We always assign a voltage drop to a resistor when a current goes through it in a closed circuit. I was wondering if this voltage drop is due to dissipated heat from the resistor or is there another reason?
The heat dissipation and volt drop are related, but I would not describe the dissipation as the cause of the drop. As electrons pass through a resistance, they lose energy as they interact with electrons in the conducting material. As energy is given up to the material, it gains thermal energy so its temperature rises. The moving electrons lose potential energy and hence there is a drop in voltage. This is similar to a gas passing through a narrow pipe, losing pressure and causing heating by friction.
You can get a voltage drop across a capacitor and current can be flowing but no power will be dissipated so no, your question....
I was wondering if this voltage drop is due to dissipated heat from the resistor
Power is a by-product (for want of a better word) of current flow and voltage i.e. power = volts x amps = heat dissipation but, this doesn't happen in a "reactive component" such as a capacitor or inductor.
Voltage is really hard to explain but can be defined as: -
The voltage between two points is equal to the work done per unit of charge against a static electric field to move the charge between two points
This isn't an intuitive answer and I struggle with it a lot but hopefully someone other than me will give a really good explanation of what voltage is.
The voltage drop times the current is the electrical power being dumped into the resistor. This causes the resistor to heat up, not the other way around. Heating a resistor won't cause current thru it or voltage across it.
The voltage drop is what resistors do when current flows thru them. One way to think about that is it's simply that way by definition of what a resistor is:
V = A * Ω
where V is Volts, A Amps, and Ω Ohms.
Another way to think about it is that the voltage is the force required to squeeze the current thru the resistor. Higher resistances resist current more, so require a larger force (more voltage) to make the same current pass thru.
To use the water analogy, a resistor is like a constriction in a pipe. More flow thru the pipe means more pressure across the constriction. Conversely, more pressure across the constriction means more flow thru it.
What is voltage?
If we drop right down to basic physics, we find that both Charge and Energy are quantities with a Conservation Law. These Laws hold in both Classical and Quantum physics, in Newtonian and General Relativity. So we can be safe in taking these as having some sort of fundamental existence.
Voltage OTOH is not mentioned at all. Voltage only appears as a defined quantity that's handy to work with, as the potential energy of an electrical field. Voltage is defined as the change of energy associated with the movement of a charge (to within a scaling factor and dimension depending on what units we are using for energy and charge and whether it's per charge or absolute).
So when we push some charge (a current flowing for some time) through a resistor, see a voltage across it, and see energy released as heat from the resistor, it's not even appropriate to ask whether the heat causes the voltage or vice versa, the voltage is just a definition of what is happening with the charge movement.
If we have a conductor through which no energy is associated with the movement of charge, then there is no voltage drop across it (@Andy), and it's called a superconductor.
An analogy is height, potential energy for a gravitational field, change of which is the energy associated with moving a mass. A superconductor is like an air table, where the mass can slide sideways without a change of potential energy. Letting it drop against a frictional restraint generates heat in the 'frictor'.
The definition of voltage and the gravity analogy works for storage of energy in capacitors, inductors, height and velocity as well, but let's keep it simple for the moment with just finite or zero resistance.
(The image is taken from here. Some attribute it to Eberhard Sengpiel).
You need to see this at an atomic level. Current flows due to the flow of electrons (in the direction opposite to its flow). Now electrons flow due to presence of valence electrons in the valence band. i.e they don't actually flow, but jump from one copper atom valence band to the next. So consider two scenarios
A copper wire across a battery(short circuit), and the potential difference across the battery terminals be 10 electrons(not the usual volt notation, simplifying the definition of voltage here). So what happens is, these 10 electrons rush into the copper wire, onto the valence band of first 10 atoms, and push the previous one's to the next 10, this follows til they come out from the other end.(for current flow, the electron that entered is not the one that comes out first). this short produces a large current(10 electrons worth large).
add a resistor to the battery and wire, now what happens is that when reaching the resistor, those 10 electrons do not have enough conductive atoms to jump onto. assuming 5 electrons get "trapped" due to lack of conductive electrons. these 5 electrons cause the voltage drop across the resistor.
The heat dissipated is due to these trapped electrons jumping and colliding with the walls of the resistor.