Here's a Question! - Scales and Static
A plastic block, which has been given a static charge, is held just above, but not touching, a digital scale that initially reads zero. What does the scale read?
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Joanna and Steve: Just science!
Joanna: Hi! I'm Joanna!
Steve: And I'm Steve!
Joanna: Here's a question for you...
Steve: I rub a plastic block on my head, giving it a static charge. I then hold the plastic block just above, but not touching, a digital scale that initially reads zero.
What does the scale read?
Joanna: Does the scale read zero?
Steve: Does the scale read a small, negative number?
Joanna: Does the scale read a small, positive number?
Steve: Or, does it depend on whether the charge on the block is positive or negative?
Joanna: Pause the video now if you'd like to think it over before we show you what happens.
Steve: If we place an object on the scale, we'd get a positive positive number. A negative number implies that something's missing or has been taken away.
Joanna: If we take a look at the scale, you'll notice that the top plate is made out of metal.
Steve: When a charged object gets close to a metal object, the electrons in the metal object move in response to the charge.
Joanna: If the plastic block is negative, the surface of the plate becomes positively charged. If the plastic block is positive, the surface of the plate becomes negatively charged. In either case, the plate is attracted to the plastic block.
Steve: This is why there's 'missing mass'. The scale's top plate is lifting up towards the plastic block. The scale isn't squished as much as it should be, so it displays a negative number.
Joanna: Thanks for watching. I hope you'll join us again soon for another question!
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