hi I did an experiment using a short piece of copper tube some carbon fibre bell wire a steel 6inch nail and a car battery
I put some carbon fibre cloth in a piece of 22mm dia copper tube and connected the copper tube to the positive side of a car battery using bell wire
I then connected another length of bell wire to a steel nail and to the negative side of the battery
I then slowly pushed the nail into the rolled up carbon fibre the nail began to melt like wax but the bell wire stayed cold unless I caught the nail against the copper then it disintegrated can anyone explain this please thanks.... kev

 

Simple resistance, current, voltage and power in a series circuit. The steel nail/carbon fiber element has a higher resistance than a short section of copper wire so at a given current most of the voltage from the battery would be across that element. The product of current and voltage is power so that would be hot if the current where high. When you shorted the nail to the copper it was eliminated from the circuit so all the current and power from the battery terminals was dissipated in the bell wire instead and pop goes the wire and/or nail.

http://www.tpub.com/neets/book1/chapter3/1-13.htm
http://www.allaboutcircuits.com/video-lectures/series-circuits-voltage-divider-equation/

 

Simple resistance, current, voltage and power in a series circuit. The steel nail/carbon fiber element has a higher resistance than a short section of copper wire so at a given current most of the voltage from the battery would be across that element. The product of current and voltage is power so that would be hot if the current where high. When you shorted the nail to the copper it was eliminated from the circuit so all the current and power from the battery terminals was dissipated in the bell wire instead and pop goes the wire and/or nail.

http://www.tpub.com/neets/book1/chapter3/1-13.htm
http://www.allaboutcircuits.com/video-lectures/series-circuits-voltage-divider-equation/

thanks for the reply but how come if an 8.5kw shower with a resistance heater coil requires thicker wire?

 

thanks for the reply but how come if an 8.5kw shower with a resistance heater coil requires thicker wire?

For the same reason, total resistance in circuit elements (#1 wire to #2 heater coil) and where you want the power to be dissipated as heat. The thicker wires (lower gauge number) have a lower resistance than the heater coil so less energy is lost in the wiring on the way to the heater resistance.