Current Definition and 1000 Threads

An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving particles are called charge carriers, which may be one of several types of particles, depending on the conductor. In electric circuits the charge carriers are often electrons moving through a wire. In semiconductors they can be electrons or holes. In a electrolyte the charge carriers are ions, while in plasma, an ionized gas, they are ions and electrons.The SI unit of electric current is the ampere, or amp, which is the flow of electric charge across a surface at the rate of one coulomb per second. The ampere (symbol: A) is an SI base unit Electric current is measured using a device called an ammeter.Electric currents create magnetic fields, which are used in motors, generators, inductors, and transformers. In ordinary conductors, they cause Joule heating, which creates light in incandescent light bulbs. Time-varying currents emit electromagnetic waves, which are used in telecommunications to broadcast information.

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  1. jisbon

    A metal bar moves upwards near a long current carrying wire....

    Not sure what to do here, but I do know that magnetic field is pointing in the paper according to the right hand grip from current carrying wire. Hence I can calculate magnetic field to be: ##B = \mu_{0} I/2\pi r=(4\pi * 10^{-7})(50)/2\pi (0.004) = 3.94784*10^{-7}T## Now from what I...
  2. F

    What is the maximum current in a superconductor loop?

    If we put an emf equal constant onto a superconductor loop(we consider the resister of emf equal zero), then what is the maximum of current in the loop?
  3. gdritz

    Molten metal temperature vs Induction Coil Current

    Hi guys, I'm new here. I have been analyzing the behavior of cast iron melts in induction furnaces and realized that at the end of the melt the current increases slightly. The power remains the same. Physically, does anyone know the reason? I imagine to be related to the magnetic permeability...
  4. bob012345

    Energy vs. Time in a Magnetic Field due to a Current Carrying Wire

    I want to know the total energy contained in a magnetic field due to a long wire (just consider a 1m segment) as a 1amp current is turned on starting at time zero. I'm assuming zero turn-on time for convenience. At t=0 the cylindrical field is formed and I wish to know the total energy as a...
  5. P

    High Current 12V DC motor speed control

    Hi. I am new here. I have a question on how to speed control a DC motor running a pump. The pump is a seawater cooling pump cooling a large V8 motor. Therefore I need to control the RPM, based on the temp of the V8 Motor. Any ideas?
  6. D

    Ranking Current Densities: How Do Different Factors Affect Current Flow?

    Homework Statement: Rank in order, from the largest to the smallest, the current densities in these four wires, that have different radius and different electric conductivity Homework Equations: current density resistivity and resistance i guess ja=I/πr2 jb=2I/πr2...
  7. S

    Reasons for the current FHA scrutiny of gifts?

    Recently, I've encountered two examples of how the Federal Housing Adminsitration requires banks to scrutinize borrowers who acknowledge receiving gifts of money to help purchase a house. The banks require an "affidavit of gift" document from the giver and a copy of a bank statement showing...
  8. A

    Rectified Current Through an Inductor

    Hello, I need to find the magnetomotive force (mmf) of an inductor using NI (turns multiplied by amperes). The set up is pretty simple, AC power supply set at 24 VAC goes through a full wave rectifier (4 diodes) and into an inductor. If the inductor has a resistance of 47 Ohms, it looks like...
  9. kolleamm

    High current power supply for servos? 40+ amps

    So I've basically built a robot that has a lot of servo motors. 14 of them can draw up to 3 amps each(max)(usually 1 amp average) at 4.8 to 7 volts. What's my best option to power it? The power source doesn't need to be connected to the robot, so weight shouldn't be an issue. My best guess is...
  10. Auto-Didact

    A Constructive QFT - current status

    I haven't been up to date on the state of the art of the field for quite some years now; the contemporaneity of my knowledge ends with the review by Rivasseau, 2000. A quick gander at the topic over at the n-Cat Lab shows that practically nothing has changed. Is anyone working in the field here...
  11. I

    How to find the Braking Torque of an Eddy Current Braking Setup?

    Homework Statement: Finding the braking torque applied on a moving disk as a result of the Eddy Currents Homework Equations: $$F = \sigma v V B^2$$ So right, Basically my problem is how to find the torque exerted by an eddy current braking setup. My setup consists of a disk rotating on an...
  12. dvscrobe

    Engineering Reverse Current PMDC Motor Braking Using Power Electronics

    Summary: How do you determine the armature current during the braking time? For a simple permanent magnet DC motor with a known resistance and inductance in series with the armature, I would like to know how to determine the armature current if there was an electronic motor drive controlling...
  13. N

    Why Is the Current Density Inside a Sphere Constant?

    Since sphere is made of l.i.h material, $$\vec{J_f}= \sigma \vec{E}$$ We compute electric field E using $$\vec{E} = -\nabla V$$ $$= -\nabla \left(V_0cos\theta\right)$$ $$= -\frac{\hat\theta}{r}\frac{{\partial}}{{\partial\theta}}\left(V_0cos\theta\right)$$ $$\vec{E}=...
  14. kolleamm

    How can I change voltage instead of current?

    I have a micro controller that reads voltages from 0V to 5V I did some tests on a potentiometer and I couldn't seem to get any readings of values in between. (range is 0-255 ,, 0 is for 0 volts and 255 is for 5V) Therefore I'm assuming the voltage is staying the same as it turns but the current...
  15. amjad-sh

    A Generation of spin and charge current due to interference and SOC

    My question may include condensed matter physics concepts but my main question is related to quantum mechanics in general, that's why I posted it here.In fact I'm working on an condensed matter physics paper, where we are dealing with a two-metal system. The interface between the two metals is...
  16. Efeguleroglu

    Why RMS for the average voltage in AC current?

    We write rms formula as $$v_{rms}=\sqrt\frac{\int_a^b[f(x)]^2dx}{|b-a|}$$ I know if we take arithmetic mean average voltage will be 0. So we want all voltage values to be positive. Why don't we do that: $$V_{average}=\frac{\int_a^b\sqrt{[f(x)]^2}dx}{|b-a|}$$ That's first what I did...
  17. J

    Voltage divider rule for CMOS in 0-state and leakage current

    I use the voltage divider rule as output voltage = Voltage across R1 = 5 * ( 0.5 / (0.5 + 20) ) This comes as 0.122 V I'm not sure why they've mentioned I leakage and does 0-state have any impact on answer.
  18. Physics lover

    Current density as a function of distance from the axis of a cylinder

    I first took out the variation of conductivity along the radius of cylinder.Also we know that J=sigmaE.Therefore i have to find variation of E also.But how will i find that as potential is also not given.Help.
  19. Oannes

    Finding the Current Through a Resistor (Working With Parallel and Series)

    Here is the actual question. And here is my attempt at a solution In Summary I did the following Found the Equivalence Resistance to Be 5.9 ohms and the Current throughout the entire resistor to be 1.53 Amperes Worked backwards from my resistor simplifications. When the resistors were in...
  20. H

    Current carrying capacity of a copper busbar in an Aircraft

    "The surface of copper naturally oxidises, forming a thin hard layer on the surface which normally prevents further oxidation. " I've seen this in a book, higher level oxygen on busbar, means lower the heat and get away with ease, Doesn't it? So, Can I say a thin hard layer musn't be used?
  21. A

    Time dependent current input in Ansys Maxwell 3D

    Hi guys, I am new to this phorum and Ansys Maxwell as well. I am trying to pass as an input a time dependent current, but I really do not know how to do it. Does anyone know how to fix this issue? Cheers, Alessio
  22. og_ogun_srvs

    Current or Power Reference Tracking with Power Electronics Converters

    Hi. This is my first post. Thanks to all who give it a look. I am working on an application that has very stringent power output requirements. There is a rechargeable battery that I am trying to manage through a bi-directional boost/buck converter as is typical of many modern-day electric...
  23. Raffallo

    Calculate the max current for a powerline

    Hello All! I'm new in this forum, so if I select wrong subject or something then sorry. I'm very, very far from physic, so I need your help. In work, I have to calculate max current for HV power line. As input have conductor temperature [*C] and material parameters (the resistance in ohm of the...
  24. Terrycho

    (Bipolar Junction Transistor) The Current Gain of Common Base Amplifier

    Everything is okay besides the current gain of this circuit, which is "g" there The formula of current gain is I_out/I_in If I call the current flows to R_e I_2, according to KCL, the following formula must be satisfied. I_in + (β+1)I_b = I_2 Therefore, to know I_in, we should use the...
  25. A

    Current in coupled loops (transformer design)

    I think I know the answer,but just to make sure. I have two coupled inductors through a core that provides better flux permeability, aka a transformer, I provide say 5v and 1000 amps AC to the primary loop, in my secondary loop I have a capacitor in series with the loop (of sufficient...
  26. Simpcar

    Discharge in a DC RC circuit and negative sign of current

    Hi everyone! I'm trying to understand why when writing the differential equation of a discharging RC circuit V_C-Ri(t)=0 \to q(t)/C - Ri(t)=0 we replace i(t) with -\frac{dq(t)}{dt}. I read many threads but I don't understand the physics behind this. The usual answers I read are something like...
  27. M

    COMSOL: How to relate terminals in electrical current?

    hello, i have a problem in comsol multiphysics, i want to relate many terminals to each other to buid a current circuit in 2D geometry. I was thinking to simulate the capacity between winding wire of a coil. So, I have to choose boundary conditions in the winding wire which are Terminals and...
  28. D

    Mutal inductance depending on current

    My frient had an experiment in lab. They measured the mutal inductance on two coils wraped around one feromagnetic core. When the current of the first coil was 1, 2, 3, 4, 5 mA the mutal inductance was 94 mH. Then they were told to rise the current to 10 mA, the mutal inductance went to 160 mH...
  29. Pushoam

    Work required to get current going

    Image from Griffith, ED, 4th ed Wm is the work needed against the back emf. If the current is steady (meaning not changing w.r.t. time) then there is no back emf. Hence, Wm = 0. But, B due to steady current is not 0. So, R.H.S is not zero. Could anyone please resolve this inconsistency?
  30. fluidistic

    A Does electron energy contribute to current

    It is often said that in metals only the electrons having an energy within ##k_BT## of ##E_F## can contribute to a current. I do not understand where this ##k_BT## comes from. I know it enters the conductivity tensor because such a tensor can be written in terms of a surface integral over the...
  31. A

    AC Induction motor startup current

    Friend gave me this task he had back in school to calculate the necessary amperage for a fuse that is put in series with one of the phases for a 3 phase induction motor. I could calculate the working current of the motor from the rated kW and cos voltage/current angle, but I couldn't figure out...
  32. Rob45327

    What are the voltage and current in motor brushes?

    Summary: I'm rewiring an antique 1 amp universal motor (AC or DC). The two field coils need new brush leads so I have to buy new wire for them and I don't know the voltage. If the motor is running on 110 Volts is it 110 volts at the brushes? The motor is speed is variable and mechanical...
  33. G

    Problems with the current design of ellipsometers

    Hi folks! For those of you who are familiar with ellipsometers, are the following some significant pains/problems with the apparatus? Are there other significant pain points? 1. The machine is too bulky. 2. The change of angle is too slow. 3. The minimum angle is limited by thickness of arms...
  34. B

    Direction of the induced current from an OCR paper -- concept question

    I am not sure in which direction this induced current would be I am assuming you have to use the dynmo rule or the right hand rule as this is the induced current, and the soft iorn core will cause a force to be produced in the coil which causes the current. If so in which direction would the...
  35. fight_club_alum

    Exploring Magnetic Forces on Current: A Visual Guide

    This is how I visualize the problem (of course I am drawing this as if it is in the z-y axis); I don't know what will be the next step. Anyone please help me. Thank you
  36. QuasarBoy543298

    I Applying an observable operator on the current state

    hey :) assume I have an operator A with |ai> eigenstates and matching ai eigenvalues, and assume my system is in state |Ψ> = Σci|ai> I know that applying the measurement that corresponds to A will collapse the system into one of the |ai>'s with probability |<Ψ|ai>|2. with that being...
  37. B

    Current Transformer Proton Accelerator

    Suppose you place a current transformer into a vacuum and flow ionized hydrogen through the center of it. The ionized hydrogen would act as the secondary winding of the current transformer. How specifically would the acceleration that the hydrogen ions experience be calculated?
  38. DaTario

    Electric current being alternated with continuous part

    Summary: In which scenario a current may exhibit alternated and continuous character together? Hi All, I would like to know in which scenario an electric current may exhibit alternated and continuous character? Something like $$ I(t) = I_0 \sin (\omega t) + I_1 $$.
  39. S

    AC Current versus AC Voltage on the Neutral Wire of AC Mains

    Sir how can one explain with the help of an analogy to a grade 10th student the reason behind not getting a shock on touching the neutral wire of the household AC circuit, even though the current flows alternately through live as well as neutral wires.
  40. Muhammad Usman

    Why is the drain current steady after pinchoff in a JFET?

    Hi, I was studying JFET and it was written that at Pinch off voltage the drain current becomes constant. And when the gate to source voltage is applied. The more negative the voltage is the current will be decreased and eventually reached to 0. I don't understand below both points. 1) At pinch...
  41. Anand Sivaram

    About Inverter Compressor: Starting Current

    If we would like to go completely off-grid with a Solar inverter: Normal refrigerators use induction motors for compressors and these induction motors do require around 5x starting current. So, if we use a refrigerator with rated power of 200W, we would require an inverter/power supply rating...
  42. M

    𝑗 - 𝜔 method alternating current circuit help

    I'm supposed to calculate the current and the phase angle. Can someone please tell me if this is the correct solution?
  43. B

    Can energy be extracted from a proton beam using a current transformer?

    Suppose there exists a proton beam that is pulsed on and off. If this beam is aimed through a current transformer as in the attached image (the proton beam would act as the main primary conductor in this case), what would be the output of the transformer, and how would it affect the beam...
  44. callie123

    Can constant electrical current heat water?

    Hi, I'm trying to make a realistic scene, in which water is kept under constant electrical charge. If the power is around 75 milliamps, and it's in a stream about twenty feet across by ten feet deep, would this affect water temperature? Thanks!
  45. Electgineer

    Is there a better way to assign current direction in circuit analysis?

    Is there like a rule of thumb, or a simple hack for assigning current direction in circuit analysis? Even when I m consistent with the direction I choose, I still get the wrong answer most of the time. Like 1. Do current source or voltage source affect when choosing current direction? 2...
  46. S

    Magnetic field of plane with thickness d with uniform surface current

    I'm confused what's meant by a uniform surface current density since this plane has a thickness, It would need a current density distributed through its cross sections, I thought.Edit: I tried solving with proper LaTeX and all my steps, but it looked awful. For outside, I got B=µ_0jd/2. for...
  47. J

    Direction of current in smaller loop on the left and on the right

    Lets say the current in the larger loop is counterclockwise. Then it doesn't matter if the smaller loop is to the right or to the left of the larger one, the induced current in it will always be clockwise. The magnetic field due to the smaller loop current always points in the negative...
  48. gibberingmouther

    Magnetic Fields from Current Carrying Wires

    I've had a lot of problems that involve a segment of current carrying wire, for example when you have a square loop of wire. I have a formula for "long" wires that is B = μ0 * I/(2 * π * d). Can I use this for shorter wire segments, and if not, what formula can I use?
  49. U

    If DC current is chopped on it's way to a load, what happens to the EM?

    I am curious what happens if you draw DC power to a load, but as it approaches the load you chop the load connection say with a SSR or switch. The power will be traveling near the speed of light, following the wire toward the load. When the load is chopped by a switch just before the pulse...
  50. S

    Torque on circular current loop

    So I'm having a little trouble getting to the solution to this question so here's my attempt at a solution and I'm not seeing the issue with it 1. I calculated the B-field due to the long wire (approximating the distance from the dipole to be equivalent to that in the centre since r>d) B=...
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