Physics 121: Electricity & Magnetism – Lecture 10

Carsten Denker

NJIT Physics DepartmentCenter for Solar–Terrestrial

Research

March 28, 2007 Center for Solar-Terrestrial Research

Magnetic Field Due to a Current

Long straight wireSemi–infinite straight wire

Circular arc

Biot–Savart’s law

Permeability constant

034

ids rdB

r

70 4 10 Tm/A

0

4

iB

R

0

2

iB

R

0

4

iB

R

March 28, 2007 Center for Solar-Terrestrial Research

Force Between two Parallel Currents

To find the force on a current–carrying wire due to a second current–carrying wire, first find the field due to the second wire at the site of the first wire. Then find the force on the first wire due to that field.

Parallel currents attract, and anti– parallel currents repel.

March 28, 2007 Center for Solar-Terrestrial Research

Ampere’s Law

Ampere’s law

Curl your right hand around the Amperian loop, with the finger pointing in the direction of integration. A current through the loop in the general direction of your outstretched thumb is assigned a plus sign, and a current generally in the opposite direction is assigned a minus sign.

0 encB ds i

March 28, 2007 Center for Solar-Terrestrial Research

Solenoids and Toroids

Ideal solenoid

Toroid

0B in

0 1

2

iNB

r

March 28, 2007 Center for Solar-Terrestrial Research

Current–Carrying Coil as a Magnetic Dipole

Current–carrying coil

032

B zz