1. 7. Power in electric circuitsV

2. Example: Two resistors, R1 = 5 , R2 = 10 , are connected in series.
The battery has voltage of V = 12 V.
a) Find the electric power delivered by the battery
b) Find the electric power dissipated in each resistor +
Power in the resistor R1:
Power in the resistor R2:
The total power:

3. Example: Two resistors, R1 = 5 , R2 = 10 , are connected in parallel.
The battery has voltage of V = 12 V.
a) Find the electric power delivered by the battery
b) Find the electric power dissipated in each resistor +
Power delivered by battery:
Power in the resistor R1:
Power in the resistor R2:
The total power:

4. 7a. Kilowatt-hours (kWh)
What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time (E = Pt).
We have been measuring energy in joules, but the electric company measures it in kilowatt-hours (kWh).
1 kWh = 1000 J/s x s = J
Example: How much energy does a typical appliance use?
Let’s look at 1000 W hair dryer. We use it for 10 minutes, electricity costs ~10 cents per kWh. How much did running the hair dryer cost?

5. 8. Alternating current DC current AC current I0 - current amplitude I0
t - time
-I0 T V0
- voltage amplitude V0 t t
-V0 T
T – period
f - frequency

6. 8a. Alternating current and RMS
It is common to describe AC current and AC voltage with, so called,
root mean square (RMS) values:
Example:
To calculate power we use RMS values of current and voltage:

7. 8b. RMS – what it is?
(Optional)