The study of the presence and flow of electric charge. Charges, currents, fields, potentials.

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207 views

Why does a fluorescent tube-lamp flicker before lighting up?

If you turn on a fluorescent tube-lamp, it flickers before lighting up. If you then turn it off and turn it back on after 2 seconds, this time it doesn't flicker but lights up straight away. If you ...
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2answers
649 views

Solving circuits using kirchhoff's laws and elimination method ( not any matrix method) [closed]

I need to solve this problem as part of my review in college physics. The task is to find the value of 5 branch currents using Kirchhoff's laws and elimination method (or maybe called elimination by ...
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1answer
72 views

Some basic questions related to electricity [closed]

Note These questions may be incorrect. 1) Why do positive charges flow from high potential to lower, but the electrons do not? Am I wrong? 2) Does battery supply charge to a conductor? I read it ...
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0answers
33 views

What mechanisms require the use of different polarities in DC welding?

In gas metal arc welding, an electric arc forms between the work piece and a consumable wire, heating the work piece and also melting the tip of this consumable wire, which is continually fed into the ...
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1answer
71 views

How dangerous is a parallel plate capacitor which is used in physics demo experiments?

How dangerous may it be when experimenting with a parallel plate capacitor typically used in physics demonstration experiments? Take for example this one (that's what I use): ...
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1answer
38 views

Using a lemon's energy [closed]

Some simple exercises i need some confirmations on. Let's say a lemon has U = 1V and an average current of 0.1mA for about an hour. Q1. How much lemons do you need to light a 100W bulb. Is the ...
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1answer
122 views

Can I feel (the results of) electromagnetic induction from overhead rail lines in the human body?

A few weeks back, I was standing at a Hornsey rail station (in the UK) which uses overhead lines, and particularly has a number of parallel rail lines, all close to the platform. While I was standing ...
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2answers
58 views

how to model the exponential growth of luminosity when you flick a light on

My original thought was: $$ L = L_0 \left( 1 - e^{-t/\tau} \right) $$ where $L$ = luminosity and $L_0$ = initial luminosity. But when I tested this I found that the time constant $\tau$ was higher ...
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1answer
87 views

What is the maximum theoretical efficiency of heat to electricity conversion?

I know that heat engines (heat to kinetic) are limited by Carnot cycle and that kinetic energy to electric energy conversion via standard generator reaches over 90%. However I would like to know ...
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1answer
109 views

Questions about Van de Graaff generator

Only asking those questions because I can't afford to get a real generator. If the voltage on the sphere was 50 kV, then I used a metal rod to discharge it and I connected a voltmeter to wire ...
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1answer
89 views

Why touching tooth filling with aluminum foil feels so weird?

I have read that there is and electrical current, but why and how? How big can this current (and voltage) be (does an amount of foil, saliva, tooth filling have any influence on this)? Can it "dry ...
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1answer
72 views

What happens when the frequency of A.C becomes infinite?

In an alternating current, the flow of electric charge periodically reverses direction, and the number of times it does that is called the frequency of the current. However, if the frequency of an AC ...
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1answer
93 views

Shot noise at high frequencies (can it really be ideal white noise?)

Quantum shot noise (either optical intensity noise or electrical current noise) described by the noise spectral density of $2 e I$ (electrically) or $2 h \nu P$ (optically). So it is white noise. I ...
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2answers
123 views

Migration of an ion in a non-conductive medium

Someone asked me why, when performing DNA electrophoresis we need to put the gel inside a conducting buffer. Couldn't we just run it in distilled water? My answer was that if we had distilled water ...
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0answers
39 views

Does the presence of atoms hinder current flow?

When current flows through a material, electric resistance is observed when the electrons collide with the material's vibrating atoms. My textbook says that when we cool the material till the point ...
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3answers
151 views

Why is the field inside a hollow sphere zero?

Say you have a hollow sphere with a uniformly distributed charge on the surface. Why is the electric field everywhere inside the sphere zero? For the centre, its easy to add the vectors from the ...
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3answers
82 views

electron flow in capacitor with dielectric

if I make a circuit with a battery and a capacitor (with a dielectric inside), how it is possible to get a current in the circuit? If electrons go from one pole of the battery and they arrive to one ...
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1answer
38 views

Capacitance of a free conductor

Consider a ideal conductor in free space.For all purposes here,the zero of the potential is taken at infinity. Suppose I give a charge $Q$ to the conductor.As a result,the conductor will have a ...
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5answers
116 views

Why is electric potential 0 in this case?

On a test, we had a question where there are 4 point charges at the vertices of a square. The 2 charges at the upper vertices have charges of +q and the 2 charges ...
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0answers
33 views

Capacitance in a capacitor

This here is a question that I just don't know what to do with. I don't know what I'm doing wrong. "A plate capacitor consists of two plates with area 2.5 cm$^2$ at a distance of 1.6 mm from each ...
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2answers
138 views

Will I get a shock when I try to use my hair-dryer under water?

Occasionally people get killed in their bathtubs by having an electrical device such as a hair-dryer take the bath with them - in movies. It seems to be a common belief that this is realistic, even ...
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1answer
68 views

How magnets create electricity in conductors?

what are the reasons for current appearing in a wire when wire is in a changing magnetic field?
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1answer
84 views

In a positively biaed PN junction, where do the injection carriers come from?

I am not quite understand i-v character of PN-junction diode. Here is the model in textbook. The PN junction diode can be divided into three regions. They are One depletion region near the PN ...
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2answers
97 views

Total Resistance in a Tetrahedal loop [closed]

Imagine if I placed resistors of 1 ohm each in shape of tertahedal. What would the total resistance be between A and D? I can't find any website that gives me a good explaination.
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2answers
103 views

What actually happens at the microscopic level in a Zener diode in reverse bias?

A Zener diode is used as a voltage stabilizer. The graph of current vs voltage of Zener diode clearly shows that there is a constant voltage across Zener after the breakdown voltage as the current ...
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4answers
201 views

Why don't positive charges move?

Charge is carried by electrons moving. The protons are always stationary. The answer I found online is the protons are stuck in the nucleus so they can't move ("strong nuclear force"). But why can't ...
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2answers
150 views

Rubbing a rod with silk?

Rubbing a glass rod with silk causes charges to be exchanged and consequently both objects get charged. Why do the objects have to be "rubbed"? I get that one has a stronger pull on the electrons ...
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2answers
38 views

Capacitance and energy

So I've been reading about capacitance (Specifically in capacitors) and I'm wondering if this reasoning is sound, and why/why not: C = Q/U U = Q/C We also know that: U = E/Q So: E/Q = Q/C Q^2 = ...
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3answers
58 views

Radiation at plate of vacuum tube

Say you have a vacuum tube, such as the kind used in old amplifiers, wherein electrons are accelerated from the cathode to the anode through an electric field. Presuming, for the sake of argument, ...
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2answers
97 views

Electric charge is lorentz invariant

I know that electric charge is lorentz invariant quantity and I can easily think of experiment to check that. Is a though experiment that can prove that also?
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2answers
72 views

How to convert RL Circuits? [closed]

How do we convert a parallel RL circuit into its equivalent series RL circuit ?
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3answers
105 views

Charge signs in current

I've had recently an argument with my friend about different charge carriers in an electric current. Suppose that electrons and holes are moving in the same direction. It effectively means we have ...
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1answer
51 views

Objects made up of electrons?

Say you have a neutral rod, and you bring a positively charged rod beside it (call the side the charged rod is brought near side A and the other side side B). The electrons from the side B will start ...
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1answer
36 views

Electromagnetism Permittivity simple question

Just a simple question regarding $\epsilon$, the absolute permittivity. I read that it measures the resistance to of certain medium to 'permit' the formation of an electrical field. Does this mean ...
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3answers
65 views

How can you have positive electricity

In a circuit, you have two opposite charges. One would be negative, which would consist of electrons. The other would be positive charge, and protons carry this. But protons aren't mobile, so how does ...
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1answer
139 views

Circuit with inductor and lightbulb when switch is turned off?

I want to know what happens with a lightbulb immediately after a switch is turned off. We're talking about a circuit with a battery (AC), an inductor, a lightbulb and a switch. I need to know it in ...
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0answers
101 views

How does the tester screwdriver work? [closed]

How does the tester screwdriver work? If I put the tester screwdriver inside a "phase" hole of an electric socket, it lits up. If I press my finger against the metal cap on top of the screwdriver. ...
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1answer
515 views

Why is stainless steel a poor conductor of electricity?

I recently had a metal plate put in my shoulder and was wondering why stainless steel isn't a good conductor (At least I hope it isn't). Does the alloy just lack free electrons? Why is that?
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1answer
77 views

what is the mechanism of water conductivity?

I'm trying to wrap my head around the kelvin water drop generator and more specifically what happens in the water before it drops. If on one side the drops of water are positive and on the other side ...
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3answers
161 views

How to calculate Standing Waves in Electrical Cables?

I have a 20 metre Coaxial Cable. I send digital signals down the cable ranging from 5 KHz to 50 KHz. I have noticed a pattern in the noise ratio, an oscillating wave. I predict this is to do with ...
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1answer
173 views

Would Beetee's electrocution plan work and kill the tributes in the lake?

In Catching Fire, the second episode of the Hunger Games trilogy, one of the characters (Beetee) proposes a plan to kill some of the remaining tributes. He proposes wrapping a special wire that he has ...
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3answers
114 views

Why do I still get static shocks when hands are wet?

Quite a few times now I have washed my hands, and while my hands are still wet I will touch something metal and get a static shock. I thought static electricity doesn't really build up when there's a ...
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2answers
84 views

Batteries connected in Parallel

"If we connect two batteries in parallel, the battery with higher voltage will discharge into the other one." What is the intuition behind this?
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1answer
33 views

Electrode potential

If I connect a zinc half cell (Left hand side) and a copper half cell (right hand side), I get an E cell value of 1.1V taken from the voltmeter. If I replace the voltmeter with a light bulb, I will ...
5
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3answers
188 views

Why don't we use beta radiation for making electricity?

Why don't we use beta radiation to make our electricity? Since it is an electron being released/made, it would make sense to try and capture it. But, we obviously don't so. What am I missing in my ...
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2answers
146 views

Physics of batteries (volts vs charge)

Car batteries are usually 12 V. What is the difference between buying a car battery and hooking up a bunch of cheap household batteries in series? Both would register at 12 V. I assume that cars need ...
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1answer
82 views

How to calculate cost of AA accumulator charge? [closed]

I would like to calculate how much it will cost to change AA accumulator. Is it really cheaper than usage of batteries (don't consider ecology question as of now)? Let's say I have the following ...
3
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2answers
268 views

Why is there no current flow when i connect two opposite terminals of two separate batteries

I don't understand this..maybe there is something wrong with my understanding of battery or electricity. I am learning about electricity more deeply this time. So please spare with me... So lets ...
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3answers
420 views

What happens when a charged (negative or positive) object touches an insulator?

I know that insulators do not conduct electricity because they do not allow the free movement of electrons. Let's assume the object that's going to touch the insulator is negatively charged. Does some ...
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1answer
360 views

Electron flow and electric potential

In the physics tutorial topic on current electricty it defines the positive terminal as the high potential terminal and the negative as the low potential. When talking about positive test charges this ...