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Physics 123
20. Magnetism
20.1 Magnets and magnetic fields
20.2 Electric currents produce magnetism
20.3 Force on current in magnetic field
20.4 Force on moving charge in magnetic field
20.5 Magnetic field due to a straight wire
20.6 Force between two parallel wires
Magnets and magnetic fields
1. Unlike poles attract2. A magnet will also attract magnetic materials that are not magnets (induced magnetism)3. Monopoles do not exist?4. Use a compass to trace the magnetic field (closed loops!)
NS
Electric Currents Produce Magnetism
Right-Hand Rule: Thumb points to current ( I) and curled fingers indicate B
I
BB
Electric Currents Produce Magnetism
Old wine … New bottle!
I
X
Electric Currents Produce Magnetism
Right Hand Rule: Thumb points to B and curled fingers indicate I
II
B
Electric Currents Produce Magnetism
II
Old wine … New bottle!
Force on current in magnetic field
I
B
XForce on wire INTO the page
Right Hand Rule: Fingers indicate I, curled fingers B and thumb points to direction of force
Force on current in magnetic field
I
B
Force = BIL
Force on current in magnetic field
I
Force = BIL sin
B
Force on current in magnetic field
I = 1.5 A, B = 0.04 T, coil is 3 cm x 2 cm.
Calculate the torque on the coil.
Torque on coil in magnetic field
F = BIL
F= 1.8 x 10 -3 N
= F x W = 3.6 x 10 -5 m.N
DC Motor
= BINA
Force on moving charge
X X X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X X X
e
Force on moving charge
X X X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X X X
X X X X X X X X X X X X
e
Force on moving charge
F = B e v
X X X X X
X X X X X
X X X X X
X X X X X
e
Electric Currents Produce Magnetism
I
BB
B = o I / 2 r
Electric Currents Produce Magnetism
II
B
B = o I / 2 r
Force between two currents
F = o I1 I2 L / 2 r
I1 I2
That’s all folks!
