I'm confused about the two terms, when voltage is applied and across a certain element in the circuit.
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\$\begingroup\$ You can apply 5V to a silicon diode, but the voltage across it will be 0.6V regardless. \$\endgroup\$– Ignacio Vazquez-AbramsAug 4, 2014 at 18:29
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1\$\begingroup\$ @IgnacioVazquez-Abrams If you connect the diode in reverse, wouldn't the voltage across it be 5V? \$\endgroup\$– NazarAug 4, 2014 at 19:55
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\$\begingroup\$ Yes, it would... But that's because for "low" voltages a reverse biased diode equals an open circuit \$\endgroup\$– Vladimir CraveroAug 4, 2014 at 19:56
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\$\begingroup\$ @IgnacioVazquez-Abrams why does the term voltage across matter? "it" being what in your statement? \$\endgroup\$– PupilAug 6, 2014 at 6:01
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6 Answers
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What Ignacio said is the core of the answer, I hope I can help you out going a bit deeper.
Generally the only distinction between "applied voltage" and "voltage across" is how you are dealing with voltage itself:
- you apply a voltage to a bipole taking a voltage source and putting it in parallel with the dipole.
- you usually measure a voltage across some dipole, putting a voltmeter in parallel with it.
That's to answer your question. Now what if you apply a voltage generator? What would the voltage across it? The answer is: there is no answer. That is a limitation of the model we are using. Ignazio makes the useful example of a diode: you apply 5V but across it there's only something like 0.7V: that's because your voltage source has an internal resistance where the remaining 4.3V drops.
Remember that most of the times when you apply a voltage to a dipole, the voltage across it will be exactly what you are applying. The two wordings though does not mean the same thing at all.
addendum
Since this is at the top now, and I've read some others very good answers, and since the question is very basic I'd like to add two words about potential, a word that every answer uses. A potential is a scalar field associated with a vector field. This vector field must be conservative for the potential to exist, and for the electric field this is true only for electrostatic fields. When things start moving around no potential can be defined. I don't want to be the fussy physicist but a professor once throw a chalk at me for this imprecision (he was quite precise) so since this might be seen from young students I though this should be pointed out.
answered Aug 4, 2014 at 18:41
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\$\begingroup\$ To be sure, I find your addendum odd and not contextually correct. There is, for example, a potential difference between the terminals of a resistor with a current through since there is a charge distribution through the resistor that gives rise to an electric field through the resistor. A charge moving through the resistor loses potential energy. Moreover, one of the assumptions of ideal circuit theory is that any changing magnetic fields threading the circuit are insignificant and thus, potentials are well defined. \$\endgroup\$ Aug 4, 2014 at 22:35
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\$\begingroup\$ @AlfredCentauri well my point is only the misuse of the word "potential". At least in italian it means a very precise thing that can not be defined for varying E fields. I'd like to see some literature about the fact that this is an assumption, maybe we're speaking of two different things. \$\endgroup\$ Aug 4, 2014 at 22:38
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\$\begingroup\$ We might be talking about different things so I'll look some references up and see if we can clear things up. \$\endgroup\$ Aug 4, 2014 at 22:38
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\$\begingroup\$ That's what I am speaking about: here. I am not arguing the fact that the word "potential" is widely used in EE, I'm just saying that it might be something that a university physics teacher wouldn't want to hear without the word electrostatic in the same sentence. That's why I'm surprised that the existance of a potential is an assumption for circuit theory, since circuit theory works in AC too of course. \$\endgroup\$ Aug 4, 2014 at 22:45
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\$\begingroup\$ Like you, I'm aware that an electric potential can only be strictly defined for a static electric field. However, in ideal circuit theory, we assume (1) changes propagate instantly (lumped element approximation), (2) no charge accumulates anywhere in the circuit, and (3) there is no magnetic coupling between circuit elements. Of course, this is not physical but, when rates of change are 'small enough' such that the assumptions above are effectively true, ideal circuit theory is a good approximation. And, as you're aware, these assumptions aren't good for, e.g., RF circuits. \$\endgroup\$ Aug 4, 2014 at 22:54
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A voltage is always across two nodes, it is the difference between the electric potentials of these two nodes. They are strictly speaking always applied by something, but we speak about applying a voltage across two nodes when we set the potentials of those two nodes by connecting them to the outputs of a voltage source, which role is to make sure the voltage across those is fixed to a known value.
The voltage of a node is often a shorthand for the potential of that node with respect to the ground of the circuit (which, as a reminder, is only a node which has been arbitrarily associated to a 0V value).
Electric potential is often compared to height in the liquid analogy where the flow of water is electric current and rocks along its path, resistance.
Reminder: a node is a uniquely defined point of interest in the circuit (a pin, the intersection of several branches etc.).
answered Aug 4, 2014 at 18:33
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The phrase "voltage across a circuit element" means precisely the potential difference between the terminals of the circuit element. One can measure this voltage with a meter.
The phrase "voltage applied to a circuit element" is less precise but I believe it means that one is driving the circuit element with a voltage source of some type and that the voltage across is, more less, fixed by this source.
The opposite of this would be the "voltage supplied by a circuit element" which would imply that the voltage across is generated by the circuit element, e.g., a battery, a charged capacitor, etc.
answered Aug 4, 2014 at 19:16
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\$\begingroup\$ Lets break this down even more by using an example. So, we can measure a circuit element like a light bulb, and find that the voltage across its terminals to a certain value, yet... the PS can apply a higher/lower value of voltage to that same bulb? \$\endgroup\$– PupilAug 7, 2014 at 20:06
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\$\begingroup\$ In a circuit I have an element that's resistance is extremely low, I'd like to increase the resistance so that more voltage can be applied to that element, yet I'm confused because the voltage across that element stays the same regardless of increasing its resistance... hope that makes sense. \$\endgroup\$– PupilAug 7, 2014 at 20:09
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1\$\begingroup\$ @Key, if the PS is effectively a voltage source (insignificant internal resistance), the PS fixes the voltage across the element. Changing the resistance of the element will only change the current through. If the PS is effectively a current source (high internal resistance), the PS fixes the current through. Changing the resistance of the element will only change the voltage across. If the PS is neither a good voltage source nor a good current source (moderate internal resistance), changing the resistance of the element will change both the voltage across and the current through. \$\endgroup\$ Aug 7, 2014 at 20:27
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\$\begingroup\$ Thank you, there seems to me a be a lot more for me to understand. \$\endgroup\$– PupilAug 7, 2014 at 21:38
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applied voltage means the voltage which is given by us to the component.
voltage across means the voltage which is reduced by the component due to the internal resistance of the component
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Applied voltage to a component is the actual voltage given to the component. Whereas voltage across a component is the voltage drop/voltage dissipated by the component. In both cases, voltage means a difference in electrical potential between two points. It is always between two points as it is only the difference between those two points that produce an electromotive force.
Now, applied voltage to a component and the voltage across a component may or may not be the same value. If you are applying 5 volts to a single resistor circuit, that resistor will get all of the 5 volts as voltage is conserved within a loop (KVL). If you now have 2 resistors in series of equal value, each resistor now gets 2.5 volts of the total 5 volts. Technically, in the latter case, a total of 2.5 volts is applied to the last resistor, and thus the voltage across it is 2.5 volts. However, there exists non-voltage driven components, meaning they're driven by current instead. A diode of any sort is a good example, where you may apply 5 volts to it, but the actual voltage drop may be something around 0.5 volts. In this case, the rest of the voltage is sent back to the source and power will be dissipated by the internal resistance of the source.
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The right way to say it would be apply voltage across something - it sounds more precise. The voltage is a difference in potential between two points. So, when they say apply voltage, they omit the word across, assuming you know between/across which two points. Commonly, this phrase is used to say 'Apply voltage to a circuit', meaning provide power to a circuit, since you know where to connect two wires. Similarly, you can say apply voltage across the circuit, but it might sound somewhat redundant, but more precise. This phrase is rather used to specifically tell between/across which points the voltage should be applied or measured. In both cases it is meant the say the same thing, but might be misunderstood.
