Electric Potential Difference

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Electric Potential Difference

• Physics A
• Current 1

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Work Potential Energy

• Work is done on an object when a force is exerted
  to move the object some distance.
• Work must be done on an object to move it in a
  direction that it would not naturally go.
• For example, you must do on a rock to lift it
  against the force of gravity.
• When you do work on the rock, energy is
  transferred from you to the rock.
• You lose energy. The rock gains the same
  amount of energy.
• The energy gained by the rock is potential (or
  stored) energy.

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Work Potential Energy

• Gravity can also do work on the rock when it
  falls to the Earth.
• Objects naturally move from high potential energy
  to low potential energy under the influence of
  the field force.

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Electric Field, Work Potential Energy

• To move a charge in an electric field against its
  natural direction of motion would require work.
• The exertion of work by an external force would
  in turn add potential energy to the object.

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Electric Potential

• Electric potential is the potential energy per
  charge.
• This means that electric potential DOES NOT
  depend on the test charge.

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Electric Potential in Circuits

• A battery powered electric circuit has locations
  of high and low potential.
• Charge moving through the wires of the circuit
  will encounter changes in electric potential as
  it traverses the circuit.
• Within the electrochemical cells of the battery,
  there is an electric field established between
  the two terminals, directed from the positive
  terminal towards the negative terminal.

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Electric Potential in Circuits

• The movement of a positive test charge through
  the cells from the negative terminal to the
  positive terminal would require work, thus
  increasing the potential energy of every Coulomb
  of charge that moves along this path.
• This corresponds to a movement of positive charge
  against the electric field.
• It is for this reason that the positive terminal
  is described as the high potential terminal.

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Electric Potential in Circuits

• Which way would a positive charge naturally move
  through a circuit?
• Negative terminal to positive terminal?
• Positive terminal to negative terminal?
• Conventional (traditional) currents are based on
  the flow of positive charges.
• Which way does conventional current flow?

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• In a certain sense, an electric circuit is
  nothing more than an energy conversion system.
• In the electrochemical cells of a battery-powered
  electric circuit, the chemical energy is used to
  do work on a positive test charge to move it from
  the low potential terminal to the high potential
  terminal.
• Chemical energy is transformed into electric
  potential energy within the internal circuit
  (i.e., the battery).

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• Once at the high potential terminal, a positive
  test charge will then move through the external
  circuit and do work upon the light bulb or the
  motor or the heater coils, transforming its
  electric potential energy into useful forms for
  which the circuit was designed.
• The positive test charge returns to the
  negative
• terminal at a low energy and low potential,
• ready to repeat the cycle all over again.

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• The quantity of electric potential is defined as
  the amount of ______.
• electric potential energy
• force acting upon a charge
• potential energy per charge
• force per charge

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• Complete the following statement
• When work is done on a positive test charge by an
  external force to move it from one location to
  another, potential energy _________ (increases,
  decreases) and electric potential _________
  (increases, decreases).

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Electric Potential Difference

• Consider the task of moving a positive
• test charge within a uniform electric
• field from location A to location B as
• shown in the diagram at the right.
• In moving the charge against the electric field
  from location A to location B, work will have to
  be done on the charge by an external force.

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• The work done on the charge changes its potential
  energy to a higher value and the amount of work
  that is done is equal to the change in the
  potential energy.
• As a result of this change in potential energy,
  there is also a difference in electric potential
  between locations A and B.
• This difference in electric potential is
  represented by the symbol ?V and is formally
  referred to as the electric potential difference.

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Electric Potential Difference

• By definition, the electric potential difference
• is the difference in electric potential (V)
• between the final and the initial location
• when work is done upon a charge to change
• its potential energy.
• In equation form, the electric potential
  difference is
• The standard metric unit on electric potential
  difference is the volt, abbreviated V.
• One Volt is equivalent to one Joule per Coulomb.
• Because electric potential difference is
  expressed in units of volts, it is sometimes
  referred to as the voltage.

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Voltage Simple Circuits

• Electric circuits are all about the movement of
  charge between varying locations and the
  corresponding loss and gain of energy that
  accompanies this movement.
• As the positive test charge moves through the
  external circuit from the positive terminal to
  the negative terminal, it decreases its electric
  potential energy and
• thus is at low potential by
• the time it returns to the
• negative terminal.

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Voltage Simple Circuits

• If a 12 volt battery is used in the circuit, then
  every coulomb of charge is gaining 12 joules of
  potential energy as it moves through the battery.
  
• And similarly, every coulomb of charge loses 12
  joules of electric potential energy as it passes
  through the external circuit.
• The loss of this electric potential
• energy in the external circuit
• results in a gain in light energy,
• thermal energy and other forms
• of non-electrical energy.

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Voltage Simple Circuits

• Electrochemical cells supply the energy to do
  work upon the charge to move it from the negative
  terminal to the positive terminal.
• By providing energy to the charge, the cell is
  capable of maintaining an electric potential
  difference across the two ends of the external
  circuit.
• Once the charge has reached the high potential
  terminal, it will naturally flow through the
  wires to the low potential terminal.

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Voltage Simple Circuits

• The movement of charge through an electric
  circuit is analogous to the movement of water at
  a water park or the movement of roller coaster
  cars at an amusement park.
• In your notes, explain one of these analogies or
  create one of your own.

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Voltage Simple Circuits

• The internal circuit is the part of the circuit
  where energy is being supplied to the charge.
• The movement of charge through the internal
  circuit requires energy since it is in a
  direction that is against the electric field.
• The external circuit is the part of the circuit
  where charge is moving outside the cells through
  the wires on its path from the high potential
  terminal to the low potential terminal.
• The movement of charge
• through the external circuit is
• natural since it is a movement
• in the direction of the
• electric field.

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Voltage Simple Circuits

• As a positive test charge moves through the
  external circuit, it encounters a variety of
  types of circuit elements. Each circuit element
  serves as an energy-transforming device. Light
  bulbs, motors, and heating elements (such as in
  toasters and hair dryers) are examples of
  energy-transforming devices. In each of these
  devices, the electrical potential energy of the
  charge is transformed into other useful (and
  non-useful) forms. For instance, in a light bulb,
  the electric potential energy of the charge is
  transformed into light energy (a useful form) and
  thermal energy (a non-useful form). The moving
  charge is doing work upon the light bulb to
  produce two different forms of energy. By doing
  so, the moving charge is losing its electric
  potential energy.

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Upon leaving the circuit element, the charge is
less energized. The location just prior to
entering the light bulb (or any circuit element)
is a high electric potential location and the
location just after leaving the light bulb (or
any circuit element) is a low electric potential
location. Referring to the diagram above,
locations A and B are high potential locations
and locations C and D are low potential
locations. The loss in electric potential while
passing through a circuit element is often
referred to as a voltage drop. By the time that
the positive test charge has returned to the
negative terminal, it is at 0 volts and is ready
to be re-energized and pumped back up to the high
voltage, positive terminal.
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• Moving an electron within an electric field would
  change the ____ the electron.
• mass of
• amount of charge on
• potential energy of  

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• If an electrical circuit were analogous to a
  water circuit at a water park, then the battery
  voltage would be comparable to _____.
• the rate at which water flows through the circuit
• the speed at which water flows through the
  circuit
• the distance that water flows through the circuit
• the water pressure between the top and bottom of
  the circuit
• the hindrance caused by obstacles in the path of
  the moving water

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• If the electrical circuit in your iPod were
  analogous to a water circuit at a water park,
  then the battery would be comparable to _____.
• the people that slide from the elevated positions
  to the ground
• the obstacles that stand in the path of the
  moving water
• the pump that moves water from the ground to the
  elevated positions
• the pipes through which water flows
• the distance that water flows through the circuit

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• Which of the following is true about the
  electrical circuit in your flashlight?
• Charge moves around the circuit very fast -
  nearly as fast as the speed of light.
• The battery supplies the charge (electrons) that
  moves through the wires.
• The battery supplies the charge (protons) that
  moves through the wires.
• The charge becomes used up as it passes through
  the light bulb.
• The battery supplies energy that raises charge
  from low to high voltage.
• ... nonsense! None of these are true.

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• If a battery provides a high voltage, it can __.
• do a lot of work over the course of its lifetime
• do a lot of work on each charge it encounters
• push a lot of charge through a circuit
• last a long time

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• Compared to point D, point A is _____ electric
  potential.
• 12 V higher in
• 12 V lower in
• exactly the same
• ... impossible to tell

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• The electric potential energy of a charge is zero
  at point _____.

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• Energy is required to force a positive test
  charge to move ___.

1. through the wire from point A to point B
2. through the light bulb from point B to point C
3. through the wire from point C to point D
4. through the battery from point D to point A

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• The energy required to move 2 C of charge
  between points D and A is ____ J.
• 0.167
• 2.0
• 6.0
• 12
• 24

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• The following circuit consists of a D-cell and a
  light bulb. Use gt, lt, and symbols to compare
  the electric potential at A to B and at C to D.
  Indicate whether the devices add energy to or
  remove energy from the charge.