# Positive and negative charges(mainly relating to copy machines)

• RuroumiKenshin
In summary, the conversation discusses the phenomenon of black colors creating a positive electrical charge while white colors create a negative charge. It is explained that this occurs in copy machines due to the photoconductive plate, which is a special type of semiconductor that only conducts electricity when light is shined on it. The process of creating these charges involves a strong voltage and the use of electrons, which are negatively charged particles. The concept of ionic bonding is also brought up but deemed irrelevant to electricity creation. The conversation then briefly touches on the definition of a semiconductor and the use of magnets in generating electricity.
RuroumiKenshin
Why is it that black colors create a positive elctrical charge, while white colors send a negative charge? Does it have to do with different wave lengths?

When light (in a copy machine) is focused on to the photoconductive drum, why and how does the black color become positively charged? I know that the ink that is applied is negatively charged, and through the principle of opposite attraction, the ink is attracted to the colors (black or whatever, as long as it isn't white). But why can't the ink get negatively charged? The black colors are already positively charged.

How do things get positively or negatively charged? I know its when an ionic bond takes place, and an atom has more electrons than protons, but how is this induced to occur? Why does this sort of reaction create a charge?

OK, the situation in copy machines doesn't have anything to do with the wavelength. It's actually arbitrary -- here's what happens. First, we charge up the drum, which I'm going to draw as a flat plate:
Code:
+ + + + + + + +     positive charge
===============     photoconductive plate (in the dark)
- - - - - - - -     negative charge
Now, the photoconductive plate is a special type of semiconductor which only conducts electricity when you shine light on it. Light from a lamp is bounced off the paper; the white reflects the light, the black doesn't. This is basically the definition of white/black -- we see something as black when there's no light coming from it, and white when it's an even mix of colors.

So the light only gets reflected off the white parts, and hits the corresponding parts of the plate. Those parts of the plate become conductive, and the opposite charges flow across it and cancel.
Code:
+ +       + +
=============
- -       - -
^^^Where the white area is in the middle.

To create the original charges, we basically create a really strong voltage that can pull electrons off the top of the plate, into some wire. The voltage is got from the electric power lines in the wall... that power is originally generated usually using some sort of magnets. See "dynamos."

Ionic bonding is something that charged particles can do, but it's not relevant to electricity creation.

What exactly is a semiconductor. I just read a definition that said

"Semi-conductors such as Silicon (see p.58) fall into this category."
'This category' being "...the conductivity of some substances varies significantly with certain conditions..."

Not a very good definition, so can you explain it to me?

That's really interesting. But how do you get the electrons into the wire? Why do you need electrons in the first place? They're negatively charged. Or do you put an extra electron in a negatively charged atom?

Umm ... i thought duplicates of the same topic in more than a single forum are not allowed in PF ... anyway ...
You may know more here , the page also include some information about how light will affect the photoconductive plate.

Staii, was this thread that you speak of, by any chance, started by me? I started a thread on this very subject, but the problem is I can't find it.

Did you check the "How Stuff Works" forum ? (Well, I did !)

Oh, I never new that forum ever existed! I'll go find it...
thanks.

## 1. What is a positive charge and how does it relate to copy machines?

A positive charge is an excess of protons in an atom or molecule. In copy machines, positive charges are used in the development process to attract toner particles to the paper, creating the image being copied.

## 2. What is a negative charge and how does it relate to copy machines?

A negative charge is an excess of electrons in an atom or molecule. In copy machines, negative charges are used in the transfer process to attract toner particles from the drum to the paper, completing the image being copied.

## 3. Why do copy machines use both positive and negative charges?

Copy machines use both positive and negative charges to attract toner particles to the paper and then transfer them to create an image. This allows for a more precise and detailed image to be produced.

## 4. Can positive and negative charges affect the quality of a copy?

Yes, the correct balance of positive and negative charges is crucial in producing a high-quality copy. If there is an imbalance, the image may appear faded or distorted.

## 5. How can I troubleshoot issues with positive and negative charges in my copy machine?

If you are experiencing issues with the quality of your copies, it is recommended to check the charge wires and cleaning blade in the development unit. These components may need to be cleaned or replaced to ensure proper charge balance.

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