##
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).
*[2.5 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

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!*
*[2 pts.]*

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

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

- 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.
- Sketch a circuit schematic of the appliances (shown as resistors),
including the wall voltage
**AND** a circuit breaker (fuse).
*[2 pts.]*

- Can both appliances operate simultaneously if the circuit breaker
is rated at 15 A?
*[2 pts.]*
*Be quantitative -- numerical calculations are required!*

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

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