fuses and Circuit Breakers

• For overloaded current flow, the path will be
  broken
• 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.

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Circuit Breaker and Fuse Box
http//www.smarthome.com/7103.html
http//jimspearsfusebox.tripod.com/help/oldfusebox
.jpg
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Exit Question Draw these two circuits using
schematic symbols.

• 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.

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Series or parallel?
http//www.school-for-champions.com/science/dc_cir
cuits.htm
http//www.inclusive.net/exemplars/exemplar_b_4_1.
shtml
http//www.fortunecity.com/greenfield/eagles/180/c
urrent.html
http//www.pz.harvard.edu/ucp/curriculum/circuits/
s5_lessonplan.htm
http//mrskingsbioweb.com/IntegratedScience.htm
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Circuits Basic Questions

• Series circuits
• 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
• 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?
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Ohms Law

• Resistance Voltage/Current
• R V / I
• 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.

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Using Ohms Law

• Use to calculate electrical resistance, amperage
  or voltage.
• Ohms 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.

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How batteries produce voltage

• 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!)

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Whats A Coulomb?

• 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)

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Electric Shock

• 5 milliamps

• 70 milliamps

0.001 amps is equal to 1 milliamp!
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Electric Shock

• For you to receive an electric shock
• 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.
• 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!
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How to Prevent Electric Shock

• 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)

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Ground Fault Current Interrupter
The GFCI senses changes in current and will trip
if the leakage current reaches 0.005 A (5
milliamps).
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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.