Is it correct to use impedance in this question?

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Homework Statement



Describe how the earth wire and fuse in an appliance with a metal casing protects the user from shocks if a short occurs to the casing.

Homework Equations



The Attempt at a Solution



I said that the earth wire provides a low impedance path to ground. As electricity flows via this low impedance path this then causes a large current draw which then causes the fuse wire to heat up and melt and breaks the circuit.

However my teacher said that I should have used the word resistance instead of impedance in my answer?

A.
 

Answers and Replies

  • #2
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Technically, yes, what we're talking about is resistance. I'd accept impedance, though the official term is resistance. Though, answering your title, no, technically it's incorrect.
 
  • #3
gneill
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Even more technically speaking, in the real world there's no such thing as a pure resistance -- even a perfectly straight piece of wire 1 cm long has *some* inductance. Capacitance, too!

The question is, how small does the reactive component have to be relative to the real component before you get to insist that it's "a resistance" and not an "impedance"? :smile:
 
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  • #4
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Even more technically speaking, in the real world there's no such thing as a pure resistance -- even a perfectly straight piece of wire 1 cm long has *some* inductance. Capacitance, too!

The question is, how small does the reactive component have to be relative to the real component before you get to insist that it's "a resistance" and not an "impedance"? :smile"

So in an ideal circuit it would be a resistance but in real life it would be impedance as there is inductance/capacitance going on as well?
 
  • #5
gneill
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So in an ideal circuit it would be a resistance but in real life it would be impedance as there is inductance/capacitance going on as well?
Yup. This is what makes circuit design at high frequencies "interesting". A design that works fine in simulation may be a bear to get working in practice as stray capacitance and inductance alter operating parameters.
 
  • #6
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Impedance is just as correct here as resistance. It's the way a professional EE would have said it.

Just because you know the dominant component is resistance that doesn't mean you can't say impedance.

All resistance is an impedance (zero imaginary part) but not all impedance is resistance.
 
  • #7
NascentOxygen
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I said that the earth wire provides a low impedance path to ground. As electricity flows via this low impedance path this then causes a large current draw which then causes the fuse wire to heat up and melt and breaks the circuit.

However my teacher said that I should have used the word resistance instead of impedance in my answer?
The use of "impedance" is slightly more technically correct than "resistance", but they should be considered equally correct. It is not sufficient that the path just have low resistance, it must also be of low impedance in order that the fuse can melt.
 

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