# Why does constant current in electrophoresis mean that the molecule moves at the same rate? [closed]

Two part question:

Part 1:

I have always learned that when running a SDS-PAGE current is what decideds how fast the macro molecules migrate.

However, if the force excerted on a molecule is equal to charge times electric field.

Then wouldn't the speed at which the protein migrates depend more on voltage then current? As the the electric field is a measure of Voltage across an area.

Also, does this change based on the type of electrophoresis? (specifically, Agarose/DNA, and native page)

Part 2:

How do you calculate the time it would take for a molecule to reach the end of a gel based on the length of the gel and either the voltage or current?

• Related – acvill Feb 23 '20 at 4:22
• Voltage is proportional to current, so it depends on both equally. Constant current is safer though, since as the resistance drops, so does the power dumped into the circuit. – user1850479 Feb 23 '20 at 5:18
• Welcome to Biology.SE! Please take the tour and then go through the help pages starting with How to Ask questions effectively on this site and edit your question accordingly. In particular, each question should be posted separately — this improves the chances that you will get answers for each question and makes the answers more accessible for future users. ——— In addition, it isn't clear to me that this is really a biology question — perhaps Physics would be more appropriate? Thanks! 😊 – tyersome Feb 23 '20 at 18:16
• For SDS-Page with a discontinuous buffer system, resistance increases as electrophoresis proceeds (see here for refs). It now depends on whether you choose to run the gel at constant-current or constant-voltage. If CC, then electrophoresis proceeds at uniform rate but the voltage will increase, thus generating more heat (which can even crack the gel plates). If CV, then the current will decrease as electrophoresis proceeds and so will the rate of migration. – user1136 Feb 25 '20 at 16:05