19 Overview

1
19 Overview

• magnetic force fields sources
• forces on charges currents
• Homework
• 3, 7, 10, 33, 45, 55, 75.

2
Magnetic Field, B
0

• SI Unit tesla, T N/Am
• Direction shown below

3
Magnetic Poles

• Do ______ have _____ electric charge
• _______________ plates easily make uniform
  electric fields.
• Uniform B field difficult to achieve

4
Magnetic Force on Charge
0

• F qvB sin ?
• ___________ points in direction of force on a
  ________________ charge (Flat RHR)
• Negative charge feels ________________ directed
  force

5
Properties of the Magnetic Force
0

• F qvB sin ?
• Proportional to charge, speed, field, sine of
  angle between v and B.
• perpendicular to both v and B.
• /

6
Ex. An object with a charge of 2.0 mC is moving
perpendicular to a magnetic field of strength
1.0 T at a speed of 1.0 x 103 m/s. a) Sketch the
situation. b) Find the magnetic force on the
particle. c) Indicate the direction of the
force on your sketch.
0
7
Applications of Magnetic Force

• electric motor
• Mass spectrometer
• magnetohydrodynamic propulsion
• colliders

8
Cyclotron Motion
0

• Magnetic force is __________________ to velocity
• causes a free charge to move in a circle
• Does no ___________

9
Cyclotron Motion
0

• qvB _____________
• radius of motion r mv/qB
• period of motion T ( vT 2pr )
• frequency 1/T

10
Ex. A particle, m 1gram, v 10m/s, q 1mC,
moves perpendicular to a uniform field B 1.0 T
directed into the screen. Sketch the motion and
calculate its radius.
0
11
Magnetic Force on a Current Carrying Wire
0

• net magnetic force net magnetic force on all
  moving charges
• qv IL
• (C)(m/s) (C/s)(m)
• F ILB sin ?
• //

12
Force on a Current Carrying Wire
0

• F ILB sin ?
• Direction is given by a right hand rule

13
Ex. A wire of length 1m is perpendicular to a
uniform magnetic field of strength 0.50 T. a)
Sketch the situation. b) Choose a direction for
the current and indicate the direction of the
force on the wire for your choice. c) If the
magnitude of the force is 10 N, find the current
in the wire.
0
14
Electric Motors
0
The ILB force causes a motor coil to turn.
Current __________________ is changed by a split
ring.
15
Torque on Coil

• area, A number of turns, N
• current in wires, I
• in field, B
• angle between coil normal B q.
• torque NIABsinq.

16
torque example

• Tesla Motors Car produces __________________
  271 mN NIAB 271 mN.
• estimated reasonable? specs
• with B 0.3T NIA 904.
• with A 0.05sq.m. NI 18,000.
• with I 20A N 900turns

17
Straight Wire Loop
0
18
Solenoid
19
Ferromagnetism

• Unmagnetized
• Induced
• Permanent

20
Summary

• F qvB sinq ILB sinq (Flat RHR)
• B fields around wires (Curled RHR)
• B field inside a solenoid is nearly uniform
• electric motors utilize B force
• charge motion (v, I) causes B fields
• //