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    A speck of dust experiences a uniform downward electric field and a uniform gravitational field.

    draw a free body force diagram.

    Why the electric force is acting upward?

    The whole question is :

    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 NC^-1 and a uniform field of strength 9.8 N kg^-1

    Draw a free body force diagram for the speck of dust.
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    are you sure that thats all the information you are given?

    maybe the dust particle is negatively charged in which case it will experience an upward force in a downward field.
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    Who is to say it is negatively charged?
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    Anyone knows?

    (Original post by mik1a)
    Who is to say it is negatively charged?
    the question does
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    (Original post by GeneralGrievous)
    the question does
    The question doesnt mention it is negative.
    How do you know that?
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    (Original post by superkillball)
    A speck of dust experiences a uniform downward electric field and a uniform gravitational field.

    draw a free body force diagram.

    Why the electric force is acting upward?

    The whole question is :

    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 NC^-1 and a uniform field of strength 9.8 N kg^-1

    Draw a free body force diagram for the speck of dust.
    yo man. electron has charge -1.6 x 10^-19, hence, since the charge of the dust particle is equal to electron, it must be negative.
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    when you draw diagrams for electric fields, the arrows show the force that will be felt by a positive charge. so, if the question says that the field is acting downwards, it means that if a positive charge was placed in the field, it would be repelled down. but since our speck of dust is negatively charged, it will be attracted upwards.

    however, our little friend also has a mass, therefore it is attracted downwards due to the gravitational field.

    so our final diagram has two forces on the particle, acting in opposite directions;
    the force caused by electric field pulls it up, and the force due to gravity, because of its mass, pulls it down.

    (Original post by superkillball)
    The question doesnt mention it is negative.
    How do you know that?
    because an electron is negative.
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    Thank you very much.
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    Ok, the question was edited so I look a bit foolish, but anyway you need to remember an electron is negative.
 
 
 

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