1. Electric Currents, Resistance & Electric Power
Chapter 25
Electric Currents, Resistance & Electric Power

2. Introduction: Our approach
Understand current and current density
current, formal current, electron current
average and instantaneous
macroscopic and microscopic
associated velocities
Understand what batteries do
Understand resistance
Calculate electric power
Describe alternating current

3. Current (macroscopically)
What is electric current? average and instantaneous?
What is the SI unit of current?
How is current related to the movement of specifically negative or positive charge?
How is current related to current density?

4. Current (microscopically)
How is current defined in terms of charge carriers moving with a drift velocity?
How is current density defined from this perspective?
How are current and current density defined when there are several kinds of charge carriers moving at different speeds?

5. Current (cont’d.)
What are the three velocities typically involved in electrical currents?
Checking out and developing our thinking about current, resistance, and voltage
Bulb brightness activity

6. Batteries What do batteries do?
How are batteries different from capacitors?

7. Resistance How is resistance defined?
What is the SI unit of resistance?
What is Ohm’s Law?
What are examples of devices that do (ohmic) and do not follow Ohm’s Law?
What is the symbol for resistance in a circuit diagram?

8. Resistance (cont’d.)
How is resistance related to the size dimensions and type of material?
How is the resistivity of a material generally related to the conductivity of the material?
How does the resistivity of a material generally depend on its temperature?

9. Electric Power
How is the power used by a device in a circuit related to the current through it and the voltage drop across it?
How can the power used be expressed without the current through it or without the voltage drop across it if the device obeys Ohm’s Law?
How can it be determined if a set of devices will overload a circuit?

10. Alternating Current
What is the typical time dependence of the voltages producing alternating current?
What is the typical time dependence of alternating current?
What is the time dependence of power used by an ohmic device in an alternating current circuit?
How are useable averages defined for alternating voltages and currents?

11. the end