Why is power equal to voltage times current?

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Discussion Overview

The discussion centers around the relationship between power, voltage, and current, specifically why power is expressed as the product of voltage and current (P=VI) rather than just current (P=I). Participants explore this concept through various analogies, mathematical reasoning, and attempts to develop an intuitive understanding of the relationship.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants express confusion about why power should depend on voltage in addition to current, suggesting that current alone seems sufficient to describe power.
  • One participant analyzes the dimensions of power, voltage, and current, demonstrating that the units align to support the equation P=VI.
  • Another participant uses a water wheel analogy to illustrate that power increases with both the height of the water (analogous to voltage) and the flow rate (analogous to current).
  • Some participants argue that voltage represents potential energy, and when multiplied by current, it yields the rate of work done, thus equating to power.
  • A later reply questions the water wheel analogy, suggesting that the efficiency of the design affects how power is generated, emphasizing that the relationship between current and voltage is not straightforward.
  • One participant provides specific examples using Ohm's law to illustrate how varying voltage affects power while keeping current constant, reinforcing the necessity of voltage in the power equation.
  • Another participant discusses the relationship between electric potential and the intensity of the electric field, further elaborating on the definitions of voltage and current.

Areas of Agreement / Disagreement

Participants generally express uncertainty and multiple competing views regarding the intuitive understanding of the relationship between power, voltage, and current. No consensus is reached on a singular explanation or model.

Contextual Notes

Limitations include varying interpretations of analogies, assumptions about the efficiency of systems, and the complexity of the underlying physics that may not be fully resolved in the discussion.

  • #31
Nugatory said:
You can write either I2 or ##I^2## and it will be a lot clearer what you mean. (try quoting this post to see what I did, or look for the sticky down in the feedback section).

Thank you. That is very helpful and I'm confidant it will make my future posts clearer, to myself as well as others..
 
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  • #32
and use V for voltage rather than E

E is more commonly use for Energy ... ie ... E = mc2

cheers
Dave
 
  • #33
davenn said:
and use V for voltage rather than E

E is more commonly use for Energy ... ie ... E = mc2

cheers
Dave

That's actually curious. I was taught Ohms Law is I = E/R but I see now that textbooks and modern reference material replace E with V as you recommended. I may be old but not so old I can't change for something half way reasonable. V it is :)
 
  • #34
The old name for "voltage" was "electromotive force", which is probably where enorbet's I = E/R came from.

The term EMF is still used sometimes (e.g. the "back EMF" in an electric motor) but being pedantic, the EMF is the "cause" of a voltage in a circuit, not the actual voltage itself. (And the name EMF is also confusing because it has nothing to do with the idea of "force" as used in mechanics.)

In the standard notation for electromagnetism E is the electric field, not the voltage. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/efromv.html
 
  • #35
emf is still used when you are referring to an induced 'voltage'. So the secondary of a transformer has an 'emf' induced and an antenna will also have an emf induced into it and a chemical cell will generate an emf. This term distinguishes this manifestation of a voltage from the Potential Difference that is actually measured in a circuit, fed from a real electrical source - one with a source resistance (so called Internal Resistance of a battery)
So PD would be equal to Emf - Current times source resistance or:
PD=rI.
I remember my Dad always used E = IR, after his college ONC and HNC courses on 'EE' (=Voltage Voltage??haha). It confused me, wham I started using V = Ir at school.
Flavours change from generation to generation. I remember , in A Level Mechanics and Physics, acceleration was always lower case f. So v=ut+ft2/2 was what we used in our equations of motion calculation.
It's a moving target, as new variables arrive and pinch the commonly used symbols.
 

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