SCEN102 MIDTERM EXAM
October 31, 1995
1. Flawed schematic of lithium
Describe five fundamental flaws in this picture, intended to
represent lithium (atomic number 3).
2. Rolling out Play-Doh
What did we learn by conducting the Play-Doh experiment?
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.
4. Internal resistance in a battery
Consider the circuit above as a model of a working flashlight.
Note that the current is specified.
- What is the voltage across the lamp?
Hint: it is NOT 1.5 V!
- What is the internal resistance r of the battery?
- Check answer! Does effective resistance yield correct current?
- What percentage of power is lost by the internal resistance?
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.
- Sketch a circuit schematic of the appliances (shown as resistors),
including the wall voltage AND a circuit breaker (fuse).
- Can both appliances operate simultaneously if the circuit breaker
is rated at 15 A?
Be quantitative -- numerical calculations are required!
- How about with a 20 A breaker?
- Why is alternating current (ac) more widely used than direct
current (dc) for power distribution?