Chapter 28 Magnetic Fields

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Chapter 28 Magnetic Fields

FIG. 28-20 B A wire (shown in cross section)

carrying current out of the page^.

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Chapter 28 Magnetic Fields

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Chapter 28 Magnetic Fields 28-9 Torque on a Current Loop

Much of the world's work B done by electric motors B due to B magnetic forces.

FIG. 28-21 B the elements of an electric motorB rectangular loop of wire B carrying a current and free to rotate about a fixed axis B immersed in a magnetic field.

Magnetic forces B on the wire produce B torque B rotates the rectangular loop.

The direction B of the current B reverses every half

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Figure 28-22a B shows a rectangular loop B of sides a Û b B carrying current i B through uniform magnetic field.

Rectangular long sides B 1 Û 3 B perpendicular to the field direction B into the page B its short sides B 2 Û 4 B not.

Figure 28-22b B shows a right-hand rule for finding the direction B normal vector n.

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Fig. 28-22c B the normal vector of the loop B shown at an arbitrary angle θ B to the direction of B magnetic field

The net force B on the loop B the vector sum of the forces acting on its four sides.

Force on B side 2 Û side 3 B have the same magnitude but opposite direction B their net force Û torque B zero

↓

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Chapter 28 Magnetic Fields

Force on B side 1 Û side 3 B have the common magnitude B iaB B do not share the same line of action B produce a net torque Ð

(b/2) sinθ

This torque B tends to rotate the loop B to align its normal vector n with the direction of the magnetic field.

If the loop of B current i B replaced with coil of N loops The total torque Ð

A (= ab) B the area enclosed by the coil.

(NiA) B grouped together B properties of the coil

Equation 28-33 holds for all flat coils B no matter what their shape B provided the magnetic field is uniform.