What is voltage drop?

**sameeralord** · Nov22-09, 11:16 PM

Thanks guys but I'm still at square one. I don't think I expressed my question properly. I understand K.. law.

Why do different resistances get different portions of the voltage. For example in 1--4 do have a voltage you need to have more electrons at 4 than 1. If the current (rate) flowing through each resistor is the same how can there be different electron differences. Basically why do different resistances have different voltage drops?

**Integral** · Nov23-09, 01:59 AM

voltage is not about the number of electrons. It is about the energy available to move electrons. Outside of the battery electrons are not stored, gained, or lost anywhere in the circuit so the number of electrons passing any point in a time interval must be constant. In other words current is constant in a series circuit.

Now do you know Ohms Law?

E=IR

This gives the voltage measured across a resistor given the current through it.

In a series circuit the current is constant and the sum of the voltage drops is equal to the source voltage.

**sophiecentaur** · Nov23-09, 04:21 AM

Originally Posted by YellowTaxi

If you could see what was actually happening inside the wires that would look like train carriges taking up the slack in their buffers ats they bump into each other. So you should in theory always have a flow of electricity after the power supply is turned off. Because the 'wave' can't move faster than light, it's not instantaneous. So I'd say there is always a build up, but you can't see it.

There may an initial build up of charge in some parts of a circuit - as when a capacitor charges up - but this cannot continue. We were discussing 'current' electricity - in which a steady state has been reached in the flow through resistance only when there is no reactive component to store energy.
If you want to show why there can't be a constant build up of charge then go and calculate the force involved when a charge of one Coulomb (that could be just 1/10Amp for 10 seconds)is brought 1m away from another 1 Coulomb. The analogy to a bicycle chain is still there; the chain stretches, initially, when you start pedalling and takes up the slack. This is equivalent to a small amount of polarisation in parts of the circuit.

And NO: 'Electricity' is not the Kinetic Motion of Electrons. They only drift through wires at a few mm per second! 'Electricity' is not really a Scientifically defined term - it just covers the general subject of Electrical Energy, Forces, Current etc.

**sophiecentaur** · Nov23-09, 04:29 AM

Originally Posted by sameeralord

Thanks guys but I'm still at square one. I don't think I expressed my question properly. I understand K.. law.

Why do different resistances get different portions of the voltage. For example in 1--4 do have a voltage you need to have more electrons at 4 than 1. If the current (rate) flowing through each resistor is the same how can there be different electron differences. Basically why do different resistances have different voltage drops?

They get different shares of the Voltage because they need different amounts of energy to shift charge through them. Once the situation has settled down (after 1ns or so) the KII situation will apply. The battery supplies just enough power to drive current through them all and each one dissipates just enough to balance the situation. If you short one of the resistors out, then the PD is shared out amongst the rest, after another ns and KII rules again.

**sophiecentaur** · Nov23-09, 04:36 AM

I know the water analogy can be full of pitfalls but consider a a length of pipe with sections of different thicknesses and stuffed with various different fillings carrying a fast flow of water. You would get different pressure drops across the thin and fat sections and the overall pressure drop would be the sum of these pressure drops. The pressure of water coming out of the end would be the input pressure minus the sum of the pressure drops. Moreover, if you open the nozzle wider and increase the flow, the output pressure would drop more and, if you shut off the nozzle, the pressure would go up to the value of the supply.

**sameeralord** · Nov23-09, 05:33 AM

When electrons go through a resistor they lose energy right. When they lose energy they would stop moving right? So how does same amount of electrons travel through each resistor if there are 3 resistors in a circuit?

Ok this is my final understanding. Tell me if this is write.

Wait is this what happens. When there are 3 resistors in a circuit. The electrons flowing for a second is the same but to maintain this they would lose different amount of energies depending on the resistance of the resistor. So example.

res 1-100 ohm
res 2- 200 ohm
res 3-300 ohm

After a minute as sophie said the electrons would come up with a constant flow rate that would allow electrons to flow throughout the circuit(let's say it is 3 electrons/s). Since resistor 1 has less resistance it loses only a little amount of energy compared to other 3 to maintain this flow(3 electrons per second).

About the kinetic energy. Since electrons are moving very slowly all the potential energy can not be converted into kinetic. So voltage is disspiated as heat instea.

Is this right!!

**sophiecentaur** · Nov23-09, 07:01 AM

That seems to be along the right lines.
The KE thing.
Just think of the vanishingly small mass that the conduction electrons constitute and then think about a mean speed of a few mm/s. (Their RMS speed due to thermal energy is much, much faster than that.) That represents a really really small amount of KE - so don't include it in any of your reasoning any more than you would include the mass of the chain when discussing the power transfer from a motorbike engine to the wheels.

The resistor heats up as a result of the 'forces' pushing the electrons through. They interact with the resistor material and transfer energy to it. If you replace a resistor with a motor, the same sort of interaction produces movement energy.

**jtbell** · Nov23-09, 07:35 AM

Originally Posted by sameeralord

When electrons go through a resistor they lose energy right. When they lose energy they would stop moving right?

No, because they lose potential energy, not kinetic energy (on the average).

Think of a block sliding down an inclined plane, whose surface produces just the right amount of friction so that the block slides at constant speed. Its kinetic energy remains constant, but it's losing gravitational potential energy as it slides downhill. That energy appears as thermal energy in the block and incline: they become warmer.

Similarly, as current flows through a resistor, the electrons move at constant speed (on the average) but lose electric potential energy. That energy appears as thermal energy in the resistor and its surroundings: they become warmer.

**YellowTaxi** · Nov23-09, 12:01 PM

Originally Posted by jtbell

No, because they lose potential energy, not kinetic energy (on the average).

Think of a block sliding down an inclined plane, whose surface produces just the right amount of friction so that the block slides at constant speed. Its kinetic energy remains constant, but it's losing gravitational potential energy as it slides downhill. That energy appears as thermal energy in the block and incline: they become warmer.

Similarly, as current flows through a resistor, the electrons move at constant speed (on the average) but lose electric potential energy. That energy appears as thermal energy in the resistor and its surroundings: they become warmer.

so that's the explicit reason why the voltage drops as you go round the circuit with a voltmeter, but if you put an ammeter in there it measures the same value all the way around ? Voltmeter measures potential (the 'height' above ground). The ammeter is measuring the kinetic of the electron flow? or have i got that wrong ?

**Astronuc** · Nov23-09, 12:06 PM

Originally Posted by YellowTaxi

so that's the explicit reason why the voltage drops as you go round the circuit with a voltmeter, but if you put an ammeter in there it measures the same value all the way around ? Voltmeter measures potential (the 'height' above ground). The ammeter is measuring the kinetic of the electron flow? or have i got that wrong ?

An ammeter measures current, or flow of charge. That has to be continuous, otherwise charge would decrease or increase locally.

The rate of flow depends on the potential difference and the resistance.

It's a bit like measuring the flow of water (e.g. down a sluice or incline) or flow of heat.

**YellowTaxi** · Nov23-09, 12:09 PM

Originally Posted by sophiecentaur

'Electricity' is not the Kinetic Motion of Electrons. They only drift through wires at a few mm per second! 'Electricity' is not really a Scientifically defined term - it just covers the general subject of Electrical Energy, Forces, Current etc.

the speed may be very slow , but generally there's a lot of them to make say one Amp.

**YellowTaxi** · Nov23-09, 12:11 PM

Originally Posted by Astronuc

An ammeter measures current, or flow of charge. That has to be continuous, otherwise charge would decrease or increase locally.

The rate of flow depends on the potential difference and the resistance.

It's a bit like measuring the flow of water (e.g. down a sluice or incline) or flow of heat.

yes exactly, so current is just a measure of the kinetic energy of the electrons (?)

**DaleSpam** · Nov23-09, 02:54 PM

Originally Posted by YellowTaxi

yes exactly, so current is just a measure of the kinetic energy of the electrons (?)

NO. Current is NOT a measure of the kinetic energy of the electrons. I mentioned this explicitly in post 9. The kinetic energy of the electrons is completely negligible for most circuits.

**Naty1** · Nov23-09, 03:18 PM

Bell's post #24 is a nice analogy.

Current (i) is a measure of the flow of electrons past a measurement point; it measures the rate of flow of charge. i = dq/dt. voltage (difference) measures the potential pushing the electrons along.

The flow of charge (movement of electrons) through a conductor is often compared with the flow of water thru a pipe; water flows thru a pipe because of a difference in presssure while electric charge flows through a conductor to to a difference in electrical pressure, an old name for voltage (electric potential)difference. With an increase in water pressure more water flows; with an increase in electric pressure(potential) more electrons flow. You can think of the rate of flow of water molecules as analogous to the rate of flow of electrons.

With water, the more resistance, the less the flow of water, say due to a smaller pipe; with electricity, the more the resistance the less the flow of electrons, perhaps due to a smaller conductor, perhaps due to a material with higher resistance (fewer free electrons to move).