1. Wednesday Oct. 5, 2011 1 PHYS 1444-004 Dr. Andrew Brandt PHYS 1444 – Section 004 Lecture #11 Wednesday October 5, 2011 Dr. Ryan Hall for Andrew Brandt Chapter 25 Electric Power

2. The Plan HW on part of Ch25 due Monday 10/10 at 5pm (note I slightly tweaked the HW, but added extra credit, so noone should be very inconvenieneced Review on Monday 10/10 Test CH 21-25 on 10/12 (up to but not including alternating circuits) Wednesday Oct. 5, 2011 2 PHYS 1444-004 Dr. Andrew Brandt

3. Wednesday Oct. 5, 2011 3 PHYS 1444-004 Dr. Andrew Brandt Electric Power How do we find out the power of an electric device? –What is definition of the power? The rate at which work is done or the energy is transferred What energy is transferred when an infinitesimal charge dq moves through a potential difference V? –dU=Vdq –If dt is the time required for an amount of charge dq to move through the potential difference V, the power P is – –Thus, we obtain. –What is the unit? –What kind of quantity is the electrical power? Scalar –P=IV can apply to any device, while the formulae involving resistance only applies to Ohmic resistors. What is this? Watts = J/s In terms of resistance

4. Wednesday Oct. 5, 2011 4 PHYS 1444-004 Dr. Andrew Brandt Example 25 – 7 Headlights: Calculate the resistance of a 40-W automobile headlight designed for a 12V battery. Since the power is 40W and the voltage is 12V, we use the formula with V and R. Solve for R

5. Wednesday Oct. 5, 2011 5 PHYS 1444-004 Dr. Andrew Brandt Power in Household Circuits Household devices usually have small resistance –But since they draw current, if they become large enough, wires can heat up (overload) and cause a fire Why is using thicker wires safer? –Thicker wires has less resistance, lower heat How do we prevent this? –Put in a switch that disconnects the circuit when overloaded Fuse or circuit breakers They open up the circuit when the current exceeds a certain value Overload

6. Wednesday Oct. 5, 2011 6 PHYS 1444-004 Dr. Andrew Brandt Example 25 – 10 Will a 30A fuse blow? Determine the total current drawn by all the devices in the circuit in the figure. The total current is the sum of current drawn by the individual devices. Solve for I BulbHeater StereoDryer Total current What is the total power?