October 31, 1995

1. Flawed schematic of lithium

[Incorrect schematic of lithium atom]

Describe five fundamental flaws in this picture, intended to represent lithium (atomic number 3). [2.5 pts.]

2. Rolling out Play-Doh

What did we learn by conducting the Play-Doh experiment? [1.5 pts.]

3. Conductor or Insulator?

Using a simple model of electronic band structure, explain why a metal is a good electrical conductor and an insulator is not. [2 pts.]

4. Internal resistance in a battery

[Schematic of flashlight circuit]

Consider the circuit above as a model of a working flashlight. Note that the current is specified.

  1. What is the voltage across the lamp? Hint: it is NOT 1.5 V! [2 pts.]

  2. What is the internal resistance r of the battery? [2 pts.]

  3. Check answer! Does effective resistance yield correct current? [2 pts.]

  4. What percentage of power is lost by the internal resistance? [2 pts.]

5. Household wiring

Consider a 1000 W coffeemaker and a 1200 W hairdryer operating in a typical U.S. household where the wall voltage is 110 V.
  1. Sketch a circuit schematic of the appliances (shown as resistors), including the wall voltage AND a circuit breaker (fuse). [2 pts.]

  2. Can both appliances operate simultaneously if the circuit breaker is rated at 15 A? [2 pts.]

    Be quantitative -- numerical calculations are required!

  3. How about with a 20 A breaker? [1 pt.]

  4. Why is alternating current (ac) more widely used than direct current (dc) for power distribution? [1 pt.]