5 . The question might as well be restated, “Name four things you could do to change the flux through the
loop,” because only a changing magnetic flux induces an EMF.
(a) Rotate the wire about an axis in the plane of the page. This will change the θ term in the
expression for magnetic flux, BA cos θ .
(b) Pull the wire out of the field. This will change the area term, because the magnetic field lines will
intersect a smaller area of the loop.
(c) Shrink or expand the loop. This also changes the area term in the equation for magnetic flux.
(d) Increase or decrease the strength of the magnetic field. This changes the B term in the flux
equation.
Rapid Review
• Magnetic fields can be drawn as loops going from the north pole of a magnet to the south pole.
• A long, straight, current-carrying wire creates a magnetic field that wraps around the wire in concentric
circles. The direction of the magnetic field is found by a right-hand rule.
• Similarly, loops of wire that carry current create magnetic fields. The direction of the magnetic field is,
again, found by a right-hand rule.
• A magnetic field exerts a force on a charged particle if that particle is moving perpendicular to the
magnetic field.
• When a charged particle moves perpendicular to a magnetic field, it ends up going in circles. This
phenomenon is the basis behind mass spectrometry.
• A changing magnetic flux creates an induced EMF, which causes current to flow in a wire.
• Lenz’s Law says that when a changing magnetic flux induces a current, the direction of that current will
be such that the magnetic field it induces is pointed in the opposite direction of the original change in
magnetic flux.
• The Biot–Savart law has as its consequence that a little element of wire carrying a current produces a
magnetic field that (1) wraps around the current element via the right-hand rule, and (2) decreases in
magnitude as 1/r 2 , r being the distance from the current element. This is applicable to Physics C only.
• Ampére’s law has as its consequence that (1) the magnetic field produced by a very long, straight
current is
outside the wire; inside the wire, the field increases linearly from zero at the wire’s center, and (2) the
magnetic field produced by a wire-wrapped torus is zero everywhere outside the torus, but nonzero
within the torus. The direction of the field inside the torus is around the donut.