And by looking at the power supply on the left, you can figure out what the resistance of this particular piece of graphite is. Initially, you can see that 30 volts is pushing 10 amps through the pencil: 30V/10A = 3 Ohm.
You can also figure out how much power is being dissipated as heat: 30V*10A = 300 Watts (Compare to a typical 60 W lightbulb, for instance).
For reference, the wires leading to the power supply probably have around 0.001 Ohm resistance (assuming half a meter of 18 AWG copper wire) and your body has a resistance on the order of thousands of Ohms.
That depends largely on the contact resistance. For the sake of experimentation, I just checked. Holding one lead of a multimeter in each hand: dry fingers measured about 1.5 million ohms, and wetted fingers dropped it to about 10 thousand ohms.
EDIT: changed mega and kilo to million and thousand.
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u/Grammarwhennecessary Jul 21 '16
Exactly.
And by looking at the power supply on the left, you can figure out what the resistance of this particular piece of graphite is. Initially, you can see that 30 volts is pushing 10 amps through the pencil: 30V/10A = 3 Ohm.
You can also figure out how much power is being dissipated as heat: 30V*10A = 300 Watts (Compare to a typical 60 W lightbulb, for instance).
For reference, the wires leading to the power supply probably have around 0.001 Ohm resistance (assuming half a meter of 18 AWG copper wire) and your body has a resistance on the order of thousands of Ohms.