# Magnetic and Electric Fields problem

• davidj
In summary, the conversation discusses the relationship between force gravity and magnetic force in Oersted's law. The goal is to calculate the required magnetic field and current needed to make a wire "float" in a specific distance from another wire.

#### davidj

Homework Statement
A straight wire of linear mass density 150 g/m has a
current of 40.0 A (supplied by a flexible connection of
negligible mass). This wire lies parallel to, and on top
of, another straight horizontal wire on a table. What
current must the bottom wire have in order to repel
and support the top wire at a separation of 4.0 cm
Relevant Equations
fg = mg
fm = qvbsin(theta)
BIL = mg
Would i assume that fg = fm (force gravity holding straight wire down is equal to the magnetic force) and isolate for I?

Are you familiar with Oersted's law?

davidj

davidj
yes i am but how you mind explaining how its applicable in this situation

davidj said:
yes i am but how you mind explaining how its applicable in this situation
The wire that is supposed to "float" is subjected to the magnetic field that comes from the wire that is lying on the table.

First, calculate what B is required for the wire with 40 A current to "float". Then you figure out what current is required in the cable lying on the table to produce this magnetic field at the desired distance.