Hello sciforums, I have a question thats been bugging me since I've known any physics. According to physicists, the total energy of any closed system is constant. I tend to agree, being able to distinguish between various forms such as kinetic, heat, potential, electrostatic and electromagnetic energies. But, being a creative thinker, I can't help but try to beat this law every now and then. One of the first methods to do this came to me when I just learned about electromagnets and electric fields, etc. It is possible to establish an electric field with the switch of a button. This can be done using a parallel plate capacitor. One of the plates becomes negatively charged while the other is positively charged. Current (if I understand correctly) leaks from one side to the other, but very slowly, especially if an insulator is used (as is done to increase the capacitance of the capacitor). In my case there is no insulator, but rather a charged bead. This bead holds an electric charge. When the capacitor is turned on, an electric field forms, and the bead, being charged, responds to this electric field. It drifts towards the capacitor plate of the opposite charge. Now, suppose this happens, and then the capacitor is turned off. I can then move my bead with no resistance back up the electric field, switch my electric field back on, and watch it migrate once again towards the opposite charge. Clearly, I do not have to do work against the electric field in between the capacitors every time I shift the bead, because the capacitor is off. So, where does the energy for my migrating bead come from? I would think its from the circuit which powers the capacitor... here is how I see it- the bead always creates an electric fields since it is permanently charged. This electric field permeates the capacitor/circuit, creating some kind of resistance in the capacitor. This way, in order for the capacitor's electric field to form, an extra bit of energy is used, which is equivalent to the kinetic energy of the migrating bead. This to me seems like the only solution- the extra energy comes from the circuit's power source. There is no other option. I guess what my question is is, what is the precise mechanism of extra energy consumption by the circuit. Any input is appreciated Please Register or Log in to view the hidden image!
You're right. The only place the energy can come from is from the battery or other power source creating the field across the capacitor. As the electric field is established across the capacitor, the combination of the capacitor+bead+field system increases in electrical potential energy, at the expense of draining an equal amount of energy from the battery (or whatever power source is being used). This electrical potential energy is converted to kinetic energy of the bead as it gains speed in the field. Of course, in any real situation, some of the chemical energy of the battery will be converted to heat as well in the process of charging the capactor.
