Procedure

Opener

Introductory question: What is a fuse? How does it work? Why would you want to use one?
Answer: Fuses are simply pieces of metal that will break when a certain current is applied. They are used to protect appliances so the extra current breaks the fuse and not the appliance.

Development

Lecture: Ohm's Law and Power
Ohm's Law: V = I*R
Ask what they think the numbers in this formula mean. They will get V (voltage, Volts) and R (resistance, Ohms), but probably not I (current, Amps).
This formula can be rearranged to solve for I or R too.
Because our students haven't mastered algebra yet, we teach the triangle method for three symbol equations:
      V
    I    R
Simply cover up the quantity of interest. For example, I = V/R.

Go over the quantities:
What is voltage? How many volts are in your house electricity (120)? How about a battery (most are 1.5V)?
What is current? The flow of electrons.
What is resistance? The slowing down of the current. Just like friction, this causes heat. A good example is a lightbulb.


Power: P = I*V
Power is what hurts you when you get shocked. You can have really high voltage, but low current and be fine. If they've ever touched their tongs to a 9V battery, they know that the increased voltage leads to an increased power they can feel.
There is a triangle for this to of course.


Problems: We just had them do these in small groups
1) Using Ohms' Law, what resistance is needed if 3 Amps are drawn at normal house voltage.
110/3 = 37 Ohm
2)  How much power would you need to break a 3 Amp fuse at house voltage? How many 60W lightbulbs would you need?
3*110 = 330 W, 6 lightbulbs

Now we can test their second calculation with the big circuit, just adding lightbulbs one at a time to get up to 360W.
 

Closure

Reflection Question: Your kitchen has at least four circuits, protected by 4 15 Amp circuit breakers. Why do you need all those circuits?
Answer: Lots of power intensive devices (microwaves, dishwasher, fridge, toaster) that would trip fewer circuits.