GP #12 Physics 112-1 F12 Page 1 of 4
Group problem #12: Ohm’s Law
An 1200W microwave, a 1.1kW toaster, and a 150W coffee maker are plugged into the same 20A and
120V circuit. What is the current drawn by each device? What is the resistance of each device? And will
this combination pop the circuit breaker (Note: the breaker flips or “pops” when the circuit attempts to
draw more current then the system is designed to carry. This opens the circuit, disrupting the current
flow, and preventing an overload of the circuit and possible fire.)
GP #12 Physics 112-1 F12 Page 2 of 4
A 1500 W electric space heater is plugged into a power strip that has a 120V and 20A capacity. You are
getting ready for a big party and are using an electric hair dryer to style your hair. Plugging the hair
dryer into the power strip, you start the hair dryer at the 600W setting. The dryer has additional
settings of 900W, and 1200W. Since you are taking Physics and it’s taking a long time to dry your hair,
you decide to calculate which power setting on the hair dryer will overload the circuit and flip the switch
on the power strip. You don’t want to get stuck with wet hair for your party!
GP #12 Physics 112-1 F12 Page 3 of 4
You have been hired as a safety advocate for an electrical company during summer break. In rare
instances electrocution has been reported at volts as low as 30V under wet conditions. Your boss
knows you took physics and asks you to determine what resistance is necessary to cause the fatal
100mA at this voltage. He additionally asks you to calculate the resistance necessary to drive the fatal
100mA at the more common 120V. The human body has a natural resistance of approximately 100,00Ω
when dry and 1,000Ω when wet from sweat or water. Does your boss have reason to be concerned?
GP #12 Physics 112-1 F12 Page 4 of 4
The resistivity of copper is 1.68x10-8 Ωm and aluminum is 2.7x10-8 Ωm. For a typical household circuit,
what type of current is drawn through copper wiring versus aluminum? A typical household circuit has a
voltage of 120V and a 12 gauge wire - wire that draws 20A and has a diameter of 2.053mm. Why is
typical household wiring copper instead of aluminum? (There is more to the problem with household
wiring than this, but this is where it all starts!)