Electricity and Ohms Law

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Electricity and Ohms Law
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Current

• Electric current- the flow of charge. Can occur
  within a material or through empty space.
• Occurs most easily in materials that are good
  conductors. Atoms hold on loosely to electrons.
  (good insulators hold on tightly to electrons)
• Generally current refers to the flow of electrons.

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Current continued

• Can also be defined as the net amount of charge
  that passes through an area (such as a
  cross-section of wire) per unit time.
• I?Q/?t
• Symbol for current I
• Unit for current Ampere (A)
• A1C/s

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For current to flow in a circuit, it is necessary
to have

• A complete circuit
• A potential difference impressed across the
  circuit
• Simple circuit diagram

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Conventional current vs. electron current

• The difference is direction
• Electron current the direction that electrons
  actually flow in a circuit
• Conventional current direction that positive
  charges would flow in a circuit. Commonly used
  (though outdated) method of discussing current.

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Microscopic view of current

• Electrons in an electrically neutral wire bounce
  around from atom to atom randomly
• When a voltage is impressed across a wire, the
  electrons are forced to move in a net direction.
• As they move, they bounce off of the atoms they
  move passed and drift slowly through the wire.
• Usually move less than 1 cm/s

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Resistance

• The property of a material that describes how
  much difficulty electrons feel or encounter as
  they are forced to move in a net direction
  through a material
• The resistance of a wire depends mainly on 4
  factors
• Type of material
• Cross-sectional area
• Length of the wire
• Temperature of the wire

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Equation for resistance

• R?(L/A)
• A-cross-sectional area
• ?-resistivity of the material
• L-length of the wire

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In Summary

• Batteries and other voltage sources produce a
  difference in electric pressure from one end of a
  circuit to the other
• This voltage causes electrons in a wire to drift
  in the same net direction
• Voltage is impressed across a wire, current flows
  through the wire
• Resistance does neither of the above and is only
  a property of a material caused by the tightness
  with which it holds its electrons.

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Continued

• Charge and current do not get used up or
  destroyed in a circuit. The energy carried on
  these charges get used up or dissipated.
• The same amount of current that leaves the
  battery on one end returns to the battery on the
  other end of the circuit.

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Ohms law

• A relationship among voltage, current , and
  resistance in basic circuitry.
• Works when discussing metal conductors, but
  inadequate for non-ohmic materials (ones that
  have variable resistance, diodes, transistors,
  etc.)
• Will be used to discuss simple, series, and
  parallel circuits.

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Compound Circuits

• Have to use the rules for series and parallel
  circuits depending on which part of the circuit
  you are studying.
• Must find the equivalent resistance of the
  parallel parts of the circuit before determining
  the total current of the circuit.

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Solve the current, v-drop across each resistor,
and Req for the circuit.
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Solve the current, v-drop across each resistor,
and Req for the circuit.
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Solve the current, v-drop across each resistor,
and Req for the circuit.
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Power in electrical circuits

• Power is the rate at which energy is converted
  from one form to another.
• In the case of mechanical systems, it was the
  rate that work was done and therefore the rate at
  which energy of motion was converted to either
  stored energy or heat.
• In electrical circuits, we generally speak of the
  power being equal to the rate at which electrical
  energy is turned to light heat.

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Power continued

• Since voltage is the energy stored on each charge
  (venergy/charge), to find the energy take
  voltage x charge. (energyV x charge).
• Power is energy/time. What we said above was that
  energy V x charge, so
• Power voltage x charge/time
• Recall that Current is the amount of charge that
  flows through a certain area in a time interval
  (charge/time)

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Drum roll please

• So, Power voltage x charge/time or
• Power voltage x current
• PIV
• Units of Watts
• See your circuit rules packets for some other
  useful rearrangements of the power formula.

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Homework

• Page 545
• Questions 1,2,3,4,5,7,8
• Problems 5-9,11,13,15-18
• Due Tuesday

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Motors vs. Generators

• Both use the relationship between electricity and
  magnetism
• Motors have electrical input and mechanical
  output
• Generators have mechanical input and electrical
  output
• What is it? game

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AC versus DC

• DC stands for direct current. Electrons flow
  through the entire circuit and back to the
  voltage source.
• Batteries produce DC
• AC stands for Alternating current
• In AC, the electrons vibrate back and forth as
  the polarity ( or positive and negative
  terminals) reverse direction.

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AC continued

• The common household outlet produces AC and the
  polarities change about 60 times each second and
  are said to have a frequency of 60 Hz.
• The voltage produced by the common household
  outlet is 120 V
• AC is modeled by a sine wave because its
  magnitude varies, growing from zero to a maximum
  positive value, then reversing so it passes zero
  once more to reach a maximum negative value

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AC voltage and current

• Both the voltage and the current follow this sine
  wave pattern and therefore reach a maximum peak
  value in the positive before returning to zero
  and then dropping to a maximum negative peak
  value.
• Because it does this complete wave 60 times each
  second, the effect of the current and voltage
  dropping to a negative value is not visible or
  noticeable to us, but can be viewed by using a
  device called an oscilloscope.
• http//www.magnet.fsu.edu/education/tutorials/java
  /ac/index.html

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AC advantages over DC

• It can be sustained at high voltages over long
  distances in contrast to DC, in which the
  electricity loses power within about a mile of
  release from the power station
• It can be stepped up or stepped down with the use
  of transformers (more about this later)
• It can be used to power devices that require a
  120V or 240 V drop.

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Household circuitry

• The electricity that enters your house runs to a
  breaker box or fuse box before being routed to
  the individual outlets and switches.
• Why do we use breakers or fuses? When do they
  blow?
• Lets think about how outlets are wired in a
  roomseries or parallel?
• What happens to the resistance of a circuit as
  more and more devices are added in parallel?

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Is the breaker box in series or parallel to the
rest of the devices on the circuit?

• What if it were in parallel?
• What about the ground?
• Most household circuits have a connection to a
  live and a neutral.
• The ground on a 3rd prong is to provide a path of
  lower resistance for the current to run through
  if a short occur and a part of the circuit
  becomes electrified that shouldnt.

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Circuits wired for 240 V

• The electricity that comes into your house has
  two terminal wires, one at 120V, one at -120V.
• For a 120V outlet, one connection is live and the
  other neutral. This produces a 120V difference
  between the 2 prongs.
• For some devices, such as air conditioners,
  electric dryers, stoves, a higher voltage is
  needed. Both prongs can be live, with one at
  120, one at -120 causing a total drop of 240V
  across the circuit.

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Diagram of a Breaker
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Electric shock

• Breakers and fuses are used to prevent
  overheating and fires, but do little to prevent
  electric shock.
• GFCIs are used to help prevent severe electric
  shock by shutting down at lower currents than an
  breaker.
• Most breakers start to open circuits between
  15-20A
• A shock of 0.001A can be felt, 0.005A hurts,
  0.010A causes muscle spasms, 0.015 causes loss of
  muscle control, and 0.070 A can be deadly,
  especially is sustained for more than 1 second.

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Ground fault circuit interrupters (GFCIs)

• Outlets or plugs with test reset buttons
• Can shut down with currents as low as 5 mA
  (0.005A) and do so in as little as 1 msec.
• Especially important in kitchens an bathrooms
  where water is present. Water lowers the
  resistance of the skin and increases the
  possibility of electric shock.

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How they work

• The major principal is electromagnetic induction
• Moving electric charges produce magnetic fields,
  and moving magnetic fields can produce current.
• GFCIs work to prevent shocks due to surfaces
  being electrified that shouldnt be, often due to
  a short circuit.

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See page 599 for diagram

• Normally, the magnetic field is cancelled because
  of the live and neutral wires running in opposite
  direction
• If ground fault occurs, the neutral wire has less
  current returning through it than leaves from the
  live.
• This coupled with the fact that AC is changing
  current produces a changing magnetic field
• This activates the solenoid circuit breaker and
  opens the circuit.

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Black box outlet plugs-transformers

• Used to step down 120 V current to a smaller
  voltage. Usually on an adapter for something that
  runs off of either batteries or outlet.
• Consists of 2 coils of wire around a laminated
  iron core.
• Primary coil has a voltage applied across it and
  causes a changing magnetic field, and will create
  an AC voltage of the same frequency in the second
  coil

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Step up and step down

• The actual output voltage depends on the number
  of coils of the output wire.
• Assuming little or no magnetic flux is lost, the
  output voltage can be determined with the
  equation from 595.
• Real transformers are usually better than 99
  efficient, so this gives accurate approximations
  of output voltage.
• To step up the voltage, the input should have
  less turns than the output.

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If your black box transformer adapter has an
input voltage of 120V and the input coil has 100
turns, how many turns must the output coil have
to produce an output voltage of 6 V?
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A very brief discussion on diodes

• Diodes are devices that only allow current to
  flow in one direction in a circuit.
• Can be used to change alternating current to
  direct current
• Light emitting diodes (LEDs) are a type of diode
  that emit photons when current flows through
  them.
• Usually emit a specific wavelength of light as
  opposed to a filament bulb which releases
  multiple wavelengths.

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Pg 609

• Question 2
• Problems 30,31