- #1

jslatane

- 11

- 0

My reason for this post is that I am giving a speech to a small class in a few days about resistors. What I wanted to do was to calculate the resistance of some real life objects that people are familiar with, just to sort of show them that resistance could be used to describe things they interact with all the time. So I figured I would start with a bare wire, the resistance of which is about as close to zero as physically possible, then the resistance of an LED, then a normal lightbulb, small appliance, large appliance, and gradually work my way up to say a house, a town, a large city, a country, etc, giving the resistances of each.

SO I started doing some calculations using numbers I found on various websites. Here they are:

E = P*T

P = I*V

R = P/(I^2)

I = P/V

R = P/(P^2/V^2)

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For bare wire:

R = ~0 ohms

For LED:

V = 3.2 volts

I = 20 mA = 0.02 amps = 2/100 amps

R = V/I = 3.2/0.02 ohms = 3.2*100/2 ohms = 320/2 ohms = 160 ohms

For standard incandescent lightbulb:

P = 100 watts

V = 120 volts

I = P/V = 100/120 amps = 1/1.2 amps = 0.833 amps

R = V/I = 120/0.833 ohms = 120*1.2 ohms = 144 ohms

For microwave:

P = 1030 watts

V = 120 volts

I = P/V = 1030/120 amps = 1.03/0.12 amps = 8.5833 amps

R = V/I = 120*0.12/1.03 ohms = 13.98 ohms

And that is as far as I got before I started wondering what was going on. For some reason I had the idea that as a device got larger, the resistance would increase. But this is not what I am seeing. Is that an unrealistic expectation? Are my calculations flawed? Are there better ways to show resistances of everyday objects? Please note that I am not asking for specific numbers or answers, thus I didn't post it in the homework section, but I can move it if need be. I would appreciate it if someone could just explain to me if my examples are valid and my methodology is realistic or flawed.

Thanks!