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!Topics

* voltage
* current
* resistance
* power (rate of consumption) – e.g., Watts, horsepower
* energy (stored; cumulative) – kwH, mAH joules
* energy density (energy stored per unit mass)
* internal resistance of battery (model)
* efficiency (power conversion)
* cost


What are metaphors for each of these things?

What are units?

What are formulas/relationships between them?

!Homework Questions

For each of these, give assumptions (foundational facts), sources of these assumptions, and reasoning from there.

# How much current does it take to operate a 60W light bulb, assuming 120v supply?  (ignore the fact that line voltage is AC — pretend it's DC.)
# Given the above, what is the equivalent resistance of the light bulb in ohms? (note: light bulbs are actually variable resistors, depending on how much current is running through them – so they're not resistors at all...)
# How much energy is stored in a standard AA alkaline cell?
Here is how to solve this. 
## Start out by noticing that the AA voltage begins at 1.5v and drops more or less linearly as the battery is used.  Normally, a product should be designed to work properly until the voltage falls below 0.9v.
## Look at the spec sheet and determine the nominal load or rate of discharge (current draw)
## Multiple by number of hours the cell is rated to be able to supply current at that rate.
## Divide by mass of cell.
# What is the relationship between the energy units watt-hours and joules.
# Find another conventional unit for energy, other than watt-hours and joules.  Explain its area of usage, and relationship between WH and J.
# If there were a 100% efficient conversion method, how much power is deliveredby the sun, onto a 1 sq meter area of land, in the Northeastern US in the summer?
# What is the energy density of a conventional automotive 12v, lead acid battery?
# Using a conventional internal combustion engine, what is the nominal energy density of gasoline?
# How much current will flow if you short out the two terminals of a standard car battery using a perfectly conductive wrench?


!Lab Experiments

These may be done in any order.  Please take notes while you're doing stuff, so you can explain what happened afterward.

# Explain how to choose the proper current-limiting resistor to operate an LED from a 5v supply.  Assume the LED diode voltage is 2v, and that the LED wishes to receive 20 mA of current.  Build the circuit and demonstrate.
# Determine how much current before the LED “derates” (that is, heats up and burns up)
# What is the wattage of the 12v halogen lamps in the "Escape from the Mall" exhibit in the lab?  (Determine from voltage and current.)
# Find the 4ohm load resistor, and run current through it.  Calculate how much heat it is dissipating.
# Destroy a 1/8 or 1/4W resistor by running too much current through it.  Be careful.  Explain how much power it was dissipating when it died.

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Page last modified on January 24, 2007, at 09:57 PM