2. POWER AND ENERGY IN ELECTRIC CIRCUITS
The resting human body uses
around 100 W of power—similar
to an incandescent light bulb.
A desktop computer + monitor
uses 150-200 W.
A laptop uses only 50 W.
3. POWER AND ENERGY IN ELECTRIC CIRCUITS
Recalling that power is the rate at which energy changes,
P = ΔE/Δt,
we can express the electric power as follows:
P = ΔE/Δt = (ΔQ)V/Δt
Knowing that the electric current is given by
I = (ΔQ)/Δt
allows us to write an expression for the electric power in terms of
the current and voltage.
6. POWER AND ENERGY IN ELECTRIC CIRCUITS
Power is the rate at which
work is done or energy
transferred or transformed.
7. ELECTRIC POWER
Thus, the electric power used by a device is equal to the
current times the voltage. For example, a current of 1 amp
flowing across a potential difference of 1 V produces a
power of 1 W.
8. ELECTRIC POWER IN THE HOME
We all use electricity in our homes.
• Are our homes wired in series or in parallel?
• How do we make sure these circuits are safe?
• How are we billed for the electricity we use?
9. ELECTRIC COMPANIES SELL ENERGY
Your local power company charges
homeowners and companies in
kilowatt-hours (kWh).
• One kilowatt-hour is 1000 W of
power used continuously for
one hour of time.
• Kilowatt-hours are units
of energy, not power!
10. POWER DISSIPATION
This power represents the energy per unit time
delivered to, or consumed by, an electrical
component having a current I and a potential
difference V.
11. QUIZ 1
Use the definition of resistance R = V / I. together with the one we
just derived (P = VI) to derive the following formula:
P = I 2R
12. QUIZ 2
Use the definition of resistance R = V / I. together with the one we
just derived (P = VI) to derive the following formula:
P = V2/ R.
13. QUIZ 3
The graph shows the V-I
characteristics of a tungsten
filament lamp. What is its power consumption
at I = 0.5 mA and at I = 1.5 mA?
14. QUIZ 4
Two 40 Ω resistors are connected in parallel to a 60-V battery.
a) How much total current flows through the circuit?
a) What is the power output of this circuit?
15. QUIZ 4
Two 40 Ω resistors are connected in parallel to a 60-V battery.
a) How much total current flows through the circuit?
a) What is the power output of this circuit?
