# Tag Info

131

The basic circuit theory "rules" you imply, are high level simplifications applicable at a large scale and at slow speeds. If you look at it close and fast enough, you could say that a current really starts to go into the obstructed path, but the electric field in front of the obstruction would build up gradually and current will start to repartition into ...

93

The idea that electricity "does not exist" is just verbal sophistry along the same lines as "matter does not exist, it is frozen energy" or, "you do not exist, you are a figment of your own imagination". At best these are all just over-dramatic and misleading ways of saying that what these things actually are is not what you ...

84

A birds legs are pretty close together. An electrical transmission wire has very little resistance. This means that the voltage as a function of distance barely changes. So the voltage difference between two birds feet is essentially 0, because the potential on each foot is practically the same. The potential difference between the wire and the ground ...

60

Perhaps you're visualizing the electron flow as if it were a series of snapshots, timed so that the snapshots all look identical. But it's more than that. The wavefunction of a moving electron is different from that of a stationary electron: it includes a nonzero velocity-associated component. It's that added component (which is always there, even in the ...

57

Electricity is not a well-defined term in physics. It's a layman's term that means something like what physicists call electrical phenomena. However it also gets used (like your first link says) for numerous specific phenomena that physicists have more specific terms for: Electric current Electrostatic potential Electric field Electric power etc. So if ...

53

Because it was defined by measurements (the force between two 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.

53

In addition to the other answers, here is something for the intuition: $$V=RI$$ More "pressure" $V$ (more correctly: higher "pressure" difference from one side to the other) is required to keep the flow $I$ of charges constant when the flow is resisted by $R$. A thin wire has higher resistance than a thick wire, $R=\rho L/A$, analogous to a "bottleneck" in ...

53

Before explaining current, we need to know what charge is, since current is the rate of flow of charge. Charge is measured in coulombs. Each coulomb IS a large group of electrons: roughly 6.24 ˟ 10^18 of them. The “rate of flow” of charge is simply charge/time and this calculation for a circuit gives you the number of coulombs that went past a point in a ...

51

Electricity isn't a gas that expands out to shock anything in contact with it. Electricity is a flow from high voltage to low voltage. Touching a charged object is only dangerous if you become a current path--if it uses you to get somewhere. Even if the earth had a net charge, you aren't providing it anywhere to go, so you will not be shocked. It's somewhat ...

51

The speed of electricity is conceptually the speed of the electromagnetic signal in the wire, which is somewhat similar to the concept of the speed of light in a transparent medium. So it is normally lower, but not too much lower than the speed of light in the vacuum. The speed also depends on the cable construction. The cable geometry and the insulation ...

51

Late last century electrical standards based on Josephson junctions became common. A Josephson junction together with an atomic clock can give an exquisitely precise voltage standard in terms of the Josephson constant. Unfortunately, the then-current definition of the volt relied on the definition of the SI kilogram, which introduced substantial uncertainty. ...

51

Current flowing in the wire is irrelevant to the danger. It's the current flowing through your body that will hurt you, and the amount of current that flows through your body will be proportional to the voltage between the wire and anything else that you happened to be touching (e.g., the ground upon which you are standing.)

45

I'll try to offer a simpler analogy of how that works. Camp A on the side of a mountain is full of hikers. There is another empty campsite B on the other side of the mountain. And there are two possible paths between A and B - over the mountain or straight through a tunnel. You order (apply voltage) the hikers (electrons) to go to camp B. While most are ...

41

Power to a water-wheel depends both on the current (amount of water delivered) and the head (vertical drop of water as it turns the wheel). So, the water analogy does have TWO variables that multiply together to make power: current, measuring (for instance) the water flow at Niagara, and vertical drop (like the height of Niagara Falls). Current is NOT the ...

41

Do not touch even the neutral wire in a live circuit! There are numerous failure modes that could make you dead wrong about not getting shocked. The neutral wire does have current going through it. However, we do not get shocked when we touch something with current going through it, we get shocked when current goes through us. In this case all of the current ...

39

Let me say in advance: You are perfectly right! The wire is a resistor, the bird is a resistor, and a bird standing on a wire with both feet down is indeed a parallel resistor to the wire. This means indeed that current flows through the bird. It is just not much, because the wire is — by design! — a very bad resistor, and birdie is by comparison (and that ...

38

The amount of heat generated by current flowing through a resistor (whether from lightning or more ordinary sources) is directly related to the power dissipated by the resistor, which is $$P = I^2 R.$$ $R$ is small for objects made from good conductors, which many metals are, and large for objects that are made from bad conductors like plastic or wood. ...

36

Oh, but you can. You can drive an high impedance input with it...including a buffer, which can then in turn be used to drive whatever you want. The more current you draw the more the voltage will droop, so you just make sure to draw as little current as possible. So that the output is, for example, 99.9% of what the divider formula says it should be. The ...

35

I am used to smoothing out badly shaped circuits by pulling the wires: Then I get a better circuit by cutting the extra wires: So there are three resistors in parallel, indicating that the current flows through three possible paths.

34

Ohm's Law is not a construct which can be derived. It is essentially a generalized observation. It is only useful for a few materials (conductors and medium resistivity), and even then virtually all of those materials show deviations from the ideal, such as temperature coefficients and breakdown voltage limits. Rather, Ohm's Law is an idealization of the ...

33

You are right, every circuit possesses some unintended capacitance, which is called "stray" capacitance. Whether or not it affects the operation of the circuit depends on the frequencies that the circuit is intended to operate at. The amount of stray capacitance that a circuit has is typically tiny, but at high enough frequencies even a very tiny amount of ...

31

resistance is due to collision with protons Actually, there are lots of materials which don't contain protons outside of atomic nuclei — e.g. steel, glass, oxygen — but all these do have resistance. Dominant factors determining resistance vary from material to material: these can be scattering of electrons/holes by motion of atomic nuclei (i.e. by phonons), ...

31

They’re describing the situation where the wires are carrying power to a load. It’s the load that (mostly) determines the current in the wires leading to it. A $1200$W oven on $120$V needs $10$A. Once the load has determined the current, the heat in the wires is given by their resistance via $I^2 R_{wire}$. A $0.02$ ohm wire to the oven will have $2$W ...

29

You could start from Drude in zero magnetic field, that equates the derivative of the momentum $\vec p$ by the electrostatic force $\vec F_{el} = q \vec E$ as a product of charge $q$ and electric field $\vec E$ minus a scattering term (with time constant $\tau$; compared to Newtons second law that does not feature the latter, crystal term): \$~~~~~~\dot {\...

29

Sometimes it is easier to understand circuitry in the context of water. What you're imagining is two tanks of water of equal size linked together by a pipe that has been sealed off. If one tank holds 5% water and the other holds 35% water, when you remove the seal, the tanks equalize and you end up with 20% in both tanks. What you're forgetting is that ...

29

Circular currents do produce EM, and indeed this is exactly how X-rays are produced by synchotrons such as the (sadly now defunct) synchotron radiation source at Daresbury. In this case the current is flowing in a vacuum not in a wire, but the principle is the same. Current flowing in loops of wire don't produce radiation in everyday life because the ...

29

But in space the resistor doesn't have anywhere to put off the heat! Actually, it does. Heat transfer can occur by three means: conduction, convection, and radiation. Very basically, heat conduction is about solid materials touching each other; convection is about gases or liquids touching the heat source; and radiation is about transmission of energy by ...

29

As you noted, particle accelerators are examples of electric currents flowing though a vacuum. In order to answer why a vacuum is considered an insulator, you have to consider where the charged particles come from. Consider the space between the plates of a capacitor that is connected to a battery. Even though there is a voltage difference between the plates,...

29

Your home circuit does not "know" how much current to deliver to each socket or appliance. The circuit supplies a constant voltage, and it is then up to each appliance to limit the current that it draws. Some simple appliances, such as lights with old incandescent bulbs or electric toasters or irons, are basically just a resistor (possible a ...

27

To be precise, current is not a vector quantity. Although current has a specific direction and magnitude, it does not obey the law of vector addition. Let me show you. Take a look at the above picture. According to Kirchhoff's current law, the sum of the currents entering the junction should be equal to sum of the currents leaving the junction (no charge ...

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