Download
1 / 58
E N D
Magnets have been known for centuries. The Chinese and Greeks knew about the “magical” properties of magnets. The ancient Greeks used a stone substance called “magnetite.” They discovered that the stone always pointed in the same direction. Later, stones of magnetite called “lodestones” were used in navigation. William Gilbert, an English physician, first proposed in 1600 that the earth itself is a magnet, and he predicted that the Earth would be found to have magnetic poles.
What is Magnetism? Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons. All magnetic phenomena result from forces between electric charges in motion.
The ends of a magnet are where the magnetic effect is the strongest. These are called “poles.” Each magnet has 2 poles – 1 north, 1 south. Like repels like… Opposites attract!
Poles of a magnet always Come in pairs! “Law of Poles” If you cut a magnet in half, S N S N S N you get 2 magnets!
No Monopoles Allowed It has not been shown to be possible to end up with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole). Note: Some theorists believe that magnetic monopoles may have been made in the early Universe. So far, none have been detected. S N
Magnetic Fields The region where the magnetic forces act is called the “magnetic field” Magnetic fields are vector quantities. The direction at any location is in the direction that the north pole of a compass would point if at that location
Field Lines of Attracting Bars Field Lines of Repelling Bars
Atoms themselves have magnetic properties due to the spin of the atom’s electrons. • Groups of atoms join so that their magnetic fields are all going in the same direction • These areas of atoms are called “domains”
When an unmagnetized substance is placed in a magnetic field, the substance can become magnetized. This happens when the spinning electrons line up in the same direction.
An unmagnetized substance looks like this… While a magnetized substance looks like this…
How to break a magnet: 1. Drop it 2. Heat it This causes the domains to become random again!
N S Magnetic Field Vectors Due to a Bar Magnet
N S
N S
N S
N S
N S
N S
N S
N S
N S
Magnetic Field Lines • The direction of the magnetic field at any point is… • tangent to the magnetic field line at that point. • the direction that the north pole of a compass would point if a compass were at that location • -defined as the direction of motion of a charged particle on which the magnetic field would not create a force.
Electromagnetism Up until 1820 everyone thought that magnetism and electricity were completely separate. But in that year, the Danish physicist Hans Oersted (1777-1851) discovered that a compass needle was deflected by an electric current. Magnetic Fields are VECTOR quantities! They are referred to as a B Field and B is the symbol used. These fields have effects on charged particles.
What does a charged particle “feel” in a magnetic field • If the charge is NOT moving there is NO force acting on the particle. • If the charge is MOVING ALONG a field line there is NO force acting on the particle. • If the charge is MOVING ACROSS • a field line, it feels a FORCE + v F B + N S
Magnetic fields produce forces on moving charged particles. The forces are perpendicular to both the velocity of the particle and the direction of the magnetic field The size of the force is proportional to the intensity of the field and the speed with which the particle is cutting across F v F v b b Note: The direction of the field and the velocity determine a plane. The force is perpendicular to that plane
The RIGHT HAND RULE Hold your right hand with your index finger straight out, your middle finger 90o from the index finger and your thumb straight up. Keep this orientation! Your index finger represents the velocity of the positively charged particle, your middle finger points the direction of the magnetic field (from the north end of a magnet) and your thumbs shows the direction of the force applied to that positively charged particle.
Field Vectors Right Up Out of Page x Down Left Into Page The convention of showing three dimensions on a two dimensional page. Examples: Find the resultant force under the given conditions… x x x
Magnets exert forces on moving particles….and as Oersted showed, moving charges also created magnetic fields and that’s what deflected Oersted’s compass. To examine the simplest case, pass a current carrying wire straight through a plane covered with compass needles. The needles line up in circles around the wire The magnetic field of a current is circular centered on the wire and lying on a plane perpendicular to the current.
You can find the direction of the magnetic field in a current carrying wire by pointing your thumb of your right hand along the direction of the flow of positive charges. Your fingers curl in the direction of the magnetic field. This is known as the RIGHT HAND RULE for current carrying wire If you look at the negative charges flowing than use the left hand rule.
Two parallel currents attract each other. The magnetic field circling each wire causes forces on the current in the other wire, pulling it closer. Andrea-Marie Ampere, discovered the force between parallel wires
If a current carrying wire is bent into a circle, a magnetic field is produced. Notice that one side looks just like a NORTH POLE – the field lines are coming out. N The other side looks like a SOUTH POLE – with field lines going in. S By winding many turns, the magnetic field is made proportionally larger. By winding turns along a cylinder, a solenoid coil is produced, with a magnetic field just like a bar magnet
Inserting an iron bar into the coil concentrates and strengthens the magnetic field, the result is an electromagnet.
Faraday Induction For 12 years after Oersted’s discovery “electricians” looked for the complimentary effect. How to make a magnetic field produce a current? In 1832 it was Michael Faraday that suggested moving the magnet! Thrusting a magnet through a loop of wire connected to a sensitive ammeter, a Galvanometer, deflects the galvanometer needle. Thus showing a current being induced.
When the magnet is held still, the meter registers no current. Faraday described this effect by saying that ELECTROMOTIVE FORCES (EMF) are generated in the wire whenever magnetic field lines cut across the wire. This is actually not a force but a potential difference measured in volts It does not matter whether the magnetic field moves or the wire moves with respect to the magnet.
When the magnet is thrust into the loop. It’s field lines cut across the wire, generating an EMF that produces a current. The same is true when the loop is moved over the magnet Although Faraday’s discovery was at first received with indifference, today nearly all our electrical power is generated by moving giant coils of wire near magnets.
Induction without Magnets Another way to induce a current in a wire is to place a second loop of wire nearby the first and energize it with a power source. When a current in the second loop is switched on or off, a current pulse is induced in the first. But when the current in the second loop is steady, no current is induced in the first loop
In the case of the two wire loops, when the current is first turned on in one loop, magnetic field lines build up, cutting across the other loop and producing an EMF. When the current is switched off, the field collapses, again cutting across the loop.
Lenz’s Law The induced emf creates a current that itself creates a secondary magnetic field. This secondary magnetic field also changes with time and thus creates a changing secondary magnetic flux. The secondary flux changes in such a way to opposes the change in flux creating the emf. Normally this means that the secondary magnetic field increases or decreases in such a way as to oppose the change in the magnetic field creating the induced emf. “Induced current flows in a direction to oppose the charge that produced it”
Magnetic Field F = q vB sinq F => Force (N) q => Charge (c) v=> Velocity of Charge particle (m/s) B => Magnetic Field (N/Am = T =Tesla) q => Angle between v and B
Magnetic Forces on Current Carrying Wire F = I L B sinq F F B I F => Force (N) v v L I => Current (A) L = vt (m) => Length of wire a charge would move in a given time B => Magnetic Field (T =Tesla) q=> Angle between v and B
Magnetic Field of an Ideal Solenoid B = m0 nI B => Magnetic Field (T =Tesla) m => 4p x 10 -7 (Tm/A) n => Linear Turn Density (N/L) # of turns per meter I => Current (A)
Induced Electromotive Forces - EMFs Magnetic Flux – A relative measure of the number of field lines passing through an area F = B A cosq Axis of Rotation F => Magnetic Flux (Tm2 = wb (weber)) B => Magnetic Field (T =Tesla) A => Area Vector (m2) q => Angle between A and B
I II III IV B A A q A A I & II If B and A are parallel, q = 0o or 180o then the Magnetic Flux is at maximum. F = B A cos0 = B A=> maximum # of lines through the loop III If B and A are perpendicular, q = 90o than the Magnetic Flux is at minimum or zero. F = B A cos 90 = 0=> No lines pass through the loop IV If B and A are between parallel and perpendicular then there is a partial Magnetic Flux. F = B A cosq => maximum # of lines > F > 0
Faradays Law of Induction The EMF induced in a coil of N loops depends on the time rate change of the number of filed lines through the loop. Time Rate Change of the Magnetic Flux x = -N DF/Dt x => Electromotive Force (v) N => Number of loops in the wire DF => Change in the flux through one loop (Tm2 = wb) Dt => Time (s)
Faradays Law of Induction The induced electromotive force (EMF) in any closed circuit is equal to the time rate change of the magnetic flux through the circuit. Or alternatively, The EMF generated is proportional to the rate of change of the magnetic flux.
Faradays Law of Induction The induced electromotive force (EMF) is not actually a force but a measure of potential difference. It is measured in volts (V)
