Download Ampere`s law

Lecture 14
Chapter 29
Ampere’s law
Course website:
http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Today we are going to discuss:
Chapter 29:
 Section 29.6 Ampere’s Law
 Section 29.5 Skip
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Bio-Savart Law
The magnetic field of a charged
particle q moving with velocity v
is given by the Biot-Savart law:
Note that the component of B parallel to the line of motion is zero.
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
ConcepTest
B field of q
A) Into the screen
What is the direction of the magnetic field at
the position of the dot?
B) Out of the screen C) Up
D) Down E) Left
Oersted Experiment
In 1819 Hans Christian Oersted discovered that an
electric current in a wire causes a compass to turn.
It was a demonstration that an electric current
produces magnetic field.
This is probably one of
the most important experiments ever done.
This observation showed that there is
connection between electricity and magnetism.
https://www.youtube.com/watch?v=Gtp51eZkwoI
(History of Electricity, start at 1.04.20)
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
While performing his electric
demonstration, Oersted noted to
his surprise that every time the
electric current was switched on,
the compass needle moved.
Let’s understand what is still missing in our magnetism picture.
Ampere’s Law
Electric Field
Magnetic Field
From Coulomb’s law
1
̂
4
Bio-Savart law
̂
4
Gauss’s Law
∙
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
There must be
something similar for B
Ampere’s Law
The line integral of the magnetic field around the curve
is given by Ampère’s law:
I1
∙
+
+
∙
I3
∙
Closed loop
(Amperian)
B
1) An Amperian loop is imaginary
2) It is a closed loop (any path can be used)
3) Choose direction (up to you).
It gives us which current is positive/negative
(use a right-hand rule:
curl your fingers in a chosen direction and an
outstretched thumb shows a positive current direction)
Amperian loop
∙
B
I2
These currents pass through the
area bounded by the loop, so
they are enclosed, Iin
I3
So I1 is positive; I2 is negative
∙
Amperian loop
Ampère’s law is very useful for a problem with a high degree of symmetry.
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
ConcepTest
Ampere’s Law
A) 0 A

The line integral of B around the
loop is 0 · 7.0 A.
Current I3 is
B) 1 A out of the screen
C) 1 A into the screen
D) 5 A out of the screen
E) 5 A into the screen
Assume I3 is out of the page
∮
∙
=7
1
Minus means our original assumption was wrong,
it is into the screen
Example
Magnetic field of a
current-carrying wire
The wire has cylindrical
symmetry so that we can
easily use Ampere’s law.
One moving charge creates
magnetic field lines centered
on the motion line:
Now we have many moving
charges (not just one). The
field pattern must be the same.
So we’ll take our Amperian
loop to be a concentric
circles of r.
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Solenoid
A solenoid is a helical coil of wire with the same
current I passing through each loop in the coil.
A uniform magnetic field can be generated
with a solenoid.
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Steps to make a solenoid
Magnetic field lines produced
with a straight wire
Let’s bend the wire into a loop
Now, let’s add more loops
A uniform magnetic field is generated with a solenoid.
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Example
The Magnetic Field
of a Solenoid
Take-home quiz
B
Along the sides (bc, da), the line integral is zero since the field is perpendicular to the path.
ds
ds
Along the bottom (ab), the line integral is zero since B  0
outside the solenoid.
∙
0
∙
B
B=0
0
∙
abcda
ds
B
0
∙
∙
∙
0
Amperian loop
abcda
∥
There are N loops with current I enclosed by an Amperian loop, so
∙
/
Uniform field
where n  N/l is the number of turns per unit length.
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Application of solenoids
This patient is undergoing magnetic
resonance imaging (MRI). The large cylinder
surrounding the patient contains a solenoid
that is wound with superconducting wire to
generate a strong uniform magnetic field.
B=1.2 T, I=100 A
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
The Magnetic Field Outside a Solenoid
•
•
The magnetic field outside a solenoid looks like that of a bar magnet.
Thus a solenoid is an electromagnet
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Electric Field
Magnetic Field
From Coulomb’s law
1
̂
4
Biot-Savart law
̂
4
Gauss’s Law
Ampere’s Law
∙
∙
So, now we know how to find magnetic fields using Bio-Savart and Ampere’s laws.
Now, the question is
“how does a magnetic field interact with material
(which consists of charges and current)?”
Magnetic force on
a moving charge
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Magnetic force on
current
Magnetic force on
a moving charge
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
The Magnetic Force on a Moving Charge
After Oersted’s discovery, there were many other experiments with magnetic fields,
currents, charges, etc. It was found that B exerts a force on a moving charge.
The magnetic force on a charge q as it moves through a magnetic field B with
velocity v is:
where  is the angle between v and B.
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics
Thank you
See you next time
PHYS.1440 Lecture14 A.Danylov
Department of Physics and Applied Physics