READING QUIZElectric currents create magnetic

fields, but magnetic fields cannot create electric currents.

1. False2. True

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Chapters 10,11,12,13,14,15

FROM LAST TIME - - -

1. Permanent magnets have opposite poles: like poles repel, opposites attract (North, South).

2. Lines of force exist around magnets (field lines).

1. A current in a wire causes a magnetic field outside of the wire.

Two parallel current-carrying wires exert an attractive force on each other when the

two currents are in the same direction.

The magnetic force exerted on the moving charges of an electric current is perpendicular to both the velocity of the charges and to the magnetic field.

Use the right hand rule VxB to fine the direction of the Force

Use the right hand rule VxB to fine the direction of the Force

If the index finger of the right hand points in the direction of the velocity of the charge, and

the middle finger in the direction of the magnetic field, the thumb indicates the direction of the magnetic force acting on a positive charge.

The forces on each segment of a current-carrying rectangular loop of wire combine to produce a

torque that tends to rotate the coil until its plane is perpendicular to the external magnetic field.

The magnetic flux through the loop of wire has its maximum value when the field lines are perpendicular to the plane of the loop. It is zero when the field lines are parallel to the

plane of the loop and do not cross the plane.

Force per unit length on parallel wires:

F

l

2k 'I1I2

r

Force on a moving charge: F = qvB,

where F is perpendicular to both v and B.

Force on a wire with current I in a perpendicular B field:

F = I LB

K = 1x10-7 N/A2

When a current-carrying wire is bent into a circular loop, the magnetic fields produced

by different segments of the wire add to produce a strong field near the center of the loop.

A current-carrying coil of wire produces a magnetic field greater than a single loop and is proportional

in strength to the number of loops in the coil.

The two most important facts about magnets:

1. Moving a coil of wire near a magnet can cause a current to flow in the wire.

2. Moving a magnet near a coil of wire can cause a current to flow in the wire.

(Faraday - magnetic induction)

A magnet moved in or out of a helical coil of wire produces an electric current in the coil.

Faraday’s Law:

Induced voltage depends on the rate of change of enclosed magnetic flux

E = /t

Note: Induced voltages can cause currents to flow in a circuit. Hence, magnetism can create electricity!

LENZ’s Law

The induced current in a loop of wire produces an a magnetic field inside the loop that opposes the change in the field producing the change.

Transformers

t

d

dt(BA) ONE COIL

V1(total) = N1 V2(total) N2 V1

N1

V2

N2