8. Power in electric circuits R V I

Example: Two resistors, R 1 = 5 , R 2 = 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 R 1 : Power in the resistor R 2 : The total power:

Example: Two resistors, R 1 = 5 , R 2 = 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 in the resistor R 1 : Power in the resistor R 2 : The total power: +

Example: V R1R1 R2R2 V R1R1 R2R2

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

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? 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. 1 kWh = 1000 J/s x s = J 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. Electric meter monitors power consumption.

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

Potential in Closed Circuit + -

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. - +