1. Admin: Mid-term date Friday March 20 th. Review lecture Monday (16 th ) Assignments : Fourth assignment is posted A couple of self-crafted Mesh analysis questions – any problems? Office hours tomorrow 2pm – 3pm Due Monday 16 th Assignment 3 feedback… more lightbulbs 26.4 Two lightbulbs of resistance R_1 and R_2 (R_2 > R_1) and a battery are all connected in series. (a) Which bulb is brighter? (b) What if they are connected in parallel?

2. Copyright © 2009 Pearson Education, Inc. A (very) practical example of circuit analysis Example 26-10: Jump starting a car. A good car battery is being used to jump start a car with a weak battery. The good battery has an emf of 12.5 V and internal resistance 0.020 Ω. Suppose the weak battery has an emf of 10.1 V and internal resistance 0.10 Ω. Each copper jumper cable is 3.0 m long and 0.50 cm in diameter, and can be attached as shown. Assume the starter motor can be represented as a resistor R s = 0.15 Ω. Determine the current through the starter motor (a) if only the weak battery is connected to it, and (b) if the good battery is also connected.

3. Copyright © 2009 Pearson Education, Inc. See page 687 for full solution I 1 =76A I 2 =-5A (charging) I 3 =71A Note how large these currents are! Are car batteries deadly?

4. Copyright © 2009 Pearson Education, Inc. A person receiving a shock has become part of a complete circuit. 26-6 Electric Hazards

5. Copyright © 2009 Pearson Education, Inc. Most people can “feel” a current of 1 mA; a few mA of current begins to be painful. Currents above 10 mA may cause uncontrollable muscle contractions, making rescue difficult. Currents around 100 mA passing through the torso can cause death by ventricular fibrillation. Higher currents may not cause fibrillation, but can cause severe burns. Household voltage can be lethal if you are wet and in good contact with the ground. Be careful! 26-6 Electric Hazards

6. Copyright © 2009 Pearson Education, Inc. A car battery delivers 12V Household voltage is 120V NIOSH: "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms,” 26-6 Electric Hazards

7. Copyright © 2009 Pearson Education, Inc. A car battery delivers 12V Household voltage is 120V NIOSH: "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms,” 26-6 Electric Hazards Household (mains) voltage can be lethal if you are wet and in good contact with the ground. Be careful!

8. Thevenin and Norton Equivalent Circuits v TH = open circuit voltage at terminal (a.k.a. port) R TH = Resistance of the network as seen from port (V m ’s, I n ’s set to zero) Any network of sources and resistors will appear to the circuit connected to it as a voltage source and a series resistance

9. Thevenin and Norton Equivalent Circuits v TH = open circuit voltage at terminal (a.k.a. port) R TH = Resistance of the network as seen from port (V m ’s, I n ’s set to zero) Léon Charles Thévenin 1857-1926

10. Norton Equivalent Circuit Any network of sources and resistors will appear to the circuit connected to it as a current source and a parallel resistance Ed Norton – Bell Labs, 1898-1983