Ohms Law

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Ohms Law

• Mitsuko J. Osugi
• Physics 409D
• Winter 2004
• UBC Physics Outreach

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Ohms Law

• Current through an ideal conductor is
  proportional to the applied voltage
• Conductor is also known as a resistor
• An ideal conductor is a material whose resistance
  does not change with temperature
• For an ohmic device,

V Voltage (Volts V) I Current (Amperes
A) R Resistance (Ohms O)
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Current and Voltage Defined

• Conventional Current (the current in electrical
  circuits)
• Flow of current from positive terminal to the
  negative terminal.
• - has units of Amperes (A) and is measured using
  ammeters.
• Voltage
• Energy required to move a charge from one point
  to another.
• - has units of Volts (V) and is measured using
  voltmeters.

Think of voltage as what pushes the electrons
along in the circuit, and current as a group of
electrons that are constantly trying to reach a
state of equilibrium.
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Ohmic Resistors

• Metals obey Ohms Law linearly so long as their
  temperature is held constant
• Their resistance values do not fluctuate with
  temperature
• i.e. the resistance for each resistor is a
  constant
• Most ohmic resistors will behave
  non-linearly outside of a given range of
  temperature, pressure, etc.

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Voltage and Current Relationship for Linear
Resistors
Voltage and current are linear when resistance is
held constant.
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Ohms Law continued
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Ohms Law continued

• The total resistance of a circuit is dependant on
  the number of resistors in the circuit and their
  configuration

Series Circuit Parallel Circuit
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Kirchhoffs Current Law

• Current into junction Current leaving junction

The amount of current that enters a junction is
equivalent to the amount of current that leaves
the junction
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Kirchhoffs Voltage Law
Sum of all voltage rises and voltage drops in a
circuit (a closed loop) equals zero

• Net Voltage for a circuit 0

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Series Circuit

• Current is constant
• Why?
• Only one path for the current to take

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Series Equivalent Circuit
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Parallel Circuit

• Voltage is constant
• Why?
• There are 3 closed loops in the circuit

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Parallel Equivalent Circuits
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• Weve now looked at how basic electrical circuits
  work with resistors that obey Ohms Law linearly.
  
• We understand quantitatively how these resistors
  work using the relationship VIR, but lets see
  qualitatively using light bulbs.

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The Light Bulb and its Components

• Has two metal contacts at the base which connect
  to the ends of an electrical circuit
• The metal contacts are attached to two stiff
  wires, which are attached to a thin metal
  filament.
• The filament is in the middle of the bulb, held
  up by a glass mount.
• The wires and the filament are housed in a glass
  bulb, which is filled with an inert gas, such as
  argon.

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Light bulbs and Power

• Power dissipated by a bulb relates to the
  brightness of the bulb.
• The higher the power, the brighter the bulb.
• Power is measured in Watts W
• For example, think of the bulbs you use at home.
  The 100W bulbs are brighter than the 50W bulbs.

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Bulbs in series experiment

• One bulb connected to the batteries. Add another
  bulb to the circuit in series.
• Q When the second bulb is added, will the bulbs
  become brighter, dimmer, or not change?
• We can use Ohms Law to approximate what will
  happen in the circuit in theory

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Bulbs in series experiment continued
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Bulbs in parallel experiment

• One bulb connected to the batteries. Add a
  second bulb to the circuit in parallel.
• Q What happens when the second bulb is added?
• ? We can use Ohms Law to approximate what will
  happen in the circuit

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Bulbs in parallel experiment continued
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Light bulbs are not linear

• The resistance of light bulbs increases with
  temperature

The filaments of light bulbs are made of
Tungsten, which is a very good conductor. It
heats up easily.
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As light bulbs warm up, their resistance
increases. If the current through them remains
constant
They glow slightly dimmer when first plugged
in. Why? The bulbs are cooler when first
plugged in so their resistance is lower. As they
heat up their resistance increases but I remains
constant ? P increases Most ohmic resistors will
behave non-linearly outside of a given range of
temperature, pressure, etc.
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Voltage versus Current for Constant Resistance
The light bulb does not have a linear
relationship. The resistance of the bulb
increases as the temperature of the bulb
increases.
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Memory Bulbs Experiment

• Touch each bulb in succession with the wire, each
  time completing the series circuit
• Q What is going to happen?
• Pay close attention to what happens to each of
  the bulbs as I close each circuit.

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Memory Bulbs Continued

• How did THAT happen??
• Temperature of bulbs increases
• ? resistance increases
• ? power dissipation (brightness) of bulbs
  increases

• Filaments stay hot after having been turned off
• In series, current through each resistor is
  constant
• smallest resistor (coolest bulb) has least power
  dissipation, therefore it is the dimmest bulb

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Conclusion

• Ohmic resistors obey Ohms Law linearly
• Resistance is affected by temperature. The
  resistance of a conductor increases as its
  temperature increases.
• Light bulbs do not obey Ohms Law linearly
• As their temperature increases, the power
  dissipated by the bulb increases
• i.e. They are brighter when they are hotter

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Youre turn to do some experiments!

• Now you get to try some experiments of your own,
  but first, a quick tutorial on the equipment you
  will be using

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The equipment youll be using

• - Voltmeter
• - Breadboard
• - Resistors
• - 9V battery
• Lets do a quick review

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How to use a voltmeter

• Voltmeter
• - connect either end of the meter to each side
  of the resistor
• If you are reading a negative value, you have the
  probes switched.
• There should be no continuity beeping. If you
  hear beeping, STOP what you are doing and ask
  someone for help!

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Voltmeter
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Measuring Voltage
Voltage Probes connect to either side of the
resistor
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Breadboards

• You encountered breadboards early in the year.
  Lets review them

The breadboard
How the holes on the top of the board are
connected
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Series
Resistors are connected such that the current can
only take one path
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Parallel
Resistors are connected such that the current can
take multiple paths
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Real data

• In reality, the data we get is not the same as
  what we get in theory.
• Why?
• Because when we calculate numbers in theory, we
  are dealing with an ideal system. In reality
  there are sources of error in every aspect, which
  make our numbers imperfect.

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Now go have fun!