1. 8. Power in electric circuitsV
Units:

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

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

4. Example (2 light bulbs labeled 100W and 50W):R1 R2 V
What power is delivered if these resistors are connected in series? V R1 R2

5. Example (2 light bulbs labeled 100W and 50W – analytical solution):

6. Example: Three wires, of the same diameter, are connected in turn between two points maintained at a constant potential difference. Their resistivities and length are: 1) ρ and L; 2) 2ρ and 2L; 3) 0.9ρ and L. Rank the wires according to the rate at which energy is transferred to thermal energy within them.
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?

7. 8a. Fuses and Circuit breakers
To prevent some damage in the electric circuit we use electric fuses.
It will blow up due to a large heat if the current flowing through it will be larger than a certain critical value (10 A, 20 A, 100 A, etc.).
Fuses are one-use items – if they blow, the fuse is destroyed and must be replaced.
Circuit breakers, which are now much more common, are switches that will open if the current is too high; they can then be reset.
Example: Consider an electric hair dryer and electric iron which have 1000 W and 1500 W power when running on 120 V
Total power:
Total current (when used simultaneously):
The fuse must to keep a current larger than 20.8 A

8. 9. EMF and terminal voltage
Definition:
Units:
An ideal emf device: r internal =0 R I
Disconnected battery: R=∞
An real emf device: r internal = 0 R I r a b
Terminal voltage:

9. Potential in Closed Circuit- +

10. Example: An electric bulb with resistance of 22 Ω is connected to the battery with emf of 12 V and internal resistance 2 Ω. Find current, terminal voltage, and potential difference across the bulb. + -