# Magnetism another “invisible” force Physics 100 Chapt 14.

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Magnetism another “invisible” force Physics 100 Chapt 14

N-poles & S-poles NS N S Unlike poles attract

N-poles & S-poles NS S N like poles repel

N-poles & S-poles can never be isolated NS NSNS Break a magnet into two pieces: A new N-pole & S-pole are formed

Compass needles If left to its own accord, the N-pole of a magnet will try to point towards the North

The Earth is a magnet South Pole North Pole N S Earth’s geographic North pole is a Magnetic S-pole geographic South pole is a magnetic N-pole

Magnetic Fields N S x Imagine a small N-pole is placed at the point N Determine the magnetic force on the imaginary N-pole B-field is in the direction of the net force

B-field from a bar magnet NS

Rules N S B-field lines come out of N-poles & go into S-poles B-field lines never end Density of B-field  strength of the field

E-field rules (reminder) E-field lines start on + charges and end on - charges Direction of E-field lines = direction of force on a + charge placed at that point (opposite for – charge) Where E-field lines are close together, the E field is large; where they are far apart, the E-field is small.

Horseshoe magnet

Earth’s B-field

Magnet arrays N N N S

Christian Oersted

Electric currents produce B-fields I B

Right-hand rule B

Current loop NS

Two current loops N S

Even more loops N S

Solenoid coil Looks like a bar magnet S N

Magnetic forces on electric currents I

Another right-hand rule I

Forces on two parallel wires II Current in same direction: wires attract B

Forces on two parallel wires I I Current in opposite directions: wires repel B

Force law of Biot & Savart I2I2 I1I1 F =  B I1I2 ldI1I2 ld l d  = 2 x 10 -7 NA2NA2

Biot & Savart example 20A F =  B I1I2 ldI1I2 ld 2m 0.01m F = 2 x 10 -7 NA2NA2 20A (20 A ) 2 2 m 0.01 m F = 2 x 10 -3 N Small, but not tiny

Electric motor I B F I F

I B

Speakers Permanent magnet Solenoid Electro-magnet