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Magnetic Fields and Electromagnetism

By: Amna, Ravina, Shivani, Riddhi, and Ismail

physics12 magnetism

on 26 February 2012

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Transcript of Magnetic Fields and Electromagnetism

So how does
the magnetic field
around the magnet look?
Magnetic Field of the Earth
Sir William Gilbert, an english physicist devised
a model to explain the Earth's magnetism
Determined that Earth's magnetic field is similar
to that of a larger magnetic bar, inclined at a slight angle
Magnetic Field of the Earth vs Magnetic Field of a Inclined Bar Magnet

Magnetic Fields and Electromagnetism
By: Amna, Ravina, Shivani,
Riddhi, and Ismail
Prior to the 19th century: Electricity and magnetism were thought to be separate until Hans Christain Oersted (Danish physicist) accidently revealed the relationship between the two.
Magnetic Field of a Straight Conductor

Electric current through a straight, long wire
Magnetic Field in a Loop
Magnetic field will appear in a circular motion if a straight wire is formed into a circular loop
Magnetic Field of a Solenoid
Right hand rule for solenoid:
Magnetic forces on conductor and moving charges
Oersted investegated intraction between a magnet and the magnetic
field of a conductor carrying a current.

French scientist Andre Marie Ampere: worked on a general relationship
between the current flowing through any conductor and the strength
of the magnetic field it produces.
He proposed the following statement:
Ampere's Law
Law also allows us to find the magnetic field strength in the core of the solenoid
using the following formula:

B= µ (NI/L)

B: magnetic field strength in the core of the solenoid in teslas (T)
I: current flowing through the coil, in amperes
L: is the length of the solenoid in metres
N: number of turns on the coil

*** The direction of the magnetic field is given by the right hand rule.

The Ampere as a unit of Electric current
Amperes Law contd.
B: is the component of B
µ: proportionality constant, called the permeability of free space, whose value
is 4π x 10^-7 T.m/A.
I : net current flowing through the area enclosed by the path
r: distance from the wire carrying the current.
Moving charges
The force that a conductor goes through in a magnetic field is because of the motion
of charged particles moving through it.

If the charged particles stationary ------> no current, no force
The ampere was introduced as the unit of electric current equivalent
to 1C of charge passing a given point in a conductor in 1 second.

1A is the current flowing through each of two long, straight, parallel conductors
1m apart in a vacuum, when the magnetic force between them is 2*10^-7 N
per metre of the length.

1 C is the charge transported by s current of 1 A in a time of 1 s.

1 C = 1 A.s

James Clerk Maxwell discovered electromagnetic waves in the 19th century
He summarized his theories in 4 basic relationships:

1) Distribution of an electric charge in space is related to the electric field it produces.
2) Magnetic field lines are continuous whereas electric field lines begin and end on electric charges
3) An electric field produce magnetic field because of the presence of the moving electric charges.
4) A changing magnetic field can produce a changing electric field
James Maxwell also predicted that as they continuously changed, these interchanging magnetic and electric fields would propagate through space in the form of an electromagnetic wave.

Came up with these characteristics for the waves:
Electromagnetic Waves

This wave was first produced and detected by Heinrich Hertz, a German physicist, in 1887 after Maxwell’s death

He also showed that these waves travelled at the speed of light and exhibited the characteristic wave phenomena: reflection, refraction, interference, and even polarization

He called this waves radio waves

Which as we know was a major discovery in the human history that lead to many great inventions. (TV, radios, cell phones and etc.)
Heinrich Herts
Applications of Electromagnetic Waves
Now a days electromagnetic waves are used in many waves such as

• Electromagnetic waves are produced whenever electric charges are accelerated.

• If the electric charge is accelerated in periodic motion,
f of electromagnetic waves produced = f of oscillation of the charge

• All electromagnetic waves travel through a vacuum at a common speed, c, and obey the wave equation
C = fλ
• Electromagnetic waves consist of oscillating electric and magnetic fields in a constant phase relation, perpendicular to each other.

• Electromagnetic waves exhibit the properties of interference, diffraction, polarization, and refraction, and can carry linear and angular momentum.
What are magnets and magnetic fields?
Has two poles: a North pole and a South pole
Law of Magnetic Poles: Opposite magnetic poles attract
and similar magnetic poles repel.
magnetic force is similar to gravitational force
it acts at a distance and therefore creats a Magnetic Field of
force around the magnet
its a vector quantity

He stated one basic principle of electromagnetism:
Moving electric charges produce a magentic field.

Magnetic field created consists of field lines that are concentric circles, with the wire at their centre
The right hand rule applies to this

If the conducter is grasped in the right hand, with the thumb
pointing in the direction of the current, the curled fingures will
point in the direction of the magnetic field lines
If a solenoid is grasped in the right hand, with the fingures curled in the direction of the electric current, the thumb will point in the direction of the magnetic field lines in its core, and, towards its north pole

Radio and T.V communications
X - rays
Gamma Radiation
Field lines inside the loop are closer to each other, therefore , stronger magnetic field on the inside than on the outside
Bar Magnet
Electromagnetic Spectrum
Maxwell predicted, and verified by Hertz, that electromagnetic waves travel at the speed of light (3,0 x 10^8 m/s)

was a paramount significance
Light was accepted to behave like wave,
but a wave of what?
Maxwell argued that light must be an electromagnetic wave
Hertz proved him right when he discovered radio waves
X - Rays
1895 - Wilhelm Roentgen discovered X - Rays

found that x- rays weren't deflected by electric or magnetic fields

concluded that they were electromagnetic waves of short wavelenght, with preperties similar to visible light

Later experiments verified that x - rays are produced when high speed electrons are decelerated very quickly in striking an obstacle

x - rays ionize the substances that go through by "knocking" electrons loose from their atoms
very powerful x - rays are used in radiation therapy

they kill cells by ionization

x - ray produced ions can interfere with the normal operation of cell

cell can become defective

x - ray radiation can potentially be dangerous and should be avoided when unnecessary
Gamma Radiation

1896 - Antoine Henri Bequerel discovered a phenomenon called radioactivity
Radioactive materials revealed that there were three distinct types:

Alpha Particles
Beta Particles
Gamma Rays
These radioactive emissions could easily be distinguised in a magnetic field:
Alpha particles being positve, and bent one way
Beta particles being negative, and are bent the other way
Gamma rays are unaffted by magnetic field, therefore neutral.
Concludes - that Gamma rays were very high frequency and electromagnetic waves emitted from the nucleus of certain radioactive substances. This is why Gamma Rays are being used in nowdays to improve technology
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