# A speck of dust that carries charge in a electric field?

• Tangeton
In summary, the electric field pattern and the sign of the charge on the surface will be different depending on the size of the speck of dust.f

#### Tangeton

A speck of dust has a mass of 1 x 10-18 kg and carries a charge equal to that of one electron. Near to the Earth's surface it experiences a uniform downward electric field od strength 100NC-1 and a uniform gravitational field of strength 9.8Nkg-1.

Draw a diagram which shows the electric field pattern and the sign of the charge on the surface.

I find my college's homeworks very confusing in wording and just generally to understand. I can do more stuff with exam Q's most of the time, but these homework bring me out of the safe zone because I can never actually understand what they are trying to say.

First of all, It says the speck of dust itself has a charge, does that mean that I can treat it as a point charge?

Secondly, if it's equal to one electron's charge, does it automatically make the charge negative? The homework sheet says "Electronic Charge = 1.6 x10-19", while the formula booklet says there is a minus before all that so I don't know whether it is a positive charge or a negative charged dust speck.

Thirdly, if it is a point charge, does it mean that it will have its own radial field that, depending on whether it is positive or negative, goes outwards or towards the charge? And that there is going to be a background electric field that is going downwards from most positive to least positive at the same time?

Fourthly, what does it mean by ''the sign of the charge on the surface''? Does it mean that I label the particle negative or positively?

Since this forum requires an attempt at answer:

Last edited:
First of all, It says the speck of dust itself has a charge, does that mean that I can treat it as a point charge?
That depends on what you want to calculate. For the sketch, it should not matter (a sphere is easier to draw clearly).
Secondly, if it's equal to one electron's charge, does it automatically make the charge negative?
Probably, as an electron has a negative charge.
Thirdly, if it is a point charge, does it mean that it will have its own radial field that, depending on whether it is positive or negative, goes outwards or towards the charge?
It will have its own electric field, independent of the size of the speck of dust. You probably don't need this field.
Fourthly, what does it mean by ''the sign of the charge on the surface''? Does it mean that I label the particle negative or positively?
Probably, but I don't see the point in this part of the homework question.

You should show the E field pointing from the Earth to the speck (quiz: why not from the speck to the earth?). The surface they refer to must be the earth, since it's obvious what the charge on the speck is. The Earth's surface in the immediate vicinity of the speck will have charge induced due to the presence of the speck.

The wording on this problem does indeed leave much to be desired.

The surface they refer to must be the earth, since it's obvious what the charge on the speck is.
Ah, good idea.
The Earth's surface in the immediate vicinity of the speck will have charge induced due to the presence of the speck.
I doubt that's relevant, considering the tiny charge of the speck.

Yes, the wording of the problem is problematic.

I doubt that's relevant, considering the tiny charge of the speck.
But, it's a tiny problem! The flux lines have to end somewhere, and that somewhere is the earth.