19.3 Electrical Resistance and Ohm

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19.3 Electrical Resistance and Ohms Law

• Key Question
• How are voltage, current, and resistance related?

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19.3 Electrical resistance

• Resistance measures how difficult it is for
  current to flow.

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19.3 Electrical Resistance

• The total amount of electrical resistance in a
  circuit determines the amount of current that in
  the circuit for a given voltage.
• The more resistance the circuit has, the less
  current that flows.

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19.3 Measuring resistance

• Set the meter to measure resistance (W).
• Set the black and red leads on opposite ends of
  the objects.

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19.3 The ohm

• Resistance is measured in ohms (W).
• One ohm is the resistance when a voltage of 1
  volt is applied with a current of 1 amp.

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19.3 Ohm's law

• German physicist Georg Ohm experimented with
  circuits to find an exact mathematical
  relationship between voltage, current and
  resistance.
• Ohm's law can be used to predict any one of the
  three variable if given the other two.

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19.3 Calculate current

• A light bulb with a resistance of 2 ohms is
  connected in a circuit that has a single 1.5-volt
  battery.
• Calculate the current that flows in the circuit.
• Assume the wires have zero resistance.

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19.3 The resistance of electrical devices

• The resistance of electrical devices ranges from
  very small (0.001 O) to very large (10106 O).
• Each device is designed with a resistance that
  allows the right amount of current to flow when
  connected to the voltage the device was designed
  for.

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19.3 Changing resistance

• The resistance of many materials, including those
  in light bulbs, increases as temperature
  increases.
• A graph of current versus voltage for a light
  bulb shows a curve.
• A device with constant resistance would show a
  straight line on this graph.

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19.3 Electrical Conductivity

• The electrical conductivity describes a
  materials ability to pass electric current.

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19.3 Conductors and insulators

• A material such as copper is called a conductor
  because it can conduct, or carry, electric
  current.
• Materials that insulate against (or block) the
  flow of current are classified as electrical
  insulators.
• Some materials are neither conductors nor
  insulators.
• These materials are named semiconductors.

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19.3 Resistors

• Electrical components called resistors can be
  used to control current.
• Resistors have striped color codes to record
  their "values" (writing on them is difficult).

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19.3 Potentiometers

• Potentiometers are a type of "variable" resistor
  that can change from low to high.
• They are wired so that as you turn the knob, it
  changes the distance the current has to flow.

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Application Hybrid Gas/Electric Cars
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20.1 Series and Parallel Circuits

• In series circuits, current can only take one
  path.
• The amount of current is the same at all points
  in a series circuit.

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20.1 Adding resistances in series

• Each resistance in a series circuit adds to the
  total resistance of the circuit.

Rtotal R1 R2 R3...
Total resistance (ohms)
Individual resistances (W)
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20.1 Total resistance in a series circuit

• Light bulbs, resistors, motors, and heaters
  usually have much greater resistance than wires
  and batteries.

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20.1 Calculate current

• How much current flows in a circuit with a
  1.5-volt battery and three 1 ohm resistances
  (bulbs) in series?

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20.1 Voltage in a series circuit

• Each separate resistance creates a voltage drop
  as the current passes through.
• As current flows along a series circuit, each
  type of resistor transforms some of the
  electrical energy into another form of energy
• Ohms law is used to calculate the voltage drop
  across each resistor.

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20.1 Series and Parallel Circuits

• In parallel circuits the current can take more
  than one path.
• Because there are multiple branches, the current
  is not the same at all points in a parallel
  circuit.

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20.1 Series and Parallel Circuits

• Sometimes these paths are called branches.
• The current through a branch is also called the
  branch current.
• When analyzing a parallel circuit, remember that
  the current always has to go somewhere.
• The total current in the circuit is the sum of
  the currents in all the branches.
• At every branch point the current flowing out
  must equal the current flowing in.
• This rule is known as Kirchhoffs current law.

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20.1 Voltage and current in a parallel circuit

• In a parallel circuit the voltage is the same
  across each branch because each branch has a low
  resistance path back to the battery.
• The amount of current in each branch in a
  parallel circuit is not necessarily the same.
• The resistance in each branch determines the
  current in that branch.

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20.1 Advantages of parallel circuits

• Parallel circuits have two big advantages over
  series circuits
• 1. Each device in the circuit sees the full
  battery voltage.
• 2. Each device in the circuit may be turned off
  independently without stopping the current
  flowing to other devices in the circuit.

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20.1 Short circuit

• A short circuit is a parallel path in a circuit
  with zero or very low resistance.
• Short circuits can be made accidentally by
  connecting a wire between two other wires at
  different voltages.
• Short circuits are dangerous because they can
  draw huge amounts of current.

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20.1 Calculate current

• Two bulbs with different resistances are
  connected in parallel to batteries with a total
  voltage of 3 volts.
• Calculate the total current supplied by the
  battery.

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20.1 Resistance in parallel circuits

• Adding resistance in parallel provides another
  path for current, and more current flows.
• When more current flows for the same voltage, the
  total resistance of the circuit decreases.
• This happens because every new path in a parallel
  circuit allows more current to flow for the same
  voltage.

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20.1 Adding resistance in parallel circuits

• A circuit contains a 2 ohm resistor and a 4 ohm
  resistor in parallel.
• Calculate the total resistance of the circuit.