Thank you for the opportunity to help you with your question!
Charge on particle must be negative as then it will experience force in upward direction balancing the mg force due to its weight. Therefore, mg=qE ( q is charge on particle, only magnitude, and E is electric field)
The magnitude of an electric field in which the electric force on a proton is equal in magnitude to its weight = E
E =1.0228756*10^-7 N/C
Please let me know if you need any clarification. I'm always happy to answer your questions.
Jan 18th, 2016
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