Magnetism

Physics Department, New York City College of Technology

Key words Magnetism Magnetic field North pole South pole Right hand rule Magnetic force Solenoid

Magnets North pole, Sourth

pole A magnet can

exert force on another one

They interact through the magnetic field B

Magnetic attraction/repulsion

Disc 19, #1

Magnetic fields A magnetic field

surrounds a magnet The unit is Tesla (T) Magnetic field lines

Direction:tangent to a field line

Strength:proportional to the number of lines per unit area

Magnetic fields around bar magnets

Disc 19, #4 Disc 19, #5

The Earth as a magnet Geographic poles Magnetic poles Why a compass

points to the North

Declination

Electric currents produce magnetic fields Right-hand rule

grasp the wire with the right hand so the thumb points in the direction of the current

fingers will encircle the wire in the direction of the magnetic field

Right hand rule

Disc 19 #7 Disc 19 #9

Magnetic force on a current A current carrying

wire experiences a force in a magnetic field

The magnitude is sinIlBF

Magnetic force on a current (continued) Right-hand rule Direction:

Perpendicular to direction of the

current direction of the

magnetic field

The definition of magnetic field

When = 90, then The magnetic field can be defined

using

The SI unit for B is tesla (T)

IlBF max

IlFB /max

Example #1

Force on a current

Force on electric charge moving in a magnetic field The magnitude is

v is the velocity B is magnetic field is the angle

between and Direction: right-

hand rule

sinvBqF

v

B

Deflected electron beam

Disc 20, #3 Disc 20, #4

Example #2

Force on a charge

Magnetic field due to a long straight wire The magnitude is

I is current r is the distance from the

wire 0 is permeability of free

space

Direction: right-hand rule

Ir

IB

20

AmT /104 70

Force between two parallel wires Magnitude:

I1 and I2 are currents d is the separation l is the length of the wire

Direction: attractive between

currents in the same direction

repulsive between currents in the opposite directions

ld

IIF 210

2

Solenoid The magnetic field

within a solenoid is

I is current N is the number of

turns l is the length of

the solenoid 0 is permeability

of free space

lINB /0

Coils with magnet

Disc 20, #12