What is Current: Definition and 1000 Discussions

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. 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...
  2. 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...
  3. 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...
  4. 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?
  5. 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...
  6. 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...
  7. 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...
  8. 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...
  9. 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...
  10. 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
  11. 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...
  12. 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?
  13. 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 $$ I(t) = I_0 \sin (\omega t) + I_1 $$.
  14. 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.
  15. 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...
  16. 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...
  17. 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?
  18. 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...
  19. 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!
  20. 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...
  21. 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...
  22. 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...
  23. 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?
  24. 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...
  25. 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=...
  26. JCMacaw

    Meaning of fuse current rating for fuse holder

    Does anybody know what the 1/10 meaning is in the current rating description for a fuse holder? 1/10-30A 600V For No. of Fuses For Fuse Current, A Voltage Wire Connection Type For DIN Rail Size, mm Features 1 1/10-30 600V DC/600V AC Screw Terminals 35 Covered Terminals...
  27. Zack K

    Current induced on a coil by a changing magnetic flux in another coil

    I will first calculate the magnetic flux of the coil in motion. $$\frac {d\phi}{dt} = -\frac {dB_{loop}}{dt}A = -\frac{d}{dt}(\frac{\mu_o}{4\pi}\frac {2\pi NR^2I}{(R^2+z^2)^{\frac{3}{2}}})A$$differentiating in terms of ##z##, we get $$\frac {d\phi}{dt} =(\frac{\mu_o}{4\pi}\frac {6\pi^2...
  28. diegbarrientos

    Influence of Core Change on Current in Solenoid Loops

    How does changing the core of the solenoid affect the current induced in the loop?
  29. F

    Engineering Help me finding the current in here please (Circuit analysis)

    Hi! I've been struggling with this. Original exercise here: Find the value of Iy when R=0. And the value of Vx when R is infinite. For the first part of the question I did this since R=0: I've tried to solve this circuit and I get that Iy and I5 are 0 A, and this can't be possible since Iy...
  30. A

    What is Current? I know it is a scalar but I found something weird....

    While I was going through "Introduction to Electrodynamics" by David J. Griffith I see the line "Current is a vector quantity". But we know it doesn't obey the vector algebra (addition ). Then how it can be a vector?... Please help me
  31. M

    Origins of magnetic fields from permanent static magnets versus current

    The videos from Veritasium explaining permanent static magnets and electromagnets were quite good I thought… But they have me a little confused with regard to the origins of magnetic fields generated by an electric current as opposed to a permanent static magnet from say iron. 1. An...
  32. M

    Why does an increase in voltage cause faster electron flow?

    In a circuit with a set resistance, let's say 1 ohm, the current flowing through the circuit is the same as the supply voltage. Why does an increase in voltage at the supply cause the electrons to flow faster?
  33. P

    Finding the current and voltage for each resistor

    Homework Statement Moderators note: link removed. All images should be uploaded to PhysicsForums. Homework Equations V=IR Series in parallel Series in current The Attempt at a Solution I managed to get the current and voltage of R1 and R2. I've been trying to get the next one R3, but haven't...
  34. 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...
  35. T

    I Relativistic form of the displacement current using the Biot-Savart Law

    The Biot-Savart law which describes a magnetic field created by a displacement current: $$\frac{dB}{dV}=\frac{\mu_0\epsilon_0}{4\pi}\frac{\frac{∂E}{∂t}×r}{r^2}$$ What's the relativistically co-variant form of this equation? Is the introduction of speed of light propagation delays enough, or...
  36. T

    Magnetic Field of a Moving Charge and the Displacement Current

    We all know a moving charge generates a magnetic field. A moving charge also generates a displacement current ∂E/∂t. Is the magnetic field generated entirely due to the presence of the displacement current, or is there an independent, separate effect which contributes to the magnetic field?
  37. MedEx

    Finding the Current in a Resistor in a Closed Circuit

    1. Homework Statement The ideal battery in Figure (a) has emf = 7.7 V. Plot 1 in Figure (b) gives the electric potential difference V that can appear across resistor 1 of the circuit versus the current i in that resistor. The scale of the V axis is set by Vs = 18.9 V, and the scale of the i...
  38. G

    I Electromagnetic Curr. in Curved Spacetime: Questions Answered

    I assume this forum to be the appropriate one, since the real problem is about covariance rather than electromagnetism. In electrodynamics in a curved background, the relation ##F^{\mu \nu} = A^{\mu , \nu} - A^{\mu , \nu}## stays in terms of ordinary derivatives. So, in particular ##F_{,\mu...
  39. A

    Tokamak plasma heating current

    I will make some assertions and then phrase a question , please correct me if necessary. So the tokamak works like a transformer where the primary is a coil winding while the secondary is a torus which contains plasma which being electrically conductive forms the secondary winding? The toroidal...
  40. callie123

    Writing: Input Wanted Electrified Water: Feasibility and Applications

    I'm working on a Scifi novel, and trying to make it somewhat...scientific! Any advice is appreciated. Here's my question: I'd like to write about a situation where a body of water is purposefully kept under electric charge for years on end. This body of water would vary in size and depth, but...
  41. R

    When current flow reach indeterminate form

    Homework Statement If the current flow, in a branch of a circuit, is a function of say (√(x + 2)-2)/(x-2) (or any such that give an indeterminate form at a certain value) of an input source current x. What current will be flowing on that part of the circuit, when the function become...
  42. Adesh

    How to solve a complex equation to get the current?

    I was reading The Feynman Lectures on physics http://www.feynmanlectures.caltech.edu/I_23.html chapter 23, section 4. In it he derives the equation for current when inductor, resistor and capacitor is connected in series with an alternating voltage source, he derives this equation:-...
  43. F

    Implement a DC current source - simulink

    Homework Statement Homework Equations - The Attempt at a Solution Here's my work : When I add the DC current source , it shows a red dotted line and won't connect to other elements . Any help would be greatly appreciated !
  44. O

    Current produced by photolectrons

    Homework Statement In perspective to the photoelectric effect I found following question: Electric current is charge flowing per unit of time. If we increase the kinetic energy of the photoelecrons (by increasing the energy of the incident photons), shouldn’t the current increase, because the...
  45. K

    B How a Secondary Resistor affects the Transformer Primary Side?

    Let's assume a 2:1 transformer which has a 100V Source connected on the primary circuit and has no/negligible resistance, on the secondary circuit a 5 Ohms resistor is connected. Using the 'Impedance Transfer/Reflection' method, the primary circuit would act as if there was a 25 Ohms resistor...
  46. Boltzman Oscillation

    In AC analysis do I alternate the current and voltage?

    Lets say I am analyzing a simple AC voltage source with a resistor. In the positive voltage peak then I will use V and I say current is flowing clockwise. When I am analyzing the -Vpk iteration then do I make the current counter clockwise too or do i keep it clockwise? Thanks.
  47. T

    Why does current initially rush into a capacitor?

    Hi all, I'm trying to wrap my head around the physics behind the current that initial flows when a voltage step is applied across a capacitor. Namely, why does a large current initially flow into a parallel plate capacitor, but a large initial current does not flow if the capacitor is replaced...
  48. F

    Is Electric armour feasible with current technology?

    I read about electric armor (also known as dynamic armor or electric reactive armor) that can protect armored vehicles from shaped charges of anti-tank weapons and I want to know if its feasible today. according to wikipedia: [Moderator: please don't do this without a link. The link is The link...
  49. Y

    Looking for a BJT with low collector leakage current

    I am looking for small signal BJT that has very low leakage current when the base is biased at about 0.2V. I have issue of transistors drawing collector current when the base is at about 0.25V that is way less than the normal turn on of 0.6V to 0.7V. I am currently using BC546 or 556, and KSA992...
  50. J8675309

    Electrical Applying current only to temporatrily demagnetize a magnet

    Ladies & Gents, Instead of applying current to a non-magnet to induce magnetism, is it possible to apply current only to temporarily de-magnetize a magnet (natural or man-made)? This would seem to use up less electricity, especially since the default mode for a desktop application I'm working...
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