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HOWTOs

# 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.

1. 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.)
2. 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...)
3. How much energy is stored in a standard AA alkaline cell?

Here is how to solve this.

1. 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.
2. Look at the spec sheet and determine the nominal load or rate of discharge (current draw)
3. Multiple by number of hours the cell is rated to be able to supply current at that rate.
4. Divide by mass of cell.
1. What is the relationship between the energy units watt-hours and joules.
2. Find another conventional unit for energy, other than watt-hours and joules. Explain its area of usage, and relationship between WH and J.
3. 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?
4. What is the energy density of a conventional automotive 12v, lead acid battery?
5. Using a conventional internal combustion engine, what is the nominal energy density of gasoline?
6. 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.

1. 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.
2. Determine how much current before the LED “derates” (that is, heats up and burns up)
3. What is the wattage of the 12v halogen lamps in the "Escape from the Mall" exhibit in the lab? (Determine from voltage and current.)
4. Find the 4ohm load resistor, and run current through it. Calculate how much heat it is dissipating.
5. 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.