Electric Currents

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Electric Currents
Electric PE and Potential Difference Electric
Current and Resistance Ohms Law, Ohmic vs.
Nonohmic Electric Circuits and EMF Potential
Divider Circuits and Sensors
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Series Circuits

• Current
• Voltage
• Resistance

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Parallel Circuits

• Current
• Voltage
• Resistance

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How can a series circuit be used as a voltage
divider?
R2 uses some of the voltage from the source,
reducing the voltage drop across R1
Useful when a resistor requires a different
voltage than the voltage source
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Example.voltage divider

• A 10? resistor is connected to a 12 volt battery.
  How much resistance must be added to the circuit
  to decrease the voltage drop in the first
  resistor to 4 volts?

A 20? resistor will use the remaining 8 volts.
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Potentiometers

• Variable potential divider
• Produces a variable power supply
• Symbol
• Need to consider the resistance of the voltmeter

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Example In the circuit at right, the voltmeter
has a resistance of 20 k?.

• Find the potential difference across the 20 k?
  resistor with the switch open.
• 2. Find the reading on the voltmeter with the
  switch closed.

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Switch open
Find current
Use current to find voltage
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Switch Closed
Find combined resistance
Find current
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• Use current to find voltage

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Sensors

• Light Dependent Resistor (LDR)
• Resistance depends on amount of light shining on
  the surface - more light means more resistance

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Sensors

• Thermistor
• Resistance depends on temperature
• Semiconducting device with a NTC (Negative
  Temperature Coefficient) - increase in
  temperature causes a decrease in resistance

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Sensors

• Strain Gauge
• Output voltage depends on small extension or
  compression resulting in a change in length

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Meters

• Voltmeter
• Measures potential difference
• Placed in parallel with component being measured
• A perfect voltmeter has infinite resistance so it
  has no effect on voltage drop in device being
  measured

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Meters

• Ammeter
• Measures current
• Placed in series at point where current is being
  measured
• A perfect ammeter has zero resistance so current
  isnt affected