16 Jan 2016

School

Department

Course

Professor

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) !