# Tag Info

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A light bulb wouldn't turn off, because no matter what direction the electricity is flowing through it, it is still electricity. It doesn't gain some anti-electricity effect. Here is an analogy with water. The water works flowing forwards and backwards. (Although in this example there is a stop.) If there is still confusion, however, remember that ...

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A negative voltage means that you have hooked your power supply across your device backwards. Purely resistive devices, like resistors and lamp filaments, don't care which way current flows through them, only how much current flows. Such devices will always have current-voltage curves which are "odd" functions, with $I(-V) = -I(V)$, as in your graph. It is ...

4

You don't need to use relativity to see what will happen in to a current in a gravitational field. We assume a wire of constant cross section with length in the vertical direction , and a constant current flowing through it. This must be done by applying an electric field $\mathbf{E}$ along the length of the wire, producing the current density (according to ...

4

They are obviously talking about "conventional" current, not the flow of electrons. When you want to know the direction of magnetic force on a current you need to use "conventional" current direction. There is an interesting corollary to this relating to the Hall effect - if current in a semiconductor is carried by "holes" the polarity of the Hall effect ...

3

The resistance of a lamp filament is not constant. As the current increases and the filament heats up the resistance increases. That's why the statement you quote is phrased that way. It means that the resistance is 4 Ohms when the current is low enough that the heating and resistance change are negligable. More verbosely it could be phrased: The limit ...

3

You can think in terms of the current density vector. Its definition: $$\mathbf J = \sum_i n_i q_i \mathbf v_i$$ Where $n_i$ is the charge carrier density in the media, $q_i$ is the charge of the charge carrier, and $\mathbf v_i$ is the average velocity of the charge carrier. Assuming we have several charge carriers (electrons, ions, etc), you have to sum ...

2

I know that Wikipedia is not the best source for reference, but according to this page the superconducting parts that are cooled by liquid nitrogen in most cases. Most of the superconductors are High TC ones, which still needs to be cooled. Here are some links related to this topic: Toy train Video Paper on HTS (High Temperature Superconducting) Maglev ...

2

It is not that there is no "interaction" - at any point in space, the two magnetic fields will add up with the resultant pointing in another direction. In other words, the magnetic field caused by the wire will appear to distort the externally imposed field. However, this distortion is radially symmetrical: you can think of the magnetic field lines as ...

2

Vacuum tubes can conduct hundreds of amps of electricity quite readily. The effect depends on heating the negative terminal so that electrons can leave the metal surface which otherwise keeps them in the surface owing to a phenomenon called work function.

2

Ampere's law (for a steady current) states that $$\oint \vec{B} \cdot d\vec{l} = \mu_0 I$$ If we consider an infinite wire, then symmetry tells us that the B-field at the point $A$ and all other points on a circle of radius $(R+y)$ is constant in magnitude and is in the azimuthal direction. Hence the magnitude of the B-field is given by $$2 \pi (R+y)B = ... 2 In general, Ampere's law does not necessarily give the value of the magnetic field. It only gives the integral of the field along a closed path. That integral can sometimes be used to deduce the magnetic field at any given point, but only if you know something about the magnetic field from symmetry or other considerations. For example, along a circular ... 2 The power rating given on lightbulbs always refers to the power at a specified operational voltage (which is always given together with the power or implied by the type of socket). The power at different voltages is not easily predictable as the resistance of the filament will vary strongly in dependence of temperature (which depends on the dissipated ... 2 Since I understand that power = V x Current, the power for a bulb can not be a constant if its resistant is assumed a constant. A normal mathematical thinking can confirm that. If the voltage and current don't change, then the power is constant. The electricity supplied from the wall is at 115V (more or less). If the resistance of the bulb is 1322 ohms, ... 2 Actually, we don't know that "filament bulb has straight Volatage vs Current graph": "The actual resistance of the filament is temperature dependent. The cold resistance of tungsten-filament lamps is about 1/15 the hot-filament resistance when the lamp is operating. For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold ... 1 The field B(z) on the axis of a circular current loop is well known. Differentiate f(z)=B(z-e/2)-B(z+e/2), and set that equal to zero. Expand the denominator to first order in ez/(z^2+a^2). The answer I got is z=(+/-) a/2. 1 Your oscilloscope is not triggering on the signal, the horizontal deflection time is set near a multiple of the period of the 50/60Hz wave and the instruments starts at slightly later phase each time it traces the waveform. That way the wave seems to move "backward" in time. If it starts tracing at a slightly earlier phase, then the wave seems like it's ... 1 In short it's low voltage. Current, voltage and power are all linked via the impedance (like resistance) of the thing in question. The voltage is a property of the grid, with something like a light bulb you can assume the grid voltage won't change. it might if you were running an industrial induction furnace maybe, but not with a light bulb. The current ... 1 It all depends how tight your coiling is. If the wire doesn't touch itself, i.e. without any contact surface excluded with itself, then the area of the wire exposed is simply the area of the wire :$$ A = 2\pi rL $$which gives immediately the length of wire you wish (with d the diameter of your wire) :$$ L = \frac{A}{d\pi}  L = ...

1

Laser Doppler velocimetry can be used. Generally it is used to measure velocity of this range for any transparent fluid flow.

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Resistance is directly proportional to the length of the wire, and inversely proportional to the cross sectional area of the wire. R = pl/A, where R is resistance, p is the material's resistance in ohms, l is the length, and A is the cross sectional area in m^2. As a wire gets longer its resistance increases, and as it gets thinner its resistance also ...

1

I understand the problem now from leongz's excellent explanation and patience in the chat discussion. Thank you very much leongz for helping me out! The capacitor is being discharged through constant current. If it starts with charge Q_0, then from the definition of current we know that the charge decreases by It after time t, where I is the constant ...

1

Consider the left branch of the bridge. The total resistance is $P + Q$, so the current is: $$I_{left} = \frac{V}{P+Q}$$ The voltage drop across $Q$ is just $V = IR$, so the voltage at the $PQ$ midpoint is: $$V_{PQ} = V_{in}\frac{Q}{P+Q}$$ We argue in the same way for the right hand branch to get the corresponding equation:  V_{RS} = ...

1

Current is a measure of charge flowing in some unit of time. The gravitational field will dilate time. The current will be reduced in the observer's point of view. $I = V/R$. Since resistance is constant, this means that voltage must have decreased in the observers point of view.

1

On the one hand, you can't solve for the magnetic field without appropriate boundary conditions (e.g. there could always be an incoming electromagnetic wave that hasn't yet impinged on your cylinder). On the other hand if you have a fixed charge and current distribution you can always use Jefimenko's equations to find a solution to Maxwell's equations, and ...

1

What's the effect of a resistor? It's a component that dissipates energy end thus lowers the voltage. So what is voltage? It's the strength of the field that moves the electrons, while current represents the number of electrons flowing through the wire. Free electrons can be stopped all together or slowed all together, but it's not possible to select only ...

1

Short answer: it depends on the impedance of the load attached to the secondary coil. A perfect transformer can be modeled as a pair of inductors with mutual inductance $M$ and self-inductances $L_1$ and $L_2$, with $M^2 = L_1 L_2$. Denote the voltage across coil 1 and the current through it as $V_1(t) = \tilde{V}_1 e^{i \omega t}$ and \$I_1(t) = ...

1

Most resistive materials have a nonzero temperature coefficient: the resistance changes with the temperature of the device. An incandescent lamp filament, which glows because it is hot, might have an temperature of several thousand kelvin. On the other hand, if you probe the lamp with an ohmmeter while it is off, you'll typically use a current of a few ...

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As you can see from an example of electrodynamics book by Griffiths: For ANY wire,equation (5.35) still holds.And you can see that for an infinite wire,θ1=π/2 and θ2=-π/2.So,in your situation you can use only θ1 οr θ2 by changing the other angle to zero.It does not matter which angle you keep,mathematically,they will both give you a B of the same magnitude ...

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