Electricity ~ Power to the People February 25, 2008

Just to review, Ohms law allows us to calculate expected current in a circuit:

Suppose we know: 

V = 12V

R = 100Ω 

I = V/R 

  = 12V / 100Ω

  = 0.12A 

  = 120mA 

Power

Power is the amount of energy dissipated by a component over a given time. Another way of saying this is that power is the rate of energy dissipation.

Units and Variable Name:

Name	Formula Symbol	Unit of Measurement
Voltage	V	V (volts)
Current	I	A (amps)
Resistance	R	Ω (ohms)
Power	P	W(watts)

Power is the product of voltage and current.

Power = Current × Voltage   so   P = I × V

The same holds true if we substitute I with V/R in the right hand triangle.

We get P = (V/R)² x R which equals P = V²/R² x R = V²R/R² = V²/R.

Summarizing, the three formulas for power are:

P = I x V or P = I² x R or P = V²/R

Example:

Take a look at the same circuit we looked at in the previous example. The applied voltage is 12 Volts and the total resistance is 100Ω. 

We know the voltage and we know the resistance. What we want to calculate is the power dissipated in the circuit. The Power formula to use is: P = V²/R.

V = 12V

R = 100Ω 

P = V²/R

  = (12V)² / 100Ω

  = 1.44W

So, the power dissipated by the circuit will be 1.44 Watts.

Ohms Law II February 25, 2008

Ohmas Law class notes.

The above notes will help you complete this assignment, which is due Wednesday Feb 27 at the beginning of class.

You may have trouble with the first couple questions which deal with Power, which we will discuss tomorrow.