Why or when does it occur in a circuit? What does it imply when you speak of a voltage drop across a resistor? (Obviously, it probably means that the current's voltage before the resistor is higher than the voltage after the resistor, but why does this drop occur?)
[Physics] When does voltage drop occur
electricityelectromagnetismvoltage
Related Solutions
Perhaps I can clarify what I'm trying to get at with the famous waterwheel analogy
99 years ago, Nehemiah Hawkins published what I think is a marginally better analogy:
Fig. 38. — Hydrostatic analogy of fall of potential in an electrical circuit.
Explanation of above diagram
- In this diagram, a pump at bottom centre is pumping water from right to left.
- The water circulates back to the start through the upper horizontal pipe marked a-b
- The height of water in the vertical columns C,m',n',o',D indicates pressure at points a,m,n,o,b
- The pressure drops from a to b due to the resistance of the narrow return path
- The pressure difference between a and b is proportional to the height difference between C and D
Analogy
- Pump = Battery
- Water = Electric charge carriers
- Pressure = Voltage
- Vertical Pipes = Voltmeters
- pipe a-b = Resistor (or series of four resistors)
Note
- A "particle" of water at a has a higher potential energy than it has when it reaches b.
There is a pressure drop across a "resistive" tube.
Voltage (electric potential) is roughly analogous to water pressure (hydrostatic potential).
If you could open a small hole at points a,m,n,o,b in the tube and hold your finger against the hole, you would be able to feel the pressure at those points is different.
The potential at some point is the amount of potential energy of a "particle" at that point.
it would help if someone could clarify in what tangible, empirical way could we see or measure that there has been an expenditure of energy by comparing a point on the circuit before the resistor and a point on the circuit after the resistor.
- Purchase a 330 ohm 1/4 watt resistor and a 9V PP3 battery
- Place the resistor across the battery terminals
- Place your finger on the resistor.
- Wait.
Once you add a changing magnetic field, the electric field no longer is conservative -- i.e. there is no longer a consistent definition of voltage!
A voltmeter measures $\int_a^b \mathbf{E} \cdot d\mathbf{s}$ along a path from $a$ to $b$. Normally this value doesn't depend on the path, so you can speak of "the" voltage drop between $a$ and $b$. In this situation, the value does depend on the path -- you can get $IR$ or $2IR$ or any other value depending on how you set up the wires of the voltmeter. For example, if you wound the voltmeter leads around the triangle several times, you might get $25IR$.
Best Answer
Voltage is the unit of electric potential, the electric potential difference (in your case, the potential difference between the two ends of resistor in a circuit) can be called the voltage drop.
The potential difference produces an electric field $\vec{E}$, and the direction of $\vec{E}$ points from high potential to low potential. The electric field applies a force on charged particles (i.e. electrons in circuits) such that the electrons are driven by this force and move, thereby producing a current. So you can see the potential (voltage) difference is the reason why there is a current. By the way, you cannot say the "current's voltage", since the current is defined as $I = dQ/dt$. That is, it only describes the flow of charge per unit time.
When electrons move through a resistor they are scattered by the other electrons and nuclei, causing the electrons lose some of their kinetic energy. But the presence of the electric field will then accelerate the electrons again. We can calculate the average kinetic energy statistically, and assume the electrons are moving at a single average velocity.
Thus after each collision there is a loss of kinetic energy (it is converted to heat) but which is recovered due to the work done by the electric field. And this work is equal to the potential energy difference. You can see that the electrons have the same kinetic energies both when they enter and when they leave the resistor, but different potential energies. So we can say the voltage drop at the two ends of a resistor is caused by the potential energy difference.