Electricity

1
Electricity

• 5.1.1 Define electric potential difference.
• 5.1.2 Determine the change in potential energy
  when a charge moves between two points at
  different potentials.
• 5.1.3 Define the electronvolt.
• 5.1.4 Solve problems involving electric potential
  difference.

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Electric Potential Energy

• Electric Field due to a charge, Q
• Electric Potential, V
• How is force related to energy?
• Work done on in moving a test charge from very
  far away to some position near Q. (Scalar Value)
• SI unit Volt. 1 Volt 1 JC-1
• Potential Difference, DV
• Electric Potential Energy, U
• What is work?

3
Electric Force, Potential, and Potential Energy
Practice

• The electric field between two parallel plates is
  100.0 NC-1. What acceleration would a charge of
  2.0mC and mass 10-3Kg experience if placed in
  this field?
• The potential at a point P is 12 V and a 3x10-8
  kg charge of 3C is placed there.
• What is the electric potential energy of the
  charge?
• What is the maximum velocity of the charge when
  released?

4
Electricity

• 5.1.5 Define electric current.
• 5.1.6 Define resistance.
• 5.1.7 Apply the equation for resistance in the
  form R(rL)/A where r is resistivity of the
  material of the resistor.
• 5.1.8 State Ohms Law.
• 5.1.9 Compare ohmic and non-ohmic behavior.
• 5.1.10 Derive and apply expressions for
  electrical power dissipation in resistors.
• 5.1.11 Solve problems involving potential
  difference, current, and resistance.

5
Electricity

• What is electric current?
• Conductors have free electrons
• Electric field must be present for current to
  flow through conductors
• Why does matter heat up when electric current
  flows through it?

6
Electricity

• Resistance
• R V/I
• RrL/A
• SI Unit Ohm (W)
• Ohms Law At constant temperature, the current
  through a conductor is proportional to the
  potential difference across it.
• How does a changing temperature affect
  resistance?

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Investigation Which resistor is more Ohmic?

• Procedure
• Build a simple circuit with one resistor and an
  ammeter.
• Vary the potential difference across the
  resistor. Measure Record the potential
  difference and current.
• Analysis, Conclusion, Evaluation
• How can you determine the ohmic nature?
• Answer the research question, with justification.
• Evaluate the procedure.

8
Electricity

• Power
• Standard Definition of power.
• Derivation of expression for power dissipated in
  a resistor.
• P IDV
• Derive alternate forms of power formula using
  Ohms Law.
• SI unit Watt

9
Electricity

• 5.2.1 Define electromotive force (emf).
• 5.2.2 Describe the concept of internal
  resistance.
• 5.2.3 Apply the equations for resistors in series
  and in parallel.
• 5.2.4 Draw Circuit diagrams
• 5.2.5 Describe the use of ideal ammeters and
  ideal voltmeters.
• 5.2.6 Describe a potential divider.
• 5.2.7 Explain the use of sensors in potential
  divider circuits.
• 5.2.8 Solve problems involving electric circuits.

10
Electricity

• Two resistors are joined as shown on the
  whiteboard. The top resistor receives a current
  of 3A What is the current in the other resistor?
  What is the current that enters as junction A?
• A resistor of resistance 12W has a current of 2.0
  A flowing through it. How much energy is
  generated in the resistor in one minute?
• A light bulb rated as 60 W at 220 V has a
  potential difference of 110 V across its ends.
  Find the power dissipated in this light bulb.

11
Electricity

• Sources of Potential Difference
• Battery, Generator, Thermocouple, photosurface
• EMF (Electromotive Force), e
• Work done by non-electrical forces to move
  charge, q, from one terminal to the other
• SI Unit Volts
• Batteries have Internal Resistance, r
• Potential Difference across batteryV e - Ir

12
Electricity

• Circuit Diagrams
• Series Circuits
• VT V1 V2 V3..
• RT R1 R2 R3.
• IT I1 I2 I3.
• Parallel Circuits
• VT V1 V2 V3..
• 1/RT 1/R1 1/R2 1/R3..
• IT I1 I2 I3..

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The resistors have values of 2W, 3W, and 4W. The
Battery provides 9V of Potential Difference.

• Circuit Diagram (Series)
• Circuit Diagram (Parallel)

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Practice

• Find the total resistance in the circuits drawn
  on the board.
• Find the current in each of the resistors in the
  circuit drawn on the board

15
Practice

• For the circuit below, find the current in the 2W
  resistor when the switch is open and when it is
  closed. (The other resistors are 4W each.)
  Whats the potential difference between the
  points A and B when the switch is open and when
  its closed. DV across the battery is 12V.

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Practice

• For light bulbs of constant resistance 60W are
  connected as shown. Find the power in each light
  bulb. If light bulb A burns out, find the power
  in each light bulb and the potential difference
  across bulb A. DV across the battery is 30V.

17
Electricity

• Potential Dividers
• Used to provide a different output voltage than
  the supplied voltage.
• Various sensors are used with Potential Dividers
• Sensors must have a variable resistance dependent
  on some measured quantity.
• Examples LDRs, thermistors (made of
  semiconductor materials), sound sensors.