So does voltage cause current or vice versa ? I am confused about it because of the following :
1> Resistor: Apply a voltage and a current will flow depending on the resistance (I = V/R). So the voltage seems to be the cause and current the effect.
2> Inductor: Change the current and you’ll see a change in voltage (V = (Omega) dI/dt). So the current seems to be the cause and voltage the effect.
3> Capacitor : Change the voltage and you’ll see a change in current (I = C dV/Dt). So the voltage seems to be the cause and current the effect.
Going by pure physics term, Voltage (or Potential Difference) seems like the force and current seems like the motion (the effect).
First things first. Voltage is a difference is a measure of the potential energy difference between to points. In order to get this energy difference electrical charges have to be moved apart physically. For example, in a battery a chemical reaction releases energy which moves the negative charges, electrons, toward one terminal making it negative and leaving the other terminal positive. This movement of charges constitutes a momentary current until the negative terminal becomes so negative that the chemical reaction in the battery can’t move any more electrons to the terminal. This is the intrinsic electrical potential of that particular reaction. So in one sense you can say that the current in the battery establishes the terminal voltage. Actually though, it is the energy in the chemistry that does it.
If a load, say a light bulb is connected to the battery, the voltage difference between the terminals results in the movement of charges through the light so in this case we can say that voltage “causes” current. However, in order to maintain that voltage at the terminals, the chemical reaction has to constantly supply electrons to the negative terminal so the flow of current within the batter “causes” the voltage at the terminals.
The voltage is the “force” which propels a “charge” current through a circuit .
If you increase the force, V, then it will drive more charge through the same circuit.
If the circuit has a very low resistance then a large current will flow even if the voltage is low.
However the amount of work that is carried out may still be small. it being the product of current*voltage=power.
In scenario 2, the the magnetic field induced in the coil is proportional to the current through it and it is the reverse voltage potential produced by this magnetic field that is responsible for limiting the current.If you increase the voltage then a larger current flows, which then produces a larger back e.m.f and this slows the rate at which the voltage across the coil builds up. So you can take the view that voltage does have and effect, without it no current would flow.
In scenario 3 the current is stored as charge on the capacitor plates, the greater the voltage driving that current, the larger the charge, hence the larger the current.
Those plates can only store a certain amount of charge, as the level of charge increases, the rate of current flow decreases, to some time when there is effectivly no current flowing, and the voltage across the capcitor is equal to the supply voltage.
In 2 and 3 there is the time element to consider hence the delta terms, but in one the circuit continues to behave in the same manner for as long as the operating conditions stay the same.
Conclusion is that if you vary any of the the properties in a circuit it will have and effect on all the others, they are interdependant.
Let’s say you connect a 1200 ohm resistor to a 24 volt constant voltage source (e.g. battery or lab power supply). 20 milliamps of current will flow through the resistor as a result of the 24 volts imposed across the resistor.
Now let’s say you connect a 1200 ohm resistor to a constant current source set to 20 milliamps. 24 volts will appear across the resistor as a result of the 20 milliamps forced through the resistor.
In the first case it would appear the current is a result of the voltage. And in the second case it would appear the voltage is a result of the current. This is more-or-less true in the practical sense. But here’s the key: the resistor doesn’t know the difference, i.e. it does not know (nor does it care) if the current is a result of the voltage or vice versa.
The way I learn was to use water and pipes as an analogy. If electricity was water and wires are pipes…
There are two traits of water coming out of a pipe. The volume of water (like current) and the pressure of the water coming out (voltage). If we connect a skinnier pipe (resistor) to the outlet, the amount of water coming out depends on the pressure (the higher the pressure, the more water that can go through the skinny pipe for a given time) and how skinny the pipe is (how much resistance of the resistor). The relationship is “amount of water coming out” = “pressure” / “size of skinny pipe” or current=voltage/resistance or I=V/R or I=E/R.
An analogy of a capacitor is a bucket. You can fill up the bucket and it holds the water. You can then pour out the water.
Jim
Okay - I was not doubting the coexistence of voltage and current. My question was simply what gives rise to what. To illustrate:
1> If F is the net force on a body of mass m then the body accelerates with an acceleration a where
F = ma
Now a force is the cause of acceleration not the other way.
2> Similarly, if the net potential difference across a resistor is V, and the resistance is R a current I flows through the resistance, where
V = RI
But unlike 2 can we say here that voltage causes the current and not the other way.
Casdave - how about an Inductor with changing flux linkage - a generator for example where the current produced is not dependent on the voltage but on the rate of change of flux linkage ?
But accelerations can cause forces. If a wheel is spinning, the rim of the wheel accelerates constantly, which applies a radial force which stretches the whole wheel outwards. The acceleration causes the force.
Or suppose you have a large mass which is drifting sideways constantly (no acceleration.) If you stand in front of it, when it strikes you it’s decelleration causes a force which throws you backwards.
Centrifugal force is a pseudo force, read any highschool physics book (It arizes from your frame your reference). Its the Centripetal force that is required for circular motion.
The moment it strikes you, it feels a force equal to the force felt by you (Newton’s first law Action and Reaction). This force causes the body to slow down.
Apart from some sort of halfway-intelligent self-regulating power supply built exclusively for the purpose, is there such a thing as a “constant current source?” Sure, there are plenty of power supplies/batteries/current sources that are limited to some maximum current, but that’s different.
Wouldn’t a regulated constant-current supply keep the current steady by just altering the driving voltage anyhow? The 24V on the resistor in this example would, of course, appear across the current source as well.
Obviously, I’m leaning toward the voltage-causes-current explanation here, but despite my best efforts, I’m no EE.
Several electronic devices are constant current sources, at least to a first approximation. By the way, constant voltage sources are also only approximately constant voltage. For example the current through the load impedance connected to the collector node of a transistor is independent of the size of the load impedance. It depends only upon the current supplied to the base of the transistor.
Now that is true only until the voltage across the load impedance equals the voltage supplied in the circuit minus the voltage across the transistor from emitter to collector. It is also only appoximately true in all cases because there are internal leakage currents in the transistor that take more or less of the collector current depending upon the voltage across the load impedance.
Likewise, the voltage across a load impedance connected to a constant voltage source is independent of the size of the load impedance. It depends only upon the voltage of the source.
However that is true only in the ideal case. All voltage sources have a series internal impedance that gradually lowers the terminal voltage as more current is drawn and the maximum current possible from such a source is V/R[sub]i[/sub] where V is the open circuit voltage and R[sub]i[/sub] is the internal impedance of the source.
For purposes of convenient analysis, any constant voltage source can be converted to a constant current source and vice versa.
In a rotating frame of reference the centrifugal force is a real force.
Plus bbeaty never said anything about centrifugal force. What he said was, “which applies a radial force which stretches the whole wheel outwards.” Please read posts more carefully before you criticize them.
They both do. The equation you need is usaually called the Lorentz equation. The force on a charged particle (say a free electron in a conducting wire) is:
F=q(E + v X B)
where q is the charge, E the electric field vector, B the magetic field vector, and v the velocity vector of the charged particle.
The Electric field vector is the change in voltage with distance (volts/meter). Also a current in a wire will create a magnectic field outside the wire (which will in turn put a force on charges moving with velocity v elsewhere).
This is a very simplified explanation. (It has been a very long time since my undergrad physics courses.) What you really need to do is look at Maxwell’s equations. The magnetic and Electric fields are tied together and cannot really be separated except in static (in time i.e. dc) circumstances. Try doing a google search for Ampere’s law and maxwells equations. I’m sure there are a bunch of sites that will tell you more that you want to know.
Can you tell of any other Radial force apart from the “centrifugal force” caused by rotation stretching the wheel outwards ? IMHO, you should read the posts more carefully.
And anyways, centripetal (or centrifugal force take u’r pic) is required for circular motion, not the other way around. The tension in a string provides the force when tie a mass to an end of it and make it go around.
I’m confused by this statement. In a rotating reference frame the only force will be a force directed inwards toward the center of rotation. This force (gravity (earth-moon), floor pressure from standing in a rotating ring (2001 and HAL9000)) will create a centripetal acceleration which will make the person accelerate toward the center of the rotation. The outward stretching mentioned by bbeaty is just inertia - “objects in motion tend to stay in motion unless acted upon by a force”. So the rotating wheel has a tangential velocity that it wants to keep, but the centripetal acceleration caused by inward radial force keeps in moving in a circle.
Thanks andy, but nothing you have said has confused me.
What I am confused about is this:
However, after some thought I am no longer confused. In a rotating reference frame the (say a room in the ring space station in 2001) the pseudo-real centrifugal force is conteracted by the normal force imposed by the space station floor leaving 0 acceleration in the frame.
