How are units combined/cancelled in this simple equation?

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Discussion Overview

The discussion revolves around the combination and cancellation of units in a specific equation involving Planck's constant and frequency. Participants explore the implications of unit definitions and the correct application of units in the context of energy calculations in physics.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes the need to understand which units are present in the equation and how they are determined.
  • Another participant asserts that the unit of frequency should be s-1, not s, suggesting a potential error in the original equation.
  • A participant explains that the second from Planck's constant cancels with the inverse second from frequency, resulting in energy units (joules) for the final answer.
  • There is a discussion about the definition of frequency as the inverse of the period, with one participant seeking clarification on whether the provided frequency can be converted to period for unit cancellation.
  • Some participants speculate that the original equation may contain a typographical error regarding the units of frequency.
  • One participant mentions that the equation relates to the energy of a photon and its frequency, indicating a conceptual link between these quantities.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the correct units for frequency, with some suggesting a typographical error while others clarify the relationship between energy and frequency. No consensus is reached on the original equation's correctness.

Contextual Notes

Participants highlight the importance of using consistent units for addition and subtraction, noting that only quantities with the same units can be combined. There is also ambiguity regarding the original presentation of the frequency unit.

Who May Find This Useful

Students new to physics, particularly those learning about energy, frequency, and unit conversions in equations.

ApoIlo
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The numerical answer is easy to find, but I'm wondering which units are in the answer and how those units are determined. Thanks in advance.

(6.63E-34 J×s)(7.09E14 s) - 2.17E-19 J =
 
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The unit of a frequency is s-1, not s.
 
Dickfore said:
The unit of a frequency is s-1, not s.

That must be a mistake on my teacher's part, then. Can you explain further? I'm very new to physics.
 
The second from the units of Planck's constant cancels with the inverse second from the unit of frequency. The product is left with a unit of energy, namely the energy of the photon with the given frequency.

You can only add and subtract quantities with the same units, namely joules. Here, you subtract the work function of a metal. Your result, the kinetic energy of the photoelectron, is also in joules.
 
If I have some kind of periodic phenomenon, the frequency is defined as the inverse of the period, the period is the time it takes for the system to complete one cycle
 
I was just given the equation alone and told to simplify it. Is (7.09E14 s) the frequency in this equation? Can I convert it to the period so that the seconds cancel or is it simply a typing error that it was not measured in inverse seconds in the first place?
 
it looks almost certainly like it is a typing error. it should be 7.09E14 s-1 Which is the frequency of the photon I presume.
 
Looks like your teacher was trying to find real equations for your exercise, but made a typo with the units.

P.s. the equation at hand is the relation between a photons energy and its frequency
 

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