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# Unit 5 Free Body/ Electric Force Q watch

1. hi, in a past paper there was a Q of the following:

A speck of dust has a mass of 1.0 × 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 of strength 100 N C-1 and a uniform gravitational field of strength 9.8 N kg-1.

Draw a free-body force diagram for the speck of dust.

The answer was that the gravitational force is downwards, but the electric force was upwards... Im really confused because I thought that the electric force would act downwards bcos the electric field strength did?
2. If they are regarding the centre of the earth as positive then then the force on the electron = QE. Since the charge on the dust is minus this gives a negative force and therefore must be away from the earth.
The electric field strength is the force per unit charge. If the amount of charge is negative then it will have negative force - a force in the opposite direction to the field lines
3. Ta very much

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