# ENGG 225 Lecture Notes - Lecture 4: T Power

9 views2 pages
16 Jan 2016
School
Department
Course
Page:
of 2
Resistors (continued)!
!
!
!
Property: voltage and current related by Ohm's Law !
""V = iR!
!
!
!
Suppose:!
!
!
!
Conductance !
!
!
!
!
!
Power and Energy!
Power is the product of voltage and current!
!
Which we may write as!
!
!
where: w = energy in Joules!
" q = change in Coulombs!
" t = time in secs!
" p = power in Watts (W)!
!
We deﬁne power in terms of the passive reference convention.!
!
!
!
!
!
!
If either current reference direction or voltage reference polarity is reversed... !
!
!
!
!
[current reference direction is in the direction of a voltage rise: from - to +]!
!
Example: !
Find the power in the circuit element at right!
(a) i = 10A, v = 12V!
!
(b) i = -10A, v = 60V!
!
Physical interpretation of the sign of p:!
"p > 0 (absorbs power)!
"p < 0 (delivers power)!
!
Consider the simple circuit:!
!
!
!
!
!
!
!
!
Power in a resistor!
!
!
!
!
!
!
!
Energy!
We have !
"Therefore, !
!
Power companies measure energy to determine monthly bill!
Cost determined by how much power used over time.!
!
Example 1:!
!
!
!
!
(a) Passive reference convention is observed, so p = vi!
!
!
(b) !