In physics, Ohm's law says that current through a wire, $I$, is directly proportional to voltage, $V$, and inversely proportional to resistance, $R$: \[I=\frac{V}{R}.\]It's also true that resistance is directly proportional to the length of the wire. We have a piece of wire. We pass $50$ volts through this wire and measure $200$ milliamps of current. If I cut the wire in half and pass $300$ volts through it, how many milliamps of current will I measure?
+98 This is not my explanation btw... credits to the owner.
"Since current is directly proportional to voltage, multiplying the voltage by a factor of 6 (going from 50V to 300V) will also multiply the current by a factor of 6. Since current is inversely proportional to resistance and resistance is directly proportional to length, halving the wire will halve the resistance and thus double the current. Therefore the new current will be 6*2=12 times the original current or 12*200=2400 milliamps." 
