The study of the presence and flow of electric charge. Charges, currents, fields, potentials.
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4answers
10k views
What is the relationship between AC frequency, volts, amps and watts?
In an alternating current, how are frequency, voltage, amperage, and watts related?
For instance, imagining the power as a sine wave, what is amperage if voltage is the amplitude? Is there a better ...
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1answer
407 views
How to teach the concepts of “Electric Potential” & “Potential Energy” to 10th grade students
I've got to teach the concepts of "Electric Potential" & "Potential Energy" to 10th grade students. I don't understand how to express these things and make them understand.
“If you can't ...
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2answers
1k views
How do I calculate the power consumed by a lightbulb?
I'm studying a lightbulb and its variable resistance, given by the expression:
$R(T) = Ro[1 + α(T-T_0)]$, where $R_0$ is the resistance of the lamp at $T_0$.
In this case, $R$ is not given by Ohm's ...
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1answer
21 views
Speakers and Static from Hands
Why is it that when you touch the a bare male end of a speaker feed that the speaker makes hissing noises? Is it just (eddy?) currents running through you?
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2answers
50 views
Different batteries connected in parallel
If we have 2 batteries one of emf x and the other is of emf y and we connect them in series we get an effective emf of x+y.
But what if we connect them in parallel, how to calculate the emf now?
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1answer
66 views
Can someone explain this intuitively please?
In a static situation we defined voltage as energy/unit charge, or j/c. As the distance between the charged particles increased, the voltage decreased. Now why do we not apply this in a simple DC ...
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1answer
114 views
The field of uniformly charged ball (without Gauss theorem)
The solution of Poisson equation is given by
$$
\mathbf E = \int \frac{\rho (\mathbf r )(\mathbf r_{0} - \mathbf r )}{|\mathbf r_{0} - \mathbf r|^{3}}d^{3}\mathbf r.
$$
I tried to use this term for a ...
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1answer
114 views
why do lightbulbs sometimes unscrew by themselves?
I've seen some light fixtures which exhibit the behavior of lightbulbs gradually unscrewing by themselves.
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2answers
2k views
current in series resistors and voltage drop in parallel resistors
when we have resistors in series, the current through all the resistors is same and the voltage drop (or simply voltage) at each resistor is different.
Question 1: it is fine that voltage drop ...
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2answers
331 views
How do electrons repel?
I understand the basics, protons have a positive charge, neutrons have no charge, and the electron has a negative charge. But looking at the lines of force from a proton, they flow outward and push ...
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3answers
136 views
Resistance of superconducting wire in parrallel with standard wire
The formula for parrallel resistors is $\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2}$
But how can you use this formula when one of the branches is a superconductor, eg:
Where the red resistor ...
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1answer
1k views
What defines the brightness of a bulb?
So I have a question. There are three identical bulbs, 2 of them are connected in parallel and the third is basically in series, on the same circuit. If the one of the lamps in parallel breaks, what ...
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1answer
398 views
Definition of Free Electrons and Mobile Charges?
Could someone please give me a good definition of the following electric terms? Despite what searching I have done, I have not come across a definition that I have found clear for me to understand:
...
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1answer
375 views
What is the capacitance of a superconductor?
I heard somewhere that the capacitance of a superconductor is much higher than regular conductors, but I haven't heard or seen anything yet proving or disproving such. So whats the truth? And why?
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1answer
2k views
Current against the inverse of resistance graph, $I = V/R +c$
If I have a plot of current ($y$ axis) against 1/Resistance ($x$ axis).
The circuit it is measured from is a simply 2 resistors connected in parallel to battery, where the potential across the ...
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4answers
3k views
domestic electric AC wiring - Live and Neutral correct or not? [closed]
On electric circuits, with $120$ or $230$ AC mains, there are the wires Live and Neutral.
The wiring isn't symmetrical : following the circuit, you cannot switch $L$ and $N$.
However, in Italy, ...
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2answers
262 views
how does a glow lamp work
First, what do you call this in English?
Second, how does it work?
Why do I have not only light at the gas barrier between the two wires?
Thanks!
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2answers
1k views
Transfer function of an RLC circuit
I'm trying to simulate an RLC circuit using transfer function. Circuit is there: http://i.stack.imgur.com/MC8ME.png (I'm a new user therefore I cannot post images)
But I can, L.Motl...
Main ...
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2answers
276 views
Maxwell equations: how to know the behavior of charge and current?
In school-level tasks, when (almost) all substances are linear, homogeneous and isotropic, we have $D=\epsilon E$, $H=B/\mu$ and thus Maxwell "in material" equations (1) say how $E$ and $B$ depend on ...
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3answers
83 views
Capacitor Charging and Discharging when connected to the ground
When we charge a capacitor using a battery and then remove the battery, the plates of capacitor becomes charged. One holds positive charge and the other one gets equal negative charge. o. k. ?
Now ...
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2answers
46 views
Light bulbs, Wattage meaning? [closed]
Light bulbs, Wattage meaning?
Two incandescent bulbs (120 V, 25 Watt) and (120 V, 500 Watt) connected to the same batteries.
Which one shines brighter? And why?
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1answer
52 views
Why do aqueous solutions always “have to be” electrically neutral?
I was reviewing some analytical chemistry and stumbled upon a section that explained the imperfection of using a salt bridge.
It said that the using dissimilar ions is a problem because in, for ...
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2answers
248 views
Why and how does symmetry work in circuits?
Why symmetry work in circuits? In my book there is no mention explanation as such for symmetry arguments and circuits. But there are circuits that are very difficult to solve without symmetry. Also I ...
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1answer
69 views
Photosensitivity of Photodiodes, lower wavelength -> lower sensitivity?
I saw a diagram of the photosensitivity (Current per Power) of a photodiode.
So there is this diagonal stating the 100% quantum efficiency.
I wondered why the sensitivity for bluer light lower ...
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2answers
309 views
Simulator for electrostatics
I am searching for a tool that visualizes the electric field/potential for simple electrostatic configurations like
conducting/polarized sphere in an uniform electric field
conducting/polarized ...
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1answer
678 views
When the electric field in an electrostatic charged conductor is zero, is the potential within also zero?
Let's say you have a point charge inside a conducting shell with an inner radius of 5cm and an outer radius of $7cm$. The point charge has a chard of $-4C$ and the shell a charge of $6C$. This means ...
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1answer
444 views
How can a portable 5V battery charge another 5V battery?
For a device that is portable and can charge an iPhone 4 or Nintendo 3DS:
http://www.amazon.com/New-Trent-iCruiser-IMP1000-Blackberry/dp/B003ZBZ64Q
I wonder, if this device outputs 5V, and iPhone or ...
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1answer
785 views
How to calculate the exit velocity of a coil gun projectile?
First off, what quantities need to be factored in? Voltage and current through the coils, the magnetism of the projectile, the magnetic fields, etc.? Next, how would you calculate the speed of the ...
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1answer
25 views
Why should a battery not give current in order to measure EMF?
Emf is the "potential difference (PD) across the terminals of a battery when it is giving no current to the circuit."
What does "when it is giving no current mean"? Will the PD across the terminals ...
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1answer
43 views
What happens when a compact fluorescent lamp implodes?
When a incandescent lamp implodes, the filament burns up and the current is interrupted.
Yesterday, a compact fluorescent lamp imploded and blew all the switches / fuses. I'm not sure if the same ...
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2answers
217 views
Electric potential at a point outside a charged sphere
The question asked me to find the potential at a distance $r$ from the center of a charged sphere, where $r>r_0$ of the sphere.
Actually, the question is answered, but what is confusing me is ...
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2answers
749 views
Physical interpretation of y-intercept in a Current vs Voltage graph
So I collected current and voltage data from a simple circuit with a power source and a resistor, using a multimeter. I created a graph for this data using excel and got the y-intercept (which is ...
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1answer
124 views
Finding the Electric Field (and other information, besides)
The problem I am working on is:
Two parallel plates having charges of equal magnitude but opposite sign are separated by 29.0 cm. Each plate has a surface charge density of 33.0 nC/m2. A proton is ...
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2answers
103 views
Under what condition charges do not flow in closed circuit?
I wanted to ask under what conditions will charges not flow in a closed circuit. Or when is current through the circuit zero even when an EMF is applied?
Like in the case of potentiometer, we say that ...
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1answer
57 views
Charged plane in an electric field acceleration
A perpendicular plane to an electric field's lines of force has more electric flux than a plane that is in parallel with the lines of force, right?
Does this mean that a charged plate would ...
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1answer
147 views
What does the current vs voltage graph of a halogen look like?
I have a halogen light with a tungsten filament. It is rated 500w at 130v but I am running it from an inverter in my car that is putting out 110v. I would like to know what the actual power usage of ...
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1answer
114 views
Electromotive force
EMF is defined as:
"The potential difference across the terminals of a battery or dry cell when it is giving no current to the circuit."
So, if current starts to flow from the ...
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1answer
115 views
Loopy lightning
What causes lightning to follow the path it does ?
picture from BBC news: http://news.bbcimg.co.uk/media/images/62891000/jpg/_62891901_untitled-1copy.jpg main page: ...
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2answers
144 views
Is it really to solve problem below by using, in the main, Gauss law?
There is an infinite cylinder surface which uniformly charged along and has a surface charge density, which can be represented as
$$
\sigma = \sigma_{0}cos(\varphi ),
$$
where $\varphi$ - polar angle ...
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3answers
2k views
Average power dissipated by a resistor on AC current
So let's say we have an AC current of 120 V at 60 Hz. Then i's waveform would be
$$f(t) = 120 \sqrt{2} \cos(2 \pi 60 t)$$ Or rather the amplitude times $\sqrt{2}$ times $\cos(2 \pi ...
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1answer
204 views
Solving by using Gauss law
Task: find the vector $ \mathbf E $ in the center of the sphere with radius $R$, which has charge volume distribution $\rho , \rho = (\mathbf a \cdot \mathbf r ), \mathbf a = \operatorname{const}, ...
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2answers
288 views
How to charge an object with electricity
I know this is a rather basic question, but how do you charge an object? Not a battery, an object. I'm guessing it involves static electricity, but I'm not sure. Some resources I've been reading talk ...
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1answer
1k views
What is the approximate electrical conductivity $\sigma$ of graphene in S/m or S/cm?
I am trying to find an approximate value of the electrical conductivity $\sigma$ of graphene in units of S/m or S/cm. This table on Wikipedia gives $\sigma$ values for a variety of materials ...
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1answer
183 views
confusion about finite line charge
My physics knowledge is (sadly) very infinitesimal and I have a question about a calculation which I think is wrong, but want to be sure.
I tried to calculate the potential of a finite line charge. ...
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1answer
141 views
Residual Resistivity in alloys and metals
Residual Resistivity
I saw that the graph of resistivity to temperature of alloys like nichrome is like so
Meaning that even at 0 K it has some resistivity just like copper :
I read some where ...
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1answer
131 views
Toroid with variable current and Ampere's Law
Say you have a current $I(t)$ (notice the time dependence) flowing through a Toroid with $N$ total loops and all the usual approximations: $(b-a) \ll r,\; B=0$ outside.
You are asked to calculate ...
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1answer
161 views
What is lambda R in Richardson's Law?
I've got to calculate the thermionic emission through a diode, so I need to use Richardson's Law. However, one thing's got me confused - according to the Wikipedia page:
$$J = A_GT^2e^\frac{-W}{kt}$$
...
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1answer
343 views
How do home-made capacitator bottles affect a tesla coil?
In the tutorial I'm following, the guy uses 6 snapple bottles to make capaciter (by covering them with foil and filling them with salt water) for a 9kV 30mA input. I have a stronger transformer (15kV ...
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2answers
761 views
Measuring the Electromotive Force and Internal Resistance of a Battery?
Here is a problem that emerged some time ago:
We needed to measure the emf(electromotive force) and internal resistance of a car battery. The only instruments were voltmeter and ammeter.
...
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1answer
1k views
When does voltage drop occur?
Why or when does it occur in a circuit? What does it imply when you speak of a voltage drop across a resistor? (Obviously, it probably means that the current's voltage before the resistor is higher ...
