1. Ch 34 Electric Current Notes General Physics March 23, 2017
Electric Current and Power
General Physics
March 23, 2017

2. Focus Questions:
What is the definition of electrical current? Formula? Units?
What supplies the electrons in an electrical circuit?
What is DC? What is AC?
What is the “normal” outlet voltage in the US?
What does a diode do?
What is drift velocity?
What is the definition of electrical power? Formula? Units?

3. Current
A sustained flow of electric charge past a point is called an electric current.
Specifically, electric current is the rate that electric charge passes a point, so Current = or
Charge
time

4. Measuring Current
If 1 Coulomb of charge (6.25 x 1018 electrons) passes a point each second, the current is 1 Ampere.
So, 1 Ampere = 1 Coulomb/sec

5. Voltage Source
A battery or electrical outlet is a source of electric potential or voltage - not charge.
The electrons that move in a conductor are supplied by the conductor - not the voltage source.

6. Electrical Resistance
Most materials offer some resistance to the flow of electric charges through them. This is called electrical resistance. However, this concept will have to wait until after spring break to be explored further…

7. Direct Current
If the voltage is maintained between two points in a circuit, charge will flow in one direction. This is called direct current (DC)
Battery-powered circuits are DC circuits.

8. Alternating Current
If the high & low voltage terminals switch locations periodically, the current will flow “back and forth” in the circuit. This is called alternating current (AC).
Circuits powered by electrical outlets are AC circuits.

9. AC in the US
In the US, current changes direction 120 times per second, for a frequency of 60 cycles per second or 60 Hertz.
Normal outlet voltage in the US is volts, although some large household appliances run on volts.

10. Converting AC to DC
AC is converted to DC using devices called diodes, which allow charges to move in only 1 direction.
Light-emitting Diodes

11. Speed of Electrons
Electrons in a circuit do not move quickly - they actually “drift” at about 1 millimeter/second (this is called drift velocity).
It is the electric field that moves quickly - at about the speed of light - through the circuit and carries the energy.

12. Electric Power – rate of doing electrical work
Power = Electrical Work ÷ Time
= Electrical Energy ÷ Time
= Voltage × Charge ÷ Time
= Voltage × Current
P = VI
1 Watt = (1 Volt)(1 Amp)
1 kilowatt = 1000 Watts
A kilowatt-hour is a unit of energy

13. Electrical Current and Power Video