Problem converting joules to dynes

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Discussion Overview

The discussion revolves around the conversion of energy units, specifically converting British thermal units (Btu) to various energy units including British gravitational, SI, and CGS. The focus is on understanding the relationship between joules, dynes, and ergs, and addressing a potential error in a provided answer sheet.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the conversion of joules to dynes, suggesting that the answer should be in ergs instead of dynes.
  • Another participant clarifies that the dyne is a unit of force in the CGS system and expresses frustration with the complexity of metric units.
  • Definitions of an erg are discussed, with one participant noting its relationship to a dyne and suggesting that the answer sheet may have omitted the necessary unit of centimeters.
  • There is a reiteration of the relationship between ergs and dynes, emphasizing that 1 erg equals 1 dyne-centimeter.

Areas of Agreement / Disagreement

Participants generally agree that there is confusion regarding the units presented in the answer sheet, particularly the distinction between ergs and dynes. However, there is no consensus on the correctness of the answer sheet itself.

Contextual Notes

Participants express uncertainty about the conversion process and the definitions of the units involved, highlighting potential omissions in the answer sheet.

Tom Hardy
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I have this question:

1. The problem
Convert 200 Btu to (a) BG (British gravitational), (b) SI and (c) cgs

The Attempt at a Solution


(a) and (b) are fine I got 155600ftlb and 200kJ but the answer the sheet gives for cgs is 211x10^10 dyne but surely the answer is 211x10^10 erg?! I can correctly convert to erg but I have no idea how to convert energy (joules) into a force (dynes).

My working was since joule is kg.m^2/s^2 to turn it into cgs units I would times by 1000 to account for kg --> cm and times by 10,000 to account for m^2-->cm^2 and I get an answer of 211x10^10 but the units is what's bothering me. Surely the units should be 'erg'? But the answer sheet is showing 'dyne', is there a way to to convert energy (joule) to force (dyne) or is there an error on the answer sheet?

Many thanks
 
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The sheet appears to be using incorrect units. The dyne is a unit of force in the CGS system. 1 dyne = 1 g-cm/s2 = 10-5 N

This is why I dislike the metric system. There are too many units to keep track of.
 
Do you know the definition of an erg?
 
Chestermiller said:
Do you know the definition of an erg?

Well, as I understand it, it has the same relationship with a dyne as Newton does with newtom metres. So basically the amount of work that is done when a force of one dyne is exerted for one centimetre, something like that at least.
 
Tom Hardy said:
Well, as I understand it, it has the same relationship with a dyne as Newton does with newtom metres. So basically the amount of work that is done when a force of one dyne is exerted for one centimetre, something like that at least.
Right you are. So 1 erg = 1 dyne - cm. They must have left out the cm part in the answer.

Chet
 
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Chestermiller said:
Right you are. So 1 erg = 1 dyne - cm. They must have left out the cm part in the answer.

Chet

Ah, that must be it. Thank you for your help!
 

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