# How to determine the magnitude of the electric force

## Homework Statement

A piece of tape is pulled from a spool and lowered toward a 170-mg scrap of paper. Only when the tape comes within 8.0 mm is the electric force magnitude great enough to overcome the gravitational force exerted by Earth on the scrap and lift it.

Determine the magnitude and direction of the electric force exerted by the tape on the paper at this distance

## Homework Equations

Coulomb's Law:
Force (electric)= k(Q1*Q2)/(r^2)

## The Attempt at a Solution

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I was not sure how to approach this problem.
But I converted 170mg=.17g and 8mm=.008m

The force is attractive, directed upward

Since k is 9*10^(9) N*(m^2)/(C^2)
and we know that the distance between the tape and the paper is .008m
I assume that we need to find the charges of the tape and the paper.

I know an electron charge is -1.602*10^(19) C

I thought that needed to see how many electrons there are in the .17g of tape. But I do not know the chemical makeup of the tape or the paper. So I don't know how many electrons there are. In other words, I am not sure how to find the charges Q1 and Q2

## Answers and Replies

TSny
Homework Helper
Gold Member
Welcome to PF!

Suppose a calculator of mass 100 g rests on a table top. How much force would be required to lift the calculator off the table?

100g= .1kg
(.1kg)(9.8m/s^2)= .98N?

Welcome to PF!

Suppose a calculator of mass 100 g rests on a table top. How much force would be required to lift the calculator off the table?
.98N
100g=.1kg
.1kg*9.8m/s^2=.98N

TSny
Homework Helper
Gold Member
100g= .1kg
(.1kg)(9.8m/s^2)= .98N?
Yes. Can you apply the same idea to your question about the electrical force of the tape?

Yes. Can you apply the same idea to your question about the electrical force of the tape?
I am not quite sure. The calculator scenario is F=ma.
The tape and paper scenario is an electrical scenario.
I am just not quite sure where the example is getting at.
I know that F(mag. of E.field)= q*E
But I am unsure of how to put these pieces together

cnh1995
Homework Helper
Gold Member
The tape and paper scenario is an electrical scenario.
The calculator had a mass of 100gm.
The paper has a mass of 170mg. How much force is required to pull the paper up? Which force is pulling the paper up?

CWatters
Science Advisor
Homework Helper
Gold Member
I am not quite sure. The calculator scenario is F=ma.
The tape and paper scenario is an electrical scenario.
I am just not quite sure where the example is getting at

Note that TSny asked you to calculate the magnitude of the force required to lift the calculator. He didn't specify what the source of that force was. Does the source of the force (eg crane, hand, rocket, magnetic, electrostatic) make a difference to the magnitude and direction?

Yes. Can you apply the same idea to your question about the electrical force of the tape?
Thank you
F(elec.)=.001666

TSny
Homework Helper
Gold Member
Thank you
F(elec.)=.001666
Good. All you need to do is add the units and the direction of the force.

• hotram
Good. All you need to do is add the units and the direction of the force.
[/Q
Would the units be Newtons or Coulombs ?

TSny
Homework Helper
Gold Member
Would the units be Newtons or Coulombs ?
Which of these corresponds to a unit for force?

Which of these corresponds to a unit for force?
haha, thanks!