When we touch anything(with small amount of current), like the end of our CPU cabinet, with our hands, it does feel like a small current is flowing through.But the same doesn't occur when touch the cabinet with our foot.Why does this happen?
I assume two things here:
- You are mainly asking about why we feel the zap sometimes and don't at other times.
- You are talking about a metal computer case and not a computer cabinet, which is usually wooden.
As to why you sometimes feel or don't feel the zap, this is primarily because of how densely packed some of your receptors are in your skin and how sensitive they are at that area. For example, your back is significantly less sensitive than your hands (palm, fingertips, back of hand, etc.) and your heel is even more insensitive than your back. You have many biological reasons why this is so but I'm not going into them here.
From a purely physical point of view, the body really doesn't vary much (nothing more than an order of magnitude) in conductivity or resistivity regardless of area chosen. Your body's composition is nowhere significantly different from the rest of it. Bone and fat have the greatest resistivity, muscle and blood the least. Surprisingly, skin isn't a great conductor because of its nature of having multiple layers of dry epidermis, dermis and fatty hypodermis. Things just do not flow and exchange as much when it comes to skin. Note that the thickness of the dead-cell layer and the wetness of skin are the greatest factors here (which is why capital punishment via electrocution needs a sponge wet with salt solution and why doctors use conducting gels for electrodes).
Note the difference between resistivity and resistance here; resistivity is an intrinsic property of the material (or composite, in this case) while resistance is an extrinsic property depending on various changeable factors like length, area, temperature, etc. Someone's resistance can be significantly changed based on where the current flows in and out. For example, someone grabbing a wire and standing on the opposite foot on the ground forces the current to travel a much longer distance through much more stuff than if the same person was insulated from the ground and touched on his skin with two wires of a circuit close together.
Yes, different tissues, and therefore body parts and organs generally have varying impedances.
Also, if it's really a current that you're sensing, then the grounding of your PC is compromised, and could pose a fire hazard.
In any case, the situation you're describing is, I believe, solved primarily by accounting for the thickness of the skin at the foot versus hand. Perhaps you should give a little more details (where exactly are you touching the PC, a flat surface or a corner, and which part of the hand/foot, ...), and try touching with different parts of the foot to see if it makes a difference.
PCs emit vibrations and varying electromagnetic fields, either or both of which could be the physics underlying your sensation. While in the latter case (EM fields) this could actually mean that your nerve cells are triggering as a response to the field, and therefore a current is involved, it doesn't imply that you're actually connected to any of the electric potentials within the cabinet.