# Surface charge decay

1. Jul 10, 2013

### Disserate

I am a chemical engineer and have little experience with complicated electrical properties. I was wondering if there was a way to calculate the rate at which the surface charge of a particle will dissipate when it comes in contact with an electrode of the opposite charge. For example a small particle of polypropylene that has a negative charge on the surface contacting an aluminum plate that has a high positive voltage on it with respect to ground. Thanks.

2. Jul 10, 2013

### Carl Pugh

There are so many variables in this example, that it may be easiest to just perform tests.

3. Jul 10, 2013

### Disserate

Well I wouldn't even know where to begin to perform tests, but I do know most of these variables. I know the voltage on the plate, the charge density of the particle along with the size, and the shape can be approximated as spherical. I can find most material properties online, but what I really need is to have a relationship that can give me the approximate amount of charge that is lost during an impact of the particle on the plate.

4. Jul 10, 2013

### dlgoff

This sounds like something to be researched. e.g. http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1387828&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D1387828

I think you'll need to know the material's (polypropylene) electrical conductivity and it appears that this is being researched also. i.e. Electrical conductivity of polypropylene

5. Jul 11, 2013

### Carl Pugh

There were some experiments where they were trying to measure the charge of one electron.
They would charge a small particle and keep it at a constant height between two plates that were at different voltages.
They then calculated the weight of the particle and the force required and the force of the electric field. From this they could calculate the charge of one electron.

If your particle only has one electron, then the rate of discharge would be instantaneous.

If it's not a secret, what are you attempting to do.

6. Jul 11, 2013

### Disserate

Basically I am trying to separate particles using charge. The particles are forced to interact with one another beforehand causing static charge to build up on them. In theory the bigger particles should charge positive and the small negative. The particles are then let to fall down between 2 high voltage electrodes, one positive and the other negative. By looking at the size of the particles underneath the electrodes, we can see if this can work. The problem is that even though we can see obvious separation at the top of the electrodes, by the time they reach the bottom they are no longer separated and are all mixed up. I was thinking that when the particles come in contact with the electrode it loses some of its charge, and by the time it gets down to the bottom, all is gone and it no longer moves towards the electrodes that it should.

7. Jul 12, 2013

### Carl Pugh

My thinking is that the opposite charged particles attract each other and by the time they get to the bottom, they have neutralized each other.

8. Jul 12, 2013

### Disserate

That was on thing that we took into account earlier, but when this happens we can see clumps forming in the mixture before we send it down between the electrodes. To counter this we don't let the particle charge enough so that the electrical force of attraction between them is significant at all.

9. Jul 12, 2013

### dlgoff

I'm curious. You mentioned polypropylene particles, so what might the other particle's material be? Or are you trying to develop a generic machine?

10. Jul 12, 2013

### Disserate

The particles are of the same material, but different sizes. This set up should also work with other materials, but they still should be the same material in different sizes.

11. Jul 12, 2013

### dlgoff

Thanks. Very interesting idea that could apply to the pharmaceutical industry.

12. Jul 13, 2013

### Carl Pugh

Why don't you just make all the particles small, then you wouldn't have to separate them?

Why not forget using charge to separate the large and small particles and just slowly put particles is a vertical shaft with a small cross draft of air. The cross draft of air will cause the small particles to drift to one side of the shaft.