Why Do We Get Zapped by Door Knobs? Understanding the Science Behind It

  • Context: High School 
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Discussion Overview

The discussion revolves around the phenomenon of experiencing electric shocks when touching door knobs, exploring the underlying physics concepts such as charge transfer, potential difference, and the role of grounding. Participants delve into theoretical explanations and practical implications, touching on concepts of electric fields and air breakdown.

Discussion Character

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

Main Points Raised

  • Some participants suggest that door knobs may not be grounded, leading to the accumulation of charge on the user, which can result in a shock when touching the knob.
  • Others explain that the Earth is a large reservoir of charge, allowing for a continuous flow of charge without significant change in potential, similar to how a door knob can also hold a large amount of charge.
  • A participant questions whether the door knob is at a lower potential than the person, indicating a desire for a more nuanced discussion of potentials and electric fields.
  • One participant emphasizes that the door knob can accept extra electrons, which leads to the transfer of charge from the person to the knob, causing the shock.
  • Another point raised involves the breakdown of air and its relation to the voltage required to create a spark, noting that the distance from the knob can be used to estimate the voltage on the body.
  • It is mentioned that the door knob acts similarly to Earth in that it can serve as a large reservoir of charges.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanisms behind the shocks, with some agreeing on the role of charge transfer and potential differences, while others raise questions about the specifics of grounding and potential levels, indicating that the discussion remains unresolved.

Contextual Notes

There are limitations in the discussion regarding assumptions about grounding, the definitions of potential, and the specifics of charge transfer mechanisms. The relationship between air breakdown and the conditions for experiencing shocks is also not fully explored.

ice109
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why do we get zapped by door knobs? they're not grounded? well maybe they are when the door is closed...
 
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What's so special about the earth?
The reason you get a shock to Earth is that it is large enough that the small amount of charge flowing from you doesn't change it's potential (voltage) so the charge can keep flowing.
Simimairly with a door knob, it's a relatively large isolated metal object so can contain a large amount of charge - so when you touch it there is a large potential difference which doesn't decrease, allowing all the charge to keep flowing from you.

It's a bit like touching a bath of hot water rather than a tiny spark. The spark is hotter but soon cools, the water can keep putting heat into you for longer.
 
mgb_phys said:
What's so special about the earth?
The reason you get a shock to Earth is that it is large enough that the small amount of charge flowing from you doesn't change it's potential (voltage) so the charge can keep flowing.
Simimairly with a door knob, it's a relatively large isolated metal object so can contain a large amount of charge - so when you touch it there is a large potential difference which doesn't decrease, allowing all the charge to keep flowing from you.

It's a bit like touching a bath of hot water rather than a tiny spark. The spark is hotter but soon cools, the water can keep putting heat into you for longer.

that which is so special about Earth is that it is a lower potential than certain things, me for example.

i don't know if you're saying the knob is at a lower potential than me or not. are you saying because it's not charged it's a priori at a lower potential? don't over simplify, i can handle talking about this in terms of potentials/fields/etc.
 
You are the one that's charged. The doorknob is a relatively large metal object that doesn't mind having some extra electrons (it would rather have the electrons than your body). This causes any extra charge that you have accumulated to transfer to the doorknob, causing a shock.

This could happen with any metal object.
 
wow. I just had the same question today.
This also has to do with breakdown of air. It is 3MVolts/meter. In dry air this is lesser.
At 1mm, 3000V is good enough to cause a breakdown(spark) between you and a large enough conductor (metal door knob).
You can roughly calculate the voltage on your body by estimating the distance from the door knob, at which the spark occurs.
The door knob is like Earth (a big reservoir of charges).
 

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