1. Magnetic Force

2. Strength of Magnetic Force
A charged particle moving in a magnetic field experiences a force that is perpendicular to BOTH the particle’s velocity and to the magnetic field itself. .
Dutch physicist

3. Strength of Magnetic Force
A charged particle moving in a magnetic field experiences a force that is perpendicular to BOTH the particle’s velocity and to the magnetic field itself.
Lorentz Force Law:
The magnitude of the magnetic force on a moving, charged particle is .
F = qvB sin q
Dutch physicist
(q is the angle between the charge’s velocity and the magnetic field)

4. F = qvB sin q
Sin 0, 180 = 0 If a charge has velocity in the same (or opposite) direction of the magnetic field, it experiences no force!
Sin 90 = 1 A charge that has velocity perpendicular to the magnetic field experiences the greatest force!

5. Question?
The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field.
Which particle experiences the greatest magnetic force?
Same B 3
F = q v B sin q 2 1

6. The direction of the magnetic force is given by the Right-Hand Rule
positive charge
► Point fingers in v (or current I) direction v B q
► Curl fingers as if rotating
vector v (current I) into B.
► Thumb is in the direction of the
force. F
negative charge
● For negative charge force is
in the opposite direction .
charge q moving with velocity v in the mag. field B
F is perpendicular to the plane of v and B

7. Right Hand Rule Practice
Convention for magnetic field direction:
x x x x x x x INTO Page
••••••••••••• OUT of Page
Right Hand Rule Practice
A proton enters a magnetic field, as shown. Which way will the electron turn? Up
Down
Out of page
Into page

8. Right Hand Rule Practice
Convention for magnetic field direction:
x x x x x x x INTO Page
••••••••••••• OUT of Page
Right Hand Rule Practice
A proton enters a magnetic field, as shown. Which way will the electron turn? Up
Down
Out of page
Into page
Put your fingers in the direction of the velocity and curl out of the page … your thumb points up

9. Right Hand Rule Practice
Convention for magnetic field direction:
x x x x x x x INTO Page
••••••••••••• OUT of Page
Right Hand Rule Practice
An electron enters a magnetic field, as shown. Which way will the electron turn? Up
Down
Out of page
Into page

10. Right Hand Rule Practice
Convention for magnetic field direction:
x x x x x x x INTO Page
••••••••••••• OUT of Page
Right Hand Rule Practice
An electron enters a magnetic field, as shown. Which way will the electron turn? Up
Down
Out of page
Into page
Remember to flip the direction of the force for negative charges

11. Right Hand Rule Practice
Convention for magnetic field direction:
x x x x x x x INTO Page
••••••••••••• OUT of Page
Right Hand Rule Practice
In which direction will wire segment B be pushed? Up
Down
Out of page
Into page
No force exists

12. Right Hand Rule Practice
Convention for magnetic field direction:
x x x x x x x INTO Page
••••••••••••• OUT of Page
Right Hand Rule Practice
In which direction will wire segment B be pushed? Up
Down
Out of page
Into page
No force exists

13. Right Hand Rule Practice
Convention for magnetic field direction:
x x x x x x x INTO Page
••••••••••••• OUT of Page
Right Hand Rule Practice
In which direction will wire segment C be pushed? Up
Down
Out of page
Into page
No force exists

14. Right Hand Rule Practice
Convention for magnetic field direction:
x x x x x x x INTO Page
••••••••••••• OUT of Page
Right Hand Rule Practice
In which direction will wire segment C be pushed? Up
Down
Out of page
Into page
No force exists
V and B are in the same direction; no force exists.

15. Magnitude of the magnetic field
We define the magnitude of the magnetic field by measuring the force on a moving charge: v B q
The SI unit of magnetic field is the Tesla (T), named after Nikola Tesla, a Croatian physicist.
1 T = 1 N·s/(C·m)

16. Magnetic Field & Magnetic Force Problems
We do:
What is the minimum magnetic field necessary to exert a 5.4 X N force on an electron moving at 2.1 X 107 m/s?

17. Magnetic Field & Magnetic Force Problems
We do:
What is the minimum magnetic field necessary to exert a 5.4 X N force on an electron moving at 2.1 X 107 m/s?
B = F / qvsinθ
B will be at a minimum when sin θ = 1
B = F / qv = 5.4X10-15N / (1.6 X C X 2.1 X 107 m/s)
B = 1.61 X 10-3 T

18. Magnetic Field & Magnetic Force Problems
You do:
What is the magnetic field necessary to exert a 5.4 X N force on an electron moving at 2.1 X 107 m/s if the magnetic field is at 45 degrees from the electron’s velocity?

19. Magnetic Field & Magnetic Force Problems
You do:
What is the magnetic field necessary to exert a 5.4 X N force on an electron moving at 2.1 X 107 m/s if the magnetic field is at 45 degrees from the electron’s velocity?
B = F / qvsinθ = 5.4X10-15N / (1.6 X C X 2.1 X 107 m/s X sin 45)
B = 2.3 X 10-3 T.

20. Magnetic Field & Magnetic Force Problems
We do and You do
What is the magnitude of the magnetic force on a proton moving at 2.5 X 105 m/s in a magnetic field of 0.5 T …
…if the velocity and magnetic field are at right angles?
… if the velocity and magnetic field are at 30°?
(c) … if the velocity is parallel to a magnetic field?

21. Magnetic Field & Magnetic Force Problems
We do and You do
What is the magnitude of the magnetic force on a proton moving at 2.5 X 105 m/s in a magnetic field of 0.5 T …
…if the velocity and magnetic field are at right angles?
… if the velocity and magnetic field are at 30°?
(c) … if the velocity is parallel to a magnetic field?
F = qvBsinθ , so
(a) when θ = 90°, F = (1.6 X C)(2.5 X 105 m/s)(0.5 T) = 2.0 X N,
(b) F = (2.0 X N) sin 30° = 1.0 X N, and
(c) F = qvB sin 0° = 0.

22. Comparison of Electric and Magnetic Forces
Using your notes, and working in small groups (3 or less), compare the electric and magnetic forces in terms of:
Magnitude
Direction
Work done
Effect on charged particles
The group with the most complete list wins a prize!

23. Comparison of Electric and Magnetic Forces
The electric force:
Felec = Eq
The magnetic force:
Fmag = qvB sin q
is always parallel to the direction
of the electric field.
acts on a charged particle
independent of the particle’s velocity
does work when moving charge:
The work, W = Fel d cosθ1, is converted into kinetic / thermal energy.
The electric field accelerates charged particles.
is always perpendicular to the direction of the
magnetic field
acts on a charged particle only when the
particle is in motion (F=0 if v=0), and only
if v and B do not point in the same or opposite
direction (sin 00 = sin = 0).
Force is perpendicular to motion so the work
done by magnetic force is zero.
W = Fmagd cosq1 = 0 (cos 900 = 0).
Change in kinetic energy of the charge is 0
In the presence of magnetic field, the moving charged particle is deflected (dotted lines)