What is Ampere: Definition and 46 Discussions

The ampere (, US: ; symbol: A), often shortened to amp, is the base unit of electric current in the International System of Units (SI). It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electromagnetism.
The International System of Units defines the ampere in terms of other base units by measuring the electromagnetic force between electrical conductors carrying electric current. The earlier CGS system had two different definitions of current, one essentially the same as the SI's and the other using electric charge as the base unit, with the unit of charge defined by measuring the force between two charged metal plates. The ampere was then defined as one coulomb of charge per second. In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second.
New definitions, in terms of invariant constants of nature, specifically the elementary charge, took effect on 20 May 2019.

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  1. Elementi

    B Understanding the Relationship Between Coulomb and Ampere in Physics

    Hi! Why coulomb is not a base unit in Physics, but the Ampere is (and ampere is coulomb/s)? Isn't it like saying that m/s is a base unit? Can someone explain it to me?
  2. T

    I Help with electromagnetics: the Poynting Vector and Ampere contradiction

    According to equation (2.33) divergence of the Poynting vector or the outflow of electromagnetic power is equal to the stored magnetic field, stored electric field and ohmic losses. My contradiction is the following: Inside a steady state DC current carrying wire, there will presumably be a...
  3. dextercioby

    I Is the new Ampère exactly the same as the old Ampère?

    The new BIPM/SI definition of the unit of electric current reads: 1 A (Ampère) is the electric current corresponding to the flow of 1/(1.602 176 634 × 10-19) elementary charges per second. The old definition (until 2019) was: 1 A (Ampère) is that constant current which, if maintained in two...
  4. S

    Physics History (Maxwell) Rotary Vectors?

    I am reading the text 'Innovations in Maxwell's Electromagnetic Theory'. on page 44 there is a discussion on Ampere's circuital law . The passage is below. I don't understand the final statement. "In general represent a kind of relationship that obtains between certain pairs of phenomena , of...
  5. kurisu

    How do I know the current's ampere value when it fluctuates rapidly?

    What "ampere value” is measured by a d-c ammeter when the current fluctuates rapidly?
  6. G

    Pre 2019 definitions for an Ampere, how is this useful in practice?

    I've been looking into definitions for SI units (pre 2019) I'm fine with length based on a certain transition of a Krypton atom and the wavelength associated with it, a spectrometer can be used to find the wavelength of the spectral line and then multiply up... but with the ampere, the idea of...
  7. Muhammad Saber

    Is Multiplying Phase Readings a Valid Method to Verify Transformer Load Balance?

    I'm working in electrical company in a department in which I receive transformer LV side ampere readings and in most of these readings the neutral has amperege due to unbalanced loading, all I need a method in which I be certain that these readings are actual measured values specially neutral...
  8. thee qs

    Ampere definition and infinite wires separated by 1 meter

    Homework Statement 3 - 2 , 1 km wires ( so acting like infinite wires ) , both have same current , no direction specified , , separated by 1 meter distance and having between them a magnetic force of module 0.02 N . find the current i= Homework Equationsthe definition of 1 ampere has same...
  9. ?

    EM fields of two opposite moving point charges

    I wrote and solved this problem but am having serious doubts about the answer I obtained. Homework Statement Two point charges \pm q move along the z-axis with velocity \pm v. If they are at the origin when t=0, what is the electric field magnitude a distance r from the z-axis? Homework...
  10. G

    Ampere turn balance (MMF balance) in transformers

    Hi all, I have been battling this topic for one year now. Frequenctly revisiting it and trying to find a clear answer on the internet and in but without finding it. Could be that my knowledge level is too low. However, I'll give it a shot here. So what I have been learned is that during...
  11. A

    Maxwell Ampere law capacitor problem

    Good day All! I have an issue solving this question According to my basic understanding, the displacement current goes from the positive plate to the negative plate, so whether the capacitor is charging or discharging the magnetic field direction should remain the same, but here is the the...
  12. K

    Why you can't measure voltage and current at the same time?

    In my physics textbook it says that ammeter in circuit C shows higher current than ammeter in circuit A and D. And the voltmeter shows higher voltage in D than in B, and C. It's explained in the book that way: some electric current is passing through the voltmeter in C, thus the ammeter will...
  13. R

    Off center hole in a current-carrying wire - magnetic field?

    Homework Statement A long copper rod 8 cm in diameter has an off-center cylindrical hole 4 cm in diameter down its full length. This conductor carries a current of 900 amps flowing in the direction “into the paper.” What is the direction, and strength in tesla, of the magnetic field at the...
  14. RoboNerd

    Multiple choice question on finding magnetic field

    Homework Statement E is the "right answer" Homework Equations B around a long wire = mu * I / [2 * pi * r] The Attempt at a SolutionOK. I know that the magnetic field lines will travel in a clockwise direction around the wire from the viewer's orientation with the current going away from...
  15. sobergeek23

    Ampere & Coulomb: Discovering Electrons in 1800's?

    ok, so i am taking an auto electrical class this semester and i can not continue reading the book/doing the homework until i understand this..my question is this: if andré marie ampére (1775-1836) discovered the electron flow, how the hell did he do it back then? how did they discover atoms and...
  16. Isaac0427

    Why is the Ampere a base unit?

    Just out of curiosity, why is the Ampere a base unit and not the Columb? It seems to me that charge is more fundamental than current. Thanks!
  17. Neil Hayes

    How to find volt drop per ampere meter?

    Homework Statement [/B] Determine the resistance of a conductor and the volt drop per ampere metre at 70°C for a 95 mm2 pvc/swa/pvc copper cable. The temperature coefficient of resistance is 0.004 per °C. Homework Equations [/B] R70 ( resistance at 70 degrees ) = R20 [ 1+( temperature...
  18. S

    When do I have a dead battery?

    Homework Statement When a certain 9.0V battery is connected to a voltmeter, the voltmeter reads 8.00V (assume that the voltmeter has an internal resistance of 10MΩ. We may conclude that (A) only 1/9 of the battery's fuel has been used, and so the battery has plenty of life left in it. (B) the...
  19. carllacan

    Relationship between Bit-Savart and Ampere laws

    I've tried to relate Biot-Savart's Law to Ampere's and I've found a contradiction, which I guess is due to a naive use of Ampere's. If ## \int \vec B · d\vec l = \mu_0 I_{enc} ## is applied to a circle of radius R around a current element ##Id\vec l## we have ## B·2\pi R = \mu_{0} I_{enc} ##...
  20. thegreengineer

    Electric circuits formulas doubts

    Well, recently I have started to deal with electronics and I've seen several new concepts such as ohm's law, parallel and series circuits, what's AC and DC and many other topics. Ok, I have no doubt on what's direct and alternate current or what a series and parallel circuits are; rather than...
  21. S

    Ampere & Biot-Savart: 2 Questions Answered

    I have 2 questions about those 2 equations. 1-) Which one came first in the history of electromagnetism? Some articles say: Biot-Savart derived their equation from Ampere's Law. Some say: Ampere derived from Biot-Savart. Which one is true? 2-) Since Ampere is special form of Biot-Savart and...
  22. Entanglement

    The original definitions of Ohm, ampere, volt and coulomb

    What's the original definition of Ohm, ampere, volt and coulomb, And which unit was defined according to the other ??!
  23. R

    How torque is converted into ampere in a generator

    As i have read on the internet that during synchronization the prime mover connected to the turbine produces a little over than required power to keep the turbine cum generator assembly moving at the synchronous speed that is regulated by the grid frequency (In India its 50Hz). As the generator...
  24. L

    Defining Ampere: Length of Wire Explained

    Homework Statement This may be a stupid question but... I don't understand why the length of the wire has to be infinite in defining the ampere... could someone explain this to me?
  25. T

    Proof of Ampere's Circuital Law: Using the Biot-Savart Law

    Where can I find a proof for the Ampere Circuital law? Wherever I look, I just find a proof for an infinitely long current carrying conductor.:confused:
  26. A

    How can ampere be considered as base quantity?

    Base Quantities are those quantities on the basics of which other quantities can be expressed. Ampere is also a base quantity.But the unit of ampere is C/s.This shows that ampere depends on time and coulomb.It make sense to consider ampere as a derived unit and coulomb as base unit.But we...
  27. S

    Help: From Ampere to Coulomb in SI units

    Hello, Could someone help me finish this train of thought? This is how we think in SI units: First, just because we like the value of 1 Ampere as it is now, we choose the force between two parallel conductors to be exactly 2×10^{-7}N= const ×\frac{1A×1A}{1m} Then, purely as choice, we decide...
  28. N

    Ampere Law in a hole Should give zero?

    Hi all, I just did an exercice and there is one point that I don't understand (about Ampere's law). Homework Statement The exercice was about a cable with given diameter a, in which a small hole with diameter b is drilled in. The axis of the cable and the hole are parallel and the...
  29. R

    Very long solenoid ampere law paradox?

    Hello I have been doing some thinking related to Ampere law and behaviour of magnetic field inside materials of different permeability. In the process, I came up with the following thought experiment: Try imagining a very long solenoid with steady current I. Turns of the solenoid are...
  30. atomqwerty

    Doubt with Ampere and Biot-Savart

    Hello, I think I'm terribly wrong by supossing Ampère-Maxwell and Biot-Savart are referred to the same concept of magnetic field B. For example, for calculating B near an infinite line, I used both, as I understood them, obtaining different expressions (see image). What is that that I don't...
  31. T

    Why can't we remove Ampere from being base unit?

    1) Analogy in F = ma From experiment, we find that F = k m a and we deliberately choose k = 1 (dimensionaless) and thus we define the unit Newton, a derived unit. 2) In the definition of ampere, it is in terms of the force per unit length between two straight parallel conductors...
  32. J

    How many Ampere in a superconducting spire?

    As far as I can understand, after inducing a current inside a superconducting ring or spire, the current keeps flowing into the ring "for ever". Is this correct? Since electrons have no "friction" against the superconducting lattice, does this mean that I can virtually accelerate them...
  33. P

    Definition of ampere in terms of a flow of electrons past a point in a wire?

    Homework Statement a.) What is the operational definition of an ampere? b.) What is the definition of an ampere in terms of a flow of electrons past a point in a wire? c.) What is the advantage of the operational definition over the definition in (b)? Homework Equations N/A...
  34. C

    Exploring Ampere's Law: Understanding External Currents

    In ampere circuital law, ∫B.dl = μo inet i includes current "only passing through the loop " also B is the net mag field at any point "all to any current anywhere" now look at the pic. In A, it is very easy to find field using ampere law ... its μi/2πr Also if in B is you apply the...
  35. R

    The Ampere in alternative units

    Could the Ampere be given in units such as N.s.m^-1 ? Would this be correct, i originally just thought it would be N.m^-1
  36. P

    Define the Ampere: Force/Length = 2*10-7N/m

    Homework Statement Two very long, thin, straight, parallel wires each carrying a current I in the same direction are separated by a distance, d. With the aid of a diagram which indicates the current directions, account for the force on each wire and show on the diagram the direction of one of...
  37. Artlav

    Is there a contradiction in Ampere's force and motion?

    I've been thinking about Ampere's force attraction between two parallel currents, and one case got me confused. Imagine two streams of similarly charged particles traveling alongside in a vaccuum, which is by definition an electric current. For the small time they would flow along until...
  38. T

    Why Ampere is Not a SI Unit? A Brief Overview

    Why ampere is not a S.I unit?
  39. E

    Ampere's Law: Questions from E.M. Purcell's Textbook

    I posted questions about this subject some time back. Didn't get them answered to my satisfaction, but did learn of an EXCELLENT textbook - E.M.Purcell's Electricity and Magnetism - thanks to you all that responded! Now I have two questions about some of his material. (1) I waded through his...
  40. L

    Ampère force law, action, reaction

    This is apparently a well known topic, but I did not know it before today. Let us consider the Ampère law for the force experience by a current element (1) in the magnetic fields of another current elment (2): \mathbf{dF}_{12} = \frac {\mu_0} {4 \pi} I_1 I_2 \frac {d \mathbf{s_2}\ \mathbf{...
  41. R

    The laws of Coulomb versus Ampere and the electromagnetic Machian paradox

    The laws of Coulomb versus Ampere and the electromagnetic Machian "paradox" Here's an apparent paradox that has been tunneling about in my little mind lately. Maybe someone out there can help me with it. Imagine if you will, a vast empty region of space devoid of any visible distant...
  42. U

    Electric Field Strength H Units - Ampere Turns/Meter?

    What are the units of the electric field strength H? I know that H = NI/L for a solenoid and I'm wondering if there is any condensed unit or if it is just ampere turns per meter.
  43. H

    Historically, what came first: coulomb unit or ampere unit?

    Historically, what came first: coulomb unit or ampere unit?
  44. H

    Anyone care to demystify ampere for the layperson?

    Hello This is a section from a textbook... "an ampere is defined as that current in each of 2 infinitely long parallel wires 1 m apart that causes an electromagnetic force of 2 x 10^-7 per meter of its length to act on each wire" Hmmm... was that in English :smile: ? q1...
  45. D

    Why Current is Chosen as Base Unit & 2E-7 N/m Force?

    Current is the rate of change of current passing throught a point each second. 1 ampere is defined as the current in two infinitely long straight wires 1m apart in a vacuum which produces a force of 2E-7 N/m on each wire. My question is why we choose current as base unit rather than charge...
  46. M

    Calculating Voltage in a Time-Varying Inductance Circuit

    Hi all, I cann't figure out the relationship between a current and the charge density. I have a current Io which circulated in a hollow cylindrique, how can I related with the charge density? Something like charge density = Io * ... Thanks in advance, Mica