1.4 The life of a molecule in a charged environment
1.4 The life of a molecule in a charged environment
What is voltage?
Potential energy is a quantitative measurement of a system’s potential to do work. For example, if you hold a spring in a stretched position and keep it that way, one could say that the spring has high potential energy. It has the potential to violently compress back into its equilibrium length the second you let it go. When that occurs, one could say that the spring has realized its potential. Similarly, if you hold a ball high up in the air, it has high potential energy. It has the potential to fall and release its potential onto the world in the form of kinetic energy. The potential energy is realized when it is let go of.
Voltage is considered a form of potential energy. It refers to the ability of a system to create electrochemical work, even though it is currently not doing any at that moment. As you have likely already encountered, like charges repel while opposite charges attract. When a negative charge accelerates towards a positive one, it is doing electrochemical work. However, if one was to place a non-permeable barrier between a negative particle and a positive one, then they would be unable to move towards each other and produce this work. They would still have the potential to accelerate towards each other, but this potential would not be realized unless barrier was lifted, or otherwise made permeable to ionically charged particles.
Electrochemical potential is termed voltage. Neurons always have a voltage across a membrane, as the inside of a neuron is more negatively charged than the outside. These negative charges would like to accelerate outside of the neuron, but they cannot because the lipid membrane is not permeable to such charges (unless it contains ion channels which allow them to pass). Theoretically, if there was no membrane, then these ions would exchange until they reach their equilibrium potential.
Voltage is measured in Volts; which is a fancy way of saying charge per meter. It denotes how much charge difference there is across a non-permeable membrane, and how far away these charges are from each other. The reference frame for the unit of Volts is completely arbitrary; one can decide to set the zero point anywhere they choose. Classically, the outside of the neuron is set as the zero point, making the inside of the neuron -70mV. However, one can easily set the zero point as the inside of the neuron, making the outside +70mV. Alternatively, the zero point can be a hallway through the membrane, making the outside +35mV and the inside -35mV. The only thing that matters is the difference between two points, which in this case, is always 70mV (between the outside and inside). Where exactly you start counting from does not matter, so long as you are communicating with people who are using the same reference point as you are.
What is current?
Current is simply the movement of charge over time. It is measured by Amperes, or how fast every charge moves per