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, V

• 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 2.0C and mass 10-

3Kg 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=(L)/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=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 = IV• 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 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

– 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 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.