# Tag Info

23

Suppose you are using a waterwheel to do some form of work (e.g. grind corn). You need a head of water to make the wheel move, and you could use either 1kg of water at a height of a million metres or you could use a million kg of water at a height of one metre. In both cases the water would do the same amount of work as it flowed through your wheel. The ...

16

Because it was defined by measurements (the force between to wire segments) that could be easily made in the laboratory at the time. The phrase is "operational definition", and it is the cause of many (most? all?) of the seemingly weird decision about fundamental units. It is why we define the second and the speed of light but derive the meter these days.

15

It's not a mistake, and conventional current is not wrong or backwards. The labeling of one polarity of charge as "positive" and the other as "negative" is totally arbitrary. It could be done either way and everything would still work out the same. Franklin didn't choose wrong; he just chose. Labeling protons as negative and electrons as positive wouldn't ...

13

The RMS (root-mean square) value of an AC voltage, which is what is represented as "110 V" or "120 V" or "240 V" is lower than the electricity's peak voltage. Alternating current has a sinusoidal voltage, that's how it alternates. So yes, it's more than it appears, but not by a terrific amount. 120 V RMS turns out to be about 170 V peak-to-ground. I ...

12

This is really the same as Adam's answer but phrased differently. Suppose you have a single wire and you connect it to a battery. Electrons start to flow, but as they do so the resistance to their flow (i.e. the resistance of the wire) generates a potential difference. The electron flow rate, i.e. the current, builds up until the potential difference is ...

10

It seems you are contrasting the speed of propagation of current with the speed of the individual charge carriers. These two things are clearly separate. There are many examples. Consider sound. A fire cracker goes off at the other end of a football field from you. You hear the sound a few 100 ms later. The air molecules that were by the firecracker ...

10

instead of thinking your body is empty and that a charged wire has to push electrons one by one through you and into the ground (blood is actually full of charge carriers), a better analogy would be a very long queue of pushy people. if the entrance to the apple store doesn't open, it doesn't matter how hard the guy at the back pushes--nothing moves. ...

8

1) If you are thinking of harmonics as sinusoidal waves, well yes, ALL waveforms are (can be described as) sum of harmonics. This is essentially the idea of the Fourier analysis. The problem is that to exactly reproduce a desired waveform you need in general an infinite number of harmonics. This is for instance the case of square waves. So in reality you ...

8

At sufficiently high voltages almost everything conducts due in part to quantum tunneling of electrons. An insulator has a breakdown voltage which is the field strength required before it will start conducting. Related to the breakdown voltage is the dielectric strength which is the minimum voltage over distance ($\mathrm{V}/\mathrm{m}$) before a material ...

7

Gregsan's and Kieran's answers are insightful analogies and the pushy electrons are certainly part of the answer. There is another aspect to the "decision" process and that is the propagation of electromagnetic waves. There is a chapter in the second volume of the Feynman Lectures on Physics - I don't have it with me but the relevant section will be just ...

6

Although the question is not clear, my guess is that you are confused with the flow of current and mean position of electrons. In case of DC, we have a continuous flow of charge from one point to another point in the conductor, any electron completes a cycle of circuit. In case of AC, there is no net displacement of charge and this may lead one in ...

6

Electrons will flow against the electric field lines because their charge is negative, and the electric field thus exerts a force $\mathbf{F}=q\mathbf{E}$ on them which is in the opposite direction. Thus electric field lines inside the wire go from the positive to the negative terminal and the electron flow goes from the negative to the positive terminal. ...

5

Electric current is the rate of flow of electric charges across any cross-sectional area of a conductor. The direction of electric current is taken as the direction of flow of positive ions or opposite to the direction of flow of free electrons. Your assumption is not necessary here... Electrons always flow from negative terminal to positive terminal. ...

5

When there is no resistance, as is the case with an ideal wire, any value of current satisfies Ohm's Law: $V = I R$ since both $V=0$ and $R=0$. UPDATE: But isn't V is like what causes the current? Perhaps a mechanical analogy of the resistor will help. Consider the dashpot where the velocity of the arm is analogous to current while the force acting ...

5

A battery is basically just a chemical reaction. At the negative (cathode) end of the battery the reaction releases electrons while at the positive (anode) end of the battery the reaction consumes electrons. As long as the external circuit allows electrons to flow from the cathode to the anode the reaction goes and the battery generates power. If you break ...

5

I think that this page explains it very well: http://www.allaboutcircuits.com/vol_1/chpt_3/3.html Direct current (DC), because it moves with continuous motion through a conductor, has the tendency to induce muscular tetanus quite readily. Alternating current (AC), because it alternately reverses direction of motion, provides brief moments of ...

5

First off, what you describe only happens for highly amplified speakers. The current change when you touch the wires is pretty tiny and you'll only hear a sound when that signal is being amplified significantly before being sent to the speakers. There are many reasons why a tiny bit of electric current flows from your body to the speaker wire so I'll only ...

5

Moving charge always produces a magnetic field. If you have a non-zero current then you have non-zero moving charge and a magnetic field will be produced. You can achieve essentially no magnetic field though by using two wires right next to each other each carrying current in the opposite directions. As long as the wires are very close and the amount of ...

5

This is your circuit: The current that comes from the source, when reaches the point that must choose it's way, sees no difference between the two paths (symmetry) , so half of it flows through one way and the other part flows in the second way. It means that, $I_1=I_2$ , So the potential difference across yellow resistors is the same. It means that the ...

5

Your problem is assuming that the charge transferred through the resistors is different. I don't know where you got that from, so I don't really know how to refute it other than by saying that since the currents must be the same, so must be the charge transfer in a given time. Edit in response to your comment: What you said is plainly not true. ...

5

Kirchhoff's loop rule is also called Kirchhoff's voltage law (KVL). Which is different from Kirchhoff's current rule which is also called Kirchhoff's current law (KCL). KVL is derived from Maxwell–Faraday equation for static magnetic field (i.e. the derivative of B with respect to time is zero). KCL is derived from charge continuity equation which is ...

5

By Ohm's law, which states V = IR, where V is the voltage accross a resistor, I the current thru it, and R the resistance. The units work out so that no additional proportionality constant is required when V is in Volts, I in Amps, and R in Ohms. For example, if the 1.5 V battery is connected to a 47 Ω resistor, then 32 mA will flow. Of course you ...

5

Your assumption that Ohm's law is fully accurate for the stun gun + human circuit isn't correct. A stun gun uses a capacitor to store charge and the capacitor is constantly being recharged to deliver a series of high-voltage pulses. A capacitor has a finite amount of charge. Once it's charged, that's it, it can never deliver more charge than that until it ...

5

Voltage is similar to height. It plays the same role for electric charge as height*gravity does for a ball on a hill. So high voltage means high potential energy the same way a ball being high up on a hill means high potential energy. Voltage is not potential energy, the same way height is not energy. However, if you have a certain amount of charge $q$, you ...

5

Yes, $\vec \jmath(x,y,z)$ should be defined as $e$ times the Schrodinger probability current. \begin{equation*} \vec \jmath = \frac{e\hbar}{2mi}\left(\Psi^* \frac{\partial \Psi }{\partial x}- \left(\frac{\partial \Psi^* }{\partial x}\right)\Psi \right) , \quad e\lt 0. \end{equation*} That's possible to explicitly see in the formalism of quantum field ...

5

Ohm's law isn't actually a law, in the sense that everything has to obey it. It's more like "This simple pattern that Ohm noticed". Devices exist that don't even come close to obeying Ohm's law, and they're used all the time. Ohm's law basically states that how well something resists current doesn't depend on how big that current is: the device isn't any ...

4

(Someone resurrected this oldie in the queue, so just to be a contrary voice...) Ben Franklin did get it wrong. He had just developed a remarkable new theory of electricity in which positive (+) and negative (-) had specific and accurate meanings, and he was unable to apply the two labels in the way he intended. In Franklin's time electricity was thought ...

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