Basic ohms law help please?

Question

okay i understand the relationship between volts amps and resistance. but im confused witha few things.

okay, a 60W globe, would have 240 volts in so is therefore using 0.25 amps. so its resistance would be volts/amps = 240/0.25= 960 ohms. 960 ohms for a globe??

now an oven would use like high amps and it would have a low resistance. so say 20 amp oven, 240 volts. therefore it would have 12 ohms of resistance. yeh?

volts always remains at 240, so a long run with smallish cable, just for an example may have 4 ohms. so does that mean it would be drawing 60 amps?? then a globe on the end with a high resistance, what happens to the current then?

and if amps and volts are directly proportional, but volts always remain constant, the only thing changing high or low wattage would be the amount of resistance?

im getting high distinctions in my class, but im just having trouble with the basic concepts for some reason.

please explain these things to me?

Answer 1

Ohms Law: Voltage = Amps * Resistance

Wattage = Amps * Volts

A 60W globe at 240V is odd, but you have calculated it correctly. In the real world, this is one of those things that is going to get hot and give you problems.

Your oven example has also been calculated correctly.

When you mention a "long run with a smallish cable" you have confused amperage with voltage drop. The smallish cable will have a greater resistance than a larger cable, but the current draw will be based upon the drop in voltage on the long run, not the wattage the appliance is using. If your voltage is going to drop more than 2% on the cable run, you have to increase the size of your cable.

Here's an example that might make it clear and leave you scratching your head at the same time. 120V circuit with a 20A breaker. You're working on a motor with the power on when you get shocked. The resistance of the circuit (with you in the middle of it) is 20 ohms (a realistic number for a ground resistance on the circuit). 120V / 20 ohms = 6A current going through your body. You're dead and the breaker never trips because it wasn't close to 20A. That's why GFCI breakers are so important.

Hope this helps.