Transcript Electricity - Schoolwires

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.
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?
Electric Force, Potential, and
Potential Energy Practice
1. The electric field between two parallel
plates is 100.0 NC-1. What acceleration
would a charge of 2.0mC and mass 103Kg experience if placed in this field?
2. 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?
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 Ohm’s 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.
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?
Electricity
• Resistance
– R = V/I
– R=rL/A
– SI Unit: Ohm (W)
• Ohm’s Law: At constant temperature, the
current through a conductor is proportional
to the potential difference across it.
– How does a changing temperature affect
resistance?
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.
Electricity
• Power
– Standard Definition of power.
– Derivation of expression for power dissipated
in a resistor.
• P = IDV
• Derive alternate forms of power formula using
Ohm’s Law.
– SI unit: Watt
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.
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.
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 battery=V = e - Ir
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…..
The resistors have values of 2W,
3W, and 4W. The Battery provides
9V of Potential Difference.
• Circuit Diagram (Series)
• Circuit Diagram (Parallel)
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
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.) What’s the potential
difference between the points A and B when the switch is
open and when its closed. DV across the battery is 12V.
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.
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.