mass Am = 60 mg on the balance pan. Find the magnetic induction
at the spot where the coil is located.
3.255. A square frame carrying a current I = 0.90 A is located
in the same plane as a long straight wire carrying a current / 0
= 5.0 A. The frame side has a length a = 8.0 cm. The axis of the
frame passing through the midpoints of opposite sides is parallel to
the wire and is separated from it by the distance which is 1 = 1.5
times greater than the side of the frame. Find:
(a) Ampere force acting on the frame;
(b) the mechanical work to be performed in order to turn the
frame through 180° about its axis, with the currents maintained
constant.
3.256. Two long parallel wires of negligible resistance are con-
nected at one end to a resistance R and at the other end to a de volt-
age source. The distance between the axes of the wires is 1 = 20 times
greater than the cross-sectional radius of each wire. At what value
of resistance R does the resultant force of interaction between the
wires turn into zero?
3.257. A direct current I flows in a long straight conductor whose
cross-section has the form of a thin half-ring of radius R. The same
current flows in the opposite direction along a thin conductor located
on the "axis" of the first conductor (point 0 in Fig. 3.61). Find the
magnetic interaction force between the given con-
ductors reduced to a unit of their length.
3.258. Two long thin parallel conductors of the -(2--).—c-
shape shown in Fig. 3.71 carry direct currents Ii
and / 2. The separation between the conductors is a,
the width of the right-hand conductor is equal to b. II I'
With both conductors lying in one plane, find the
magnetic interaction force between them reduced
to a unit of their length.
3.259. A system consists of two parallel planes
carrying currents producing a uniform magnetic Fig. 3.71.
field of induction B between the planes. Outside
this space there is no magnetic field. Find the magnetic force acting
per unit area of each plane.
3.260. A conducting current-carrying plane is placed in an external
uniform magnetic field. As a result, the magnetic induction becomes51 82 81 82 fif
0(a) (c)
Fig. 3.72.142