What is Elecricity: Definition and 97 Discussions

A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost fully segregated from the outside environment. Third rail systems are always supplied from direct current electricity.
The third-rail system of electrification is not related to the third rail used in dual gauge railways.

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  1. M

    Could there ever be a chromo-electric field weapon?

    In Star Trek, they mention a Chromo-electric pulse and Chromo-electric force field. I know chromo-electric fields hold the nuclear force together with gluons but could a highly advanced civilization use chromo-electric fields for weapons or protection?
  2. agatha

    Electronic Circuits - Calculate voltage

    TL;DR Summary: Electronic Circuits How do I calculate the voltage v1 using KCL? I don't understand how I should express the current I3 to get the equation right. I already watched this video but I don't know what to do when there are two resistors.
  3. D

    I How does the velocity of electrons depend on the voltage across a conductor?

    How exactly (what is the function?) the velocity of the electrical conductor's electrons depends on the voltage of the conductor?
  4. S

    B Single phase induction motor Torque under increased frequency and Same Current

    Hi , Consider Single phase induction motor. What will be torque of induction motor, if we increase supply frequency 10 times, if the motor uses same supply current. Let's say Torque (Supply frequency 50 Hz, Supply current 10 Amperes ) = N What is Torque (Supply frequency 500 Hz, Supply...
  5. T

    I Why is not possible to store electric energy from a Lightning?

    That question has been puzzling me recently. But I know there is a simple answer to it.
  6. jackkk_gatz

    Electric heat generation from parallel rods carrying current in an oil bath

    I know that ##\dot e_{gen} = \frac {R_{e}i^2} {\pi r^2 L}## the thing is I don't know the value of i. I didn't write it, but another thing that the problem asks is to determine the flow of electric current. The model I came up with using heat generation due to electrical current is $$ \frac...
  7. P

    I What is the benefit of measuring both direct and reverse current?

    What is it's benefit?
  8. sinus

    I The Method of Images (Electromagnetism)

    Can anyone explain to me why grounded means zero electric potential. I confuse what's the relation between infinite ground conducting plane and its electric potential (the method of images). I have a several question: 1. Why the conductor plane must be infinite, while in reality there's no...
  9. C

    Finding Area of Ring Segment to Find Electric Field of Disk

    Hi! For this problem, Why is the area of each ring segment dA equal to (2π)(r)(dr)? However, according to google the area of a ring segment (Annulus) is, Many thanks!
  10. Viona

    I A smaller cross-sectional area increases the resistance of a conductor?

    why smaller cross-sectional area increases the resistance of a conductor?
  11. samy4408

    I Why in an electric circuit with only a generator the resistance R=infinity?

    Why in an electric circuit with only a generator the resistance R=infinity? [Newbie's post edited by a Mentor to delete extraneous embellishment]
  12. A

    Do concentric coils steal power from one another?

    The amount of power generated by a coil in a changing magnetic field is determined by the area perpendicular to the field, the field strength, and how quickly those values change over time, and is multiplied by the number of loops in the coil. But that made me wonder what would happen if one...
  13. A

    Electromagnetism - movement of a coil in a magnetic field

    So my idea was that to reach the equilibrium position, the final moment of force has to be 0 (so in the end the forces will “eliminate” each other). And I found the equation Fm=B*I*l*sinα, which should characterize the force, which affects wire with the current in a magnetic field, and Fleming’s...
  14. Raz Matazz

    I Why do electrons never stop moving?

    Whether in vacuum inside a CRT, or in a metal, a potential difference has to be applied for free electrons to move from the negative to the positive terminal. However, the reality is that free electrons are still moving when no voltage is present at the 2 ends of a metal conductor. In this...
  15. Kaushik

    Facing issues in understanding a Purely Inductive Circuit

    Consider a purely inductive circuit with an inductor of self-inductance ##L## and sinusoidally varying AC source of peak voltage ##V_{m}##. First of all, Why is ##V_{ac} = ε_{ind}## (where ##ε_{ind}## is the back emf)? Now, at ## t=0 ##, voltage is increasing at a fast rate. Hence, due to the...
  16. curiosissimo

    Using Kirchhoff's laws on this current division problem

    The main problem is the verse of the 2 currents ## i1 ## and ## i2 ## . I think they both go clockwise, so in the node A we should have ##i1+i2=20A##. Now let's apply the second law (going clockwise). So ##ΔV1-R1*i3 - R2*i4 + ΔV2=0##, where ##i3## and ##i4## are the currents which pass through...
  17. halleff

    Physical/conceptual reason for inductor voltage step response

    I'm trying to understand the physical reason why when you drive an ideal inductor (no series resistance) with an ideal voltage step input (no series resistance), e.g. some Vin(t) = V0u(t), the output current will be a linear ramp. I can see how to derive this from the inductor equation, v = L...
  18. gumthakka

    Current and stationary charges

    I've learned that moving charges produce magnetic fields which in turn affect other charges in motion. After seeing explanations that point to special relativity, I am kind of confused. Can **ALL** magnetic fields be accounted as some kind of electric field from a particular reference frame...
  19. N

    Question about electrical networks

    Suppose we have a connected electrical network with only resistors. We also assume that each vertex is connected to at least ##k## vertices, where ##k\geq2##. Suppose all resistors in the network has resistance between 1 and 2. Suppose we flow 1 unit of current from vertex ##a## to vertex ##b##...
  20. S

    Finding Equivalent Resistance Using Symmetry and Kirchhoff's Laws

    By the use of symmetry, I have found that equal currents will flow through opposite edges like A-C and C'A', but still, the use of kirchhoffs current and voltage laws lead to four equations in four variables which I am not able to solve, neither I am sure that the four equations which I have...
  21. M

    Two batteries and two resistors circuit - wrong possible answers ?

    It's ABCD question with answers attached. Problem is that, I don't think that neither of those are correct. From KVL the equation seems to be i = (E2-E1)/(2r1+2r2 + R). I've attached images of the circuit, answers, and my solution. Can someone please point me if I am wrong, and if yes, where's...
  22. xWaldorf

    How does a current "know" where to go

    Let's say we have a circuit, and in that circuit there's a resistor, and there's a wire that goes around it so that the current can flow freely without getting to the resistor. my question is this: how does the current "knows" to flow towards the around the resistor? how does it know not to get...
  23. J

    Interaction energy of two interpenetrating spheres of uniform charge density

    I am trying to calculate the interaction energy of two interpenetrating spheres of uniform charge density. Here is my work: First I want to calculate the electric potential of one sphere as following; $$\Phi(\mathbf{r})=\frac{1}{4 \pi \epsilon_{0}} \int...
  24. nineteen

    What is the standard definition for what an "electric source" is?

    The syllabus states : "Explain that the potential difference which is needed to apply across any electric circuit to flow current is supplied by an electric source". To explain this and make notes, I want to first understand what an electric source particularly is. And how all those electric...
  25. D

    Physics Homework about using Electricity to Heat Flowing Water

    This is the question: You want to make an electric instantaneous water heater in which 5.0 liters of water flows past a resistance per minute and heats water from 10.0 ° C to 45.0 ° C. Calculate the magnitude of the resistance to use and the amperage. The flow-through is connected to 230V So...
  26. J

    Point Charge above an Infinite Plate

    Merged thread deleted first post next post is the first post.
  27. yakhassan

    I want to study electrical electronics but i don't know about it

    Summary:: I am a student and i plan to study electrical electronics but i don't know how to go about it.Anyone who is into it can you please guide me on how to become one I am a student and i plan to study electrical electronics but i don't know how to go about it.Anyone who is into it can you...
  28. O

    R_th wrong while applying series and parallel simple theory

    Summary:: Trying to find Rth but I do not get the same value as the one from the solution. [moderator: moved from a technical forum. No template.] I am trying to find Rth to solve this problem, however once I simplified it, I get a value of 700.745 Ω while in the solution, the answer is...
  29. N

    Finding the currents in this circuit (2 voltage sources and 3 resistors)

    In my attempt, I tried 1) I1 = I2 + I3 Then set up these two equations based on Kirchhoff's second rule: 2) U1 = R1 (I2) + R3 (I3) and 3) U1 + U2 = R1 (I1) + R2 (I3). From what we have 10 = 0.1* I_2 + 0.2 * I_3 22 = 0.1* I_2 + 0.02*I_3 I_3 = 50 - 0.5 I_2 That means I_2 = 233.3 A I_3 = -66.7A...
  30. bagasme

    Wheatstone Bridge: Substitution Resistance Formula Derivation?

    Hello, In high school, I had been taught about finding substitution resistance from Wheatstone bridge. The formula: a. If the cross product of ##R1## and ##R3## is same as ##R2## and ##R4##, the galvanometer in the middle (##R_5##) can be omitted and use series-parallel principle to solve for...
  31. bagasme

    B Different Sign Conventions used with Kirchhoff's Law?

    Hello, As far as I know, the standard sign convention for Kirchhoff's law are: For resistor: the potential difference (voltage) is negative if the chosen loop direction is the same as chosen current direction through the resistor, positive otherwise. For battery: the voltage is positive if the...
  32. Fdtroya

    Node Counting Circuit for Standing Wave Generator

    Summary: Make a circuit that counts the number of nodes in a standing wave generator I have a an assignment to complete this Term, the assignment consist in making a machine that would produce standing waves in various frequencies, (for example the typical motor connect to a string with...
  33. peace

    Mutual inductance between a very long, straight wire and a Semicircular loop

    I think I have to assume a point like P in the semicircle. The point in terms of r and θ: P (r,θ). So the magnetic field at that point: B = µI/2π(R+rcosθ) . So the magnetic flux: Φ = ∫B.dA= µI/2π ∫∫ rdrdθ / R+rcosθ . Is this the correct solution?
  34. P

    Why is the Lorentz force linear with velocity?

    I am working on derivation of Lorentz force. (I know that Lorentz force is in some sense definition of fields, but still there is nontrivial dependence on velocity). I want to derive that the force is linear in components of velocity, so for example $$F_x=q(E+Av_x + Bv_y + Cv_z ),$$where ##A...
  35. maxipoblete

    Capacitance of a circular capacitor with two parallel dielectrics

    Hello, the problem is better illustrated at the picture below. The capacitor is isolated, with an initial charge Q0. I understand that Q0 does not distribute along the plates homogeneously. How could it be solved with the equivalent parallel circuit?
  36. Eugene567

    What forces are acting on electron in a circuit?

    What forces are acting on electron in a circuit? What is the source of such kind of forces? Do electrons act on each other in electric current? If i have an ordinary electric circuit with only one resistor, then if i raise resistance, the electric current goes down. As I understand it happens...
  37. Eucliwood

    Why does the voltage across a capacitor start from zero and then start rising?

    I suspect that at first the electrons on the other side of the plate in the capacitors are pushed to the light bulb that is the plate becomes more positive to compensate for the increase of electron on the other plate. Was this right? And can anyone refer me to a study or law regarding it?
  38. Kasiopea

    Why is the x-component of the electric field at point P positive?

    I got the answer: E(P)=(k*Q)/(3î )−(k*Q*ĵ) /2 Can someone confirm if this is the right answer?
  39. Renewable_energyguy

    Making electricity by means of my old bike

    going off the summary I want some feedback and any opinion that may improve this project, please and thank you.
  40. cianfa72

    Electric potential difference between a battery's + terminal and the ground

    Hi, I've a question about electricity in the following scenario: consider an accumulator (e.g. a 9V battery) and an analog/digital voltmeter having a probe connected to the accumulator + clamp and the other to the ground (for instance connecting it to a metal rod stuck in the ground). Do you...
  41. B

    A question about magnetism that causes a wheel-loop to rotate

    This question is from 1977 AP Physics C so I suppose it would be clear enough, but I am confused about question c. Question a is easy (it rotates counterclockwise), question b too (Στ=6*rxF=6*r x (I*i x B)=0.06). Question C is where I am stuck. The diagram provided with the question looks like...
  42. Tiqo

    Electrical Repair of an old JJ LlOYD T20K tensile machine

    Hello, I am currently a mechatronics engineering student, I am working on a school project: retrofitting a tensile machine. The goal of the project is to improve the human-machine interface as it is a very old machine, it is a JJ LOYD T20K, my problem is that the machine no longer works(it...
  43. N

    Deriving Voltage from the Inductor Equation

    Homework Statement L = 20mH = 20 x 10-3 H i = 40 mA for t≤0 i = A1e-10,000t + A2e-40,000t A for t≥0 The voltage at t=0 is 28 V. I have to find the equation for the voltage for t>0. Then I have to find the time when power is zero. Homework Equations v(t) = L* di/dt p(t) = L*i* di/dt The...
  44. Aya Elsayed

    I Grounding Process: Neutralizing Negatively Charged Objects

    Let us consider a negatively charged object. We want to neutralize this object, so we touched it by our hands to remove the excess of negative charges by Grounding. Now the question is : Will our body (The Ground) be negatively charged as it gained those negative charges? then why it is called...
  45. T

    How does mechanical torque affect generator volts and amps?

    Hello! today I have a question regarding the most efficient way to generate electricity by moving a magnet across a coil. The theoretical magnet would be moving in a circular motion fixed to the outer circumference of a driver wheel like in many simple generators, how does the physical torque...
  46. K

    A challenging capacitor problem

    1. Two identical flat plate capacitors are inserted in each other. First, none of the plates was charged, but then they have been connected to sources of current, keeping the constant voltage ## V_1 ## and ##V_2 ##. Find the potential difference between inner plates, which are kept at the...
  47. Mutatis

    Find the electric field at an arbitrary point

    Homework Statement A distribution of charge with spherical symmetry has volumetric density given by: $$ \rho(r) = \rho_0 e^{ \frac {-r} {a} }, \left( 0 \leq r < \infty \right); $$ where ##\rho_0## and ##a## is constant. a) Find the total charge b) Find ##\vec E## in an arbitrary point...
  48. P

    What is the most efficient way to create steam with the desired properties?

    Hello! new here. recently I've developed an interest in the way electricity is produced, I've seen some mechanisms, most of them are steam turbine based. i have a few questions about the topic, mainly about thermodynamics. if i want to create enough energy to move a steam turbine that requires...
  49. T

    Confused by capacitors and potential energy

    HiI was reading about capacitors and potential energy. But the equation seems counter to how i thought. For potential energy you have: U = Q^2 / 2C or U = CV^2 / 2 But doesn't this suggest you lose potential energy the more capacitance you have? Since in the first equation as C increases U...
  50. V

    A few questions concerning voltage, current and resistance

    [ mentor note: post adjusted to conform to homework template and some spellings fixed ] 1. The Problem I have some few questions that connect voltage, current and resistance. Imagine that we have a circuit that consists of a battery (5V), 2 resistors. Now once we close the circuit, battery...
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