Electricity and Ohms Law

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Electricity and Ohms Law

• Completing the Circuit

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What is Electricity?

• Common definitions
• The power used to run electric devices
• Anything having to with electric power
• Electromagnetism is the scientific study of
  electric charges
• All electrical phenomenon are caused by electric
  charges, usually tiny particles called electrons

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Electric Charge

• Electrons have a negative electric charge
• An object can have three types of electric charge
• Negative lots of extra electrons
• Neutral just the right amount of electrons
• Positive not enough electrons
• Alike charges repel, opposite charges attract

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Conductors and Insulators

• In a conductor, electrons can move freely through
  the material
• Examples
• Most metals (copper, silver and gold are some of
  the best conductors)
• Salt water
• In an insulator, electrons have difficulty moving
  through the material
• Examples
• Most plastics and ceramics
• Distilled water
• Air

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Electric Current

• The flow of electrons is called electric current
• The unit of electric current is the ampere or amp
  for short
• The symbol for amp is A
• An amp is the measure of how much charge flows by
  per second

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Potential Difference (Voltage)

• A potential difference occurs when there are more
  electric charges in one location than in another
• The unit of potential difference is a volt
• The symbol for volt is V
• Example excess electrons will be attracted to
  the positive plate because of the potential
  difference

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A More Relevant Example

• In a household electric outlet, the potential
  difference (voltage) between the two slots is 120
  volts
• Because of a power plant many miles away,
  electrons are under pressure to move from one
  slot to the next

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Electrical Circuits

• An electrical circuit is formed when electrons
  are given a path to move across a potential
  difference
• Generally, conducting (metal) wires are used to
  complete the circuit

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Electrical Resistance

• Electrical resistance is the measure of the
  resistance of part of a circuit to the flow of
  electricity (current)
• Electrical resistance is measured in ohms
• Ohms are often represented by the symbol omega
  (?)
• Example the filament of a 100 W lightbulb
  presents a resistance of about 140 ? to the flow
  of electricity

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The Water Pressure Model

• A helpful way of thinking about current, voltage
  and resistance is to compare electricity to the
  flow of water
• voltage (i.e. potential difference) is analogous
  to the water pressure
• current is analogous to the flow of water
• resistance is analogous to the diameter of the
  pipe (e.g. a small pipe allows much less water to
  flow than a large pipe)

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Ohms Law

• Voltage, current and resistance in an electric
  circuit can be related using Ohms Law
• Ohms law states that
• V I x R
• where
• V is the voltage (in volts)
• I is the current (in amps)
• R is the resistance (in ohms)

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Example 1

• If 4 amps of current are flowing through a
  toaster connected to a 120 V outlet, what is the
  resistance of the toaster?
• Answer Rearrange V I x R to get R V/I
• Thus, R (120 V)/(4 A) 30 ?

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Example 2

• If a lightbulb connected to an outlet has a
  resistance of 144 ?, what current is flowing
  through the bulb?
• Answer Rearrange V I x R to get I V/R
• Thus,
• I (120 V)/(144 ?)
• 0.83 A

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Home Wiring Parallel Circuits

• A single switch can be used to turn on multiple
  lights or appliances if they are connected in
  parallel
• The voltage is the same for each light or
  appliance, although the current increases with
  each new addition
• A fuse is designed to break if the current gets
  too large

Fuse
Voltage
Lights
Switch
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Home Wiring Series Circuits

• If a lights or appliances are connected in series
  they will not experience the same voltage
• Since most lights and appliances are designed to
  work at a specific voltage, they cannot be
  connected in series
• The resistance of lights or appliances connected
  in series is the sum of their resistances

Fuse
Voltage
Lights
Switch
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Example

• What is the total resistance of three 100 W light
  bulbs (each 144 ?) in series?
• Answer 144 144 432 ?
• What is the total current through three 100 W
  light bulbs?
• Answer I (120 V)/(432 ?) 0.27 A
• Note this is significantly less than the 0.83 A
  that will flow through a single bulb (i.e. the
  bulbs will be dimmer)