Chpt' 30: Sources of Magnetism

1
Chpt. 30 Sources of Magnetism

• Basic Principles of Electromagnetism
• Hans Oersted discovers in 1819 that a compass
  needle is deflected by a wire carrying a current.
• Andre Ampère discovers that all magnetic
  phenomena, including permanent magnets, are due
  to current loops at an atomic level.

2
Right hand rule for current loop

• Wrap your right hand fingers around the current
  loop in the direction of the current and the
  thumb will point out the direction of the
  magnetic field .

3
Right hand rule for straight wire

• Point your right hand thumb along the
  direction of current and your fingers will wrap
  around the direction of the magnetic field.

• SEE ACTIVE FIG. 30.9 at pse6.com
• Harvard Lecture question 1

4
SUMMARY OF RIGHT HAND RULES

• RHR1 FOR A CURRENT LOOP, WRAP RIGHT HAND
  AROUND CURRENT FLOW AND YOUR THUMB WILL POINT IN
  DIRECTION OF MAGNETIC FIELD B
• RHR2 FOR A STRAIGHT WIRE, POINT THUMB IN
  DIRECTION OF CURRENT AND YOUR FINGERS WILL WRAP
  AROUND THE MAGNETIC FIELD LINES (WHICH ARE
  CIRCLES)

5
Magnetic Force between two parallel conductors
6
Procedure for solving configuration problems

• (1) draw all forces acting on the correct current
• (2) calc. the absolute value (magnitude) of each
  force from formula
• (3) determine the angle from 0-360 for each
  force vector
• (4) calc. x and y components of each vector
• F F (cosq i sin q j )
• (5) add all x and y components to get x and y
  components of net (resultant) force vector
• (6) calc. the magnitude of resultant vector using
  Pythagorean thm. and angle using arctan (Ry/Rx)

7
The Biot-Savart Law
8
Example 30.2 Magnetic Field of Curved Wire
Segment
CHALLENGE PROBLEM REPEAT FOR A COMPLETE
CIRCULAR ARC.
9
Challenge Prob. B on x axis

• 3) Special result
• If x 0 B m0I/2R (center) (checks out)
• If x infinite B k/x3 (magnetic dipole)

10
Ampères Law
SEE ACTIVE FIG. 30.8 at pse6.com
11
Example 30.4 Magnetic Field of Long
Current-Carrying Wire
12
Calc. of B for config. of wires
CALCULATE NET MAGNETIC FIELD B AT P

• Problem Two wires carry currents of 10 A in
  opposite directions. I1 10 A k and I2 - 10
  A k . Wire 1 is located at x - 2 m while wire
  2 is located at x 3 m . Calc. the net
  magnitude field at the pt. x 0 m.
• Hint use right hand rule. Each B field will be
  in the /- j direction.

13
Procedure to solve B config. Prob
14
The Magnetic Field of a Solenoid
15
Link to return to Phy2049 classnotes homepage

• Link here