Current Electricity

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Current Electricity

• Current flows through a conductor.
• Requires a potential difference.
• Potential Difference is known as voltage.
• Current flows from High Voltage to Low Voltage

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Voltage and Current

• Voltage is to electric current as pressure is to
  water current.
• Water flows from high pressure to low pressure.
• Heat flows from hot to cold.
• Current flows from high voltage to low voltage.
• A greater voltage induces a
  greater current

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Household Outlets

• The potential difference in a common outlet is
  120V.
• This is the voltage or electrical pressure.

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Generation of Current

• To create water pressure we use a pump.
• To create electrical pressure we use a battery
  or generator.
• Must maintain a continuous flow of electrons.
• Generate current at source current moves toward
  sink.

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Direct vs. Alternating Current

• A battery creates Direct Current (DC).
• A continuous flow of electrons from a source to a
  sink
• Generators create Alternating Current (AC)
• A magnetic field causes electrons in a wire to
  vibrate back and forth
• No continuous flow
• Electric outlet in your house.
• Generator/power plant does not supply electrons,
  supplies energy to move electrons.

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Electric Circuits

• A closed loop or continuous path through which
  electrons can flow.
• Consists of a charge pump such as a battery or
  generator.
• In DC circuits, the pump supplies electrons.
• In AC circuits, the pump supplies the energy to
  cause the electrons to vibrate and generate power.

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Electric Current

• The rate of flow of electricity.
• Coulombs per second
• Amperes (amps, A)
• Current is measured with an ammeter,
• If a wire carries a current of 10A, 10 coulombs
  of charge pass by every second!
• That is 6.25 x 1019 electrons per second!
• 625,000,000,000,000,000,000 electrons per second!

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Resistance

• What can be done to a pipe to increase the
  resistance to flow of water?
• Decrease the diameter of the pipe
• Place objects in the path of water flow
• If resistance to the flow of water increases,
  what happens to the overall current in volume per
  second?

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Resistance

• The same is true for Electricity.
• The following factors affect resistance through a
  circuit.
• The kind of metal (conductivity)
• Length of wire. Longer wires have greater
  resistance.
• Thickness of the wire. A thicker wire has less
  resistance.
• Temperature of the wire. Higher temperatures
  cause an increase in resistance.

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Resistance

• The units on resistance are Ohms (?).
• Wires used to connect electrical devices have low
  resistances.
• Resistors are designed to have a specific
  resistance to reduce the amount of current going
  to a specific part of a circuit.

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Resistors

• A potentiometer is a variable resistor that gives
  a smooth continuous change in current.
• Dimmer on a light
• Speed adjustment on fan.
• Volume adjustor on TV

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Resistance and Your Body

• Dry Skin 500,000 ?
• Wet Skin 100 ?
• Rubber soles of shoes can resist 120V but wet
  feet on bathroom floor cannot.

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Effect of Various Electric Currents on the Body

• 0.001 A can be felt
• 0.005 A Painful
• 0.010 A Involuntary muscle contractions
• 0.015 A Loss of muscle control
• 0.070 A Serious, possibly fatal

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Ohms Law

• Georg Simon Ohm
• Current in a circuit is directly proportional to
  the voltage.
• Current in a circuit is inversely proportional to
  the resistance.
• V IR
• Electrical outlet provides 120V. Can use
  resistors to vary current to different devices.

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Ohms Law Examples

• A circuit draws 1amp of current and provides 4
  ohms of resistance. What is the voltage across
  the circuit?
• A 9 volt battery is connected to a circuit that
  has 3 ohms of resistance. What is the current
  through the circuit?
• How much resistance is contained in a circuit
  that draws .001 amps of current and has a
  potential difference of 15V?

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PowerFrom First Semester

• Work divided by time.
• Joules per second (Watts).
• Rate at which work is done.
• Rate at which energy is transformed.

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Electric Power

• Rate at which energy is converted by an electric
  circuit into another form of energy.
• P VI, P I2R, P V2/R
• Units Watts

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Power Examples

• How much power is generated by a light bulb that
  draws 1.0 amps of current from a 120V outlet?
• How much power is generated by a circuit that
  draws .003 amps of current but has 150 ohms of
  resistance?

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Electric Power

• To supply energy at a high rate to a large
  population. . .
• Can decrease resistance of the wire.
• This would require greater diameter and heavier
  wires.
• This would lead to a great increase in current
  and heat production.
• P I2R
• Can increase voltage to allow same power to be
  transmitted but at a lower current P
  IV.Electricity transmitted at up to 120,000V!

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Your Electric Bill

• You are charged for the energy you consume, not
  the power.
• Power is the rate at which energy is used.
• Power is measured in kilowatts (thousands of
  watts).
• Energy is measured in Kilowatt-hours (kWh)

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Power Calculations

• In the same way. . .
• Power is the rate of energy use, but we want to
  know how much energy was consumed.
• If given rate of energy use and time period of
  usage
• Energy use rate of energy consumption (kW) x
  time of use (hours).
• E Pt
• Energy use is measured in kilowatt-hours (kWh)

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Example

• How much would you be charged for using a 60 Watt
  light bulb for 10 hours if electricity costs .07
  per kWh?
• E PT
• E 0.06kW x 10h
• E 0.6kWh
• 0.6kWh x .07/kWh
• 0.04

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Incandescent Bulbs
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Compact Fluorescent Bulbs
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Diagramming Circuits

• When drawing circuits, we will use diagrams and
  symbols rather than realistic pictures. (see
  notes)
• Even though it is the electrons moving, we define
  current as flowing in the direction a positive
  charge would move (?-)

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Fuses and Circuit Breakers

• A byproduct of current flowing through wires is
  heat.
• Too much current, produces heat heat produces a
  fire.

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Fuses and Circuit Breakers

• Fuses melt, break the circuit and stop the flow
  of electricity.
• Fuses must be replaced.
• Circuit breakers are switches that open the
  circuit, stopping the flow of electricity.
• Do not have to be replaced.

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Types of Circuits

• Series Circuit
• Electrical current flows through each component
  one after another.
• There is ONE path through which current can flow.
• Parallel Circuit
• Circuit in which there are two or more paths
  through which current can flow.

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Drawing Circuits

• Battery
• Lamp
• Conductor
• Switch
• Fuse
• Capacitor

• Resistor
• Potentiometer
• Ground
• Voltmeter
• Ammeter
• Electric Connection