Electricity Define electric potential difference Determine the change in potential energy when a charge moves between two points at different potentials Define the electronvolt 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,  V Electric Potential Energy, U –What is work?

Electric Force, Potential, and Potential Energy Practice 1.The electric field between two parallel plates is NC -1. What acceleration would a charge of 2.0  C and mass Kg 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 Define electric current Define resistance Apply the equation for resistance in the form R=(  L)/A where r is resistivity of the material of the resistor State Ohm’s Law Compare ohmic and non-ohmic behavior Derive and apply expressions for electrical power dissipation in resistors 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=  L/A –SI Unit: Ohm (  ) 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 = I  V Derive alternate forms of power formula using Ohm’s Law. –SI unit: Watt

Electricity Define electromotive force (emf) Describe the concept of internal resistance Apply the equations for resistors in series and in parallel Draw Circuit diagrams Describe the use of ideal ammeters and ideal voltmeters Describe a potential divider Explain the use of sensors in potential divider circuits 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 12  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),  –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 =  - Ir

Electricity Circuit Diagrams Series Circuits –V T = V 1 + V 2 + V 3 ….. –R T = R 1 + R 2 + R 3 …. –I T = I 1 =I 2 = I 3 ……. Parallel Circuits –V T = V 1 = V 2 = V 3 ….. –1/R T = 1/R 1 + 1/R 2 + 1/R 3 ….. –I T = I 1 + I 2 + I 3 …..

The resistors have values of 2 , 3 , and 4 . 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 2  resistor when the switch is open and when it is closed. (The other resistors are 4  each.) What’s the potential difference between the points A and B when the switch is open and when its closed.  V across the battery is 12V.

Practice For light bulbs of constant resistance 60  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.  V 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.