Electromagnetism. Magnets Made from alloys of iron, nickel and cobalt Each Magnet has two poles! Magnetic poles are areas of concentrated magnetic force

Electromagnetism

Magnets•Made from alloys of iron, nickel and cobalt

Each Magnet has two poles!•Magnetic poles are areas of concentrated

magnetic force•When left to rotate freely,

▫One pole will seek (point to) the northerly direction on Earth – referred to as North Pole

▫One pole will seek the southerly direction on Earth – referred to as South Pole

Laws of Magnetic Poles•Opposite magnetic poles attract

•Similar magnetic poles repel

•Forces of attraction and repulsion are felt “at a distance” – affect each other before they touch (like gravity)

•Magnetic Field of Force – the space around a magnet in which magnetic forces are exerted

Characteristics of Magnetic Field Lines

1) The spacing of the lines indicates the relative strength of the force▫The closer the lines are the greater the force

2) Outside a magnet, lines are concentrated at the poles. ▫They are closer inside the magnet itself

Characteristics of Magnetic Field Lines

3) By convention, the lines proceed from S to N inside a magnet and from N to S outside a magnet

▫ All lines form closed loops

4) Lines do not cross each other

Domain theory of Magnets•Atoms in ferromagnetic materials can be thought of as tiny magnets with N and S poles – these tiny atomic magnets are called dipoles

•Each dipole can affect its neighbor, causing their dipoles to line up in the same direction – when this happens, called an electric domain

•Nickel, cobalt and any alloy containing iron, nickel or cobalt can become magnetized by bringing them close to a magnet – these materials are called FERROMAGNETIC

Unmagnetized Domain•dipoles lined up in each domain (group) but each domain is pointing randomly in different directions

•When an unmagnetized piece of iron is placed in a magnetic field (near another magnet) – the dipoles act like small compasses and rotate until they are all aligned with the magnetic field

Magnetized Domain

•Dipoles all line up in the same direction causing one end to become the north pole and one end to become the south pole

Effects of the Domain Theory1) Magnetic Induction

•A permanent magnet brought close to a piece of unmagnetized iron can force the poles of individual domains to align, turning it into a magnet

•Magnetic Induction – process of magnetizing an object from a distance

Effects of the Domain TheoryMagnetic Induction

• Soft Iron: iron that loses its magnetism as the magnet moves away

•Hard Iron: iron that retains its magnetism as the magnet moves away

Effects of the Domain Theory2) Demagnetization• Dropping or heating a magnet will demagnetize it – dipoles will go back into a random alignment

Effects of the Domain Theory3) Reverse Magnetization•If a bar magnet is placed in a strong enough magnetic field of opposite polarity, its domains can switch – the N-pole becomes S-pole, etc

Effects of the Domain Theory4) Breaking a Bar Magnet

• Breaking a bar magnet creates 2 magnets with the same poles/alignment of dipoles

Effects of the Domain Theory5) Magnetic Saturation•Occurs when the maximum number of poles are aligned, this determines the maximum strength of the magnet

Effects of the Domain Theory6) Induced Magnetism by Earth• If a piece of iron is held in Earth’s magnetic field pointing north and its atoms are agitated by heating or mechanical vibrations, its dipoles can align, creating a magnet

•Steel columns in buildings, ships and railway tracks are often magnetized

Principle of Electromagnetism:Moving electric charges produce a magnetic field.

Right-Hand Rule for a Straight Conductor:If a conductor is grasped in the right hand, with the thumb pointing in the direction of the current and the curled fingers point in the direction of the magnetic field lines

RHR straight conductor SUMMARY•Fingers B (magnetic field)•Thumb I (current)

8.1 Electromagnetism

Solenoid: a coil of wire

Right-Hand Rule for a Solenoid:If a solenoid is grasped in theright hand, with curled fingers point in the direction of the electric current and the thumb pointing in the direction of the magnetic field lines in its core.

RHR Solenoid SUMMARY•Thumb B (magnetic field)•Fingers I (current)

8.1 Electromagnetism

Note: The RHR is for conventional current flow; meaning the charge is positive.

The Geographic North pole is the Earth's magnetic south pole. The magnetic poles have switched back and forth through out Earth's history.

8.1 Practice Questions

Page 391 Questions 1-5

8.2 Magnetic Force on Moving Charges

The size of a magnetic force on a charged particle is:

1. directly proportional to the size of the magnetic field, velocity, and the charge of the particle.

2. is dependent on the angle, between the direction of the magnetic field and the direction of the velocity. The force is proportional to sin.

Fm = qvBsin where Fm = magnitude of the magnetic force (in N)

q = magnitude of charge (in C)v = speed (in m/s)B = magnitude of the magnet field (in T)

= angle between the direction of magnetic field and velocity


Thus,

B = __Fm__ qvsin


Note: The SI unit of magnetic field strength is Tesla.1T = 1 Tesla1 T = 1 kg/(C•s)


Motor Principle (Right Hand Rule):•Thumb – direction of current, I•Index finger (pointer finger) – direction of magnetic field

•Middle finger – direction of magnetic force [or use out from palm]



Example #1: An electron moves into a magnetic field of 0.50T [forwards] at a velocity of 5.0Mm/s[leftwards]. Calculate the magnetic force.

CATHODE TUBES


•Read Paragraph on Cathode tube on page 396.


•Thompson’s research lead to the development of the mass spectrometer, used cathode tube to deflect electrons. The electrons get deflected along a circular arc of radius, r.


Derive starting at net force.


Note for q=e, q_ = _ε_m B2r


•Note: e/m = 1.76 X 1011 C/kg


Example #2: Find the mass of an electron knowing e = 1.6 X 10-19 C and e/m = 1.76 X 1011 C/kg.

Example #3: Calculate the mass of fluorine-ion (charge of -1e) that has a charge of 1.602 X 10-19 C and is accelerated into a mass

spectrometer, through an electric

field with a magnitude of 2.23 X 105N/C into a 2.00T field. The radius of the curved path is 1.1 cm. (Solution on Blackboard)

Copy the following in your notes

8.2 Practice Questions

•Page 376 Q1-5 •Page 402 Q1,2,7

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Electromagnetism. Magnets Made from alloys of iron, nickel and cobalt Each Magnet has two poles! Magnetic poles are areas of concentrated magnetic force