Magnets and Magnetic Fields
Physics 1-2 Chapter 21
How did magnets get their name?
• First discovered about 3000 years ago
• Magnesia, Greece• First naturally
occurring magnetic rock, lodestone
• Made up of iron-based material
Where do magnetic fields come from?
• All magnetic fields arise from electric currents.
• In the case of permanent magnets in ferromagnetic materials, the currents are from unpaired electrons orbiting the nucleus.
Magnetic Poles
• Magnets have a north pole and a south pole
• Like charges:– Opposite poles attract– Like poles repel
• Can’t isolate south pole from north pole– If magnet is cut, each piece will still have two
poles
What are magnetic domains?Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms.
Properties of Magnets
• Permanent ALL the time, called permanent magnets– Example: lodestones
• Classified as magnetically hard or soft
• Soft magnets:– Example: iron– Easily magnetized– Loses its magnetic properties easily
Properties of Magnets• Hard magnets:
– Example: cobalt, nickel– More difficult to magnetize– Don’t lose magnetism easily
• Soft magnets:– Example: Iron in a staple or in a nail.– Easy to magnetize– Can be easily demagnetized by physical
shock or heating
Magnets exert magnetic forces on each other
• Example:– When magnet is lowered into bucket
of nails, it can pick up a chain of nails
– Each nail is temporarily magnetized by nail above it (exert magnetic force on nail below it)
– Limit to how long chain of nails can be
– The farther from the magnet smaller magnetic force
Magnets exert magnetic forces on each other
• Eventually, magnetic force not strong enough to overcome force of gravity bottom nails fall
Magnetic Fields
• Force exerted by magnets acts at a distance• Example:
– Move south pole of magnet toward another south pole
– Magnet will move away
• Other forces act at a distance:– Gravitational forces, force between electric
charges
Magnetic Fields
• ALL magnets produce a magnetic field
• Strength of magnetic field depends on:– Material magnet made from– How much object is magnetized– How far from magnet.
• Magnetic field lines used to represent magnetic field – Like electric field lines represent electric field
Magnetic Field Lines
• Direction is defined as the direction that the north pole of a compass will point at that location. ( go from N to S )
• Form closed loops• Field lines that are closer together
strong magnetic field• Field line that are farther apart weak
magnetic field• Magnetic field strongest near poles
Magnetic Field Lines
How do compasses work?
• Analyze magnetic field’s direction
• Compass: magnet on top of pivot
• Aligns with Earth’s magnetic field
• Can be used to determine direction as Earth acts like a giant bar magnet
Earth’s Magnetic Field• Earth’s magnetic poles not same as
geographic poles
• Geographic north pole (Canada) magnetic south pole
• Geographic south pole (Antarctica) magnetic north pole
• Poles of magnet named for geographic pole they point to
– N: “north-seeking” pole
– S: “south-seeking” pole
Earth’s Magnetic Field
Earth’s Magnetic Field • Source of magnetism is unknown
– Earth’s core made mostly of iron but too hot to have magnetic properties
– Circulation of ions or electrons in liquid layer of Earth’s core?
• Direction of Earth’s magnetism has changed– 20 reversals in last 5 million years – We are due for a reversal in the next few thousand
years!
• Aurora Borealis/Australis:– Solar wind (charged particles emitted from sun) is
deflected by Earth’s magnetic field
Aurora Borealis- “Northern lights”
Aurora Australis
• Aurora Australis
Auroras• Auroras are only visible at night in extreme
northern or southern latitudes.
• In cases of unusually high solar activity, the auroras may be visible further south.
Ch21.2 : Electromagnetism
• There is a magnetic field associated with any current (there is no magnetic field without a current!)
• The magnetic field lines are co-encentric circles around a straight wire. The field line direction is given by the right hand rule. Thumb points in the direction of the current and fingers gripping the straight wire point in the direction of the field.
Right hand rule (P770, hons P662):
Solenoid
• A long helically wound, insulated electric wire. The magnetic field is concentrated within the coil. It is further concentrated when a ferromagnetic material is placed inside the coil.
• Electromagnet: A magnet that consists of a solenoid and a ferromagnetic core. The magnetic field can be switched on and off with the flow of electric current.
Solenoid
21.3 – Relationship between current and magnetic field
• A charge moving (a current) through a magnetic field experiences a force.
• F = B q v Sin θ– F is force in Newtons
– B is magnetic field strength in Tesla, T
– q is charge in coulombs and,
– V is the velocity of the charge
– Θ is the angle between mag field and motion direction.
Right hand rule shows direction of force (P774, P650 Hons) on a positevely charged particle USE LEFT HAND FOR ELECTRON
v
B
F
Homework :
• Hons. P679 Q1,2,3 (use Voltage to calculate v first), 4, 5, 6. Draw picture!)
• Reg. P 775 Q1 to 5. Draw a picture!
Force on a current carrying conductor
• F = B I l– Where I is the current and l is the
length of the conductor– There is therefore a force between
any 2 current carrying conductors (note the demo)
– Do Q 1 – 5 page 778– AND P779 1-5
• The force on a current carrying conductor has uses in motors, moving coil meters (any meter with a needle) and in any device where electrical energy converts to kinetic energy ……where motion is produced.
Chapter 22 Induced current• If a conductor is placed in a varying
magnetic field, a voltage is induced in the conductor. (Faradays First Law)
• If the conductor can form a circuit, a ______ will flow.
• This induced voltage (emf) can happen in one of the following ways:
• 1) Move the conductor into or out of the field.
Inducing voltage (contd.)• 2) Circuit is rotated in the field (angle
between conductor and field changes)
• 3) Change the intensity of the magnetic field.
• Before we go and do numerical problems based on this idea do some concept problems on the previous topics:
• P769 Q1-4
• P779 Q1-5
Practical applications of electromagnetic induction
• Motor: This is more of an application of F = BIL . A current flowing through a loop of wire between two magnet poles experiences a force that causes rotation (See and understand demo). When the motor turns 180°, a commutator (switch) changes the direction of the current so that the force is now changed 180°, and rotation continues.
• A moving coil meter (galvanometer) is like a motor without the commutator and it also has a spring to return it to zero.
• Generator: Identical to motor in construction BUT: the coil is forced to rotate with the magnetic field by an outside force (ex. A turbine) and the induced voltage causes a current to flow.
• Speaker – works due to F = BIL. • Sound is just a pattern of changing
pressure (vibration).• A loudspeaker or headphone has a wire
coil placed in a permanent magnetic field. Current passed through the coil causes the coil to experience a ______ .
• If the current changes at the same rate as the sound, the speaker coil and the permanent magnet interact to vibrate the coil at the same frequency as the desired sound.
• Microphone – identical to the speaker construction but the coil moves due to sound vibrations, causing a current to be induced in the coil. This current can than be recorded or increased (amplified).
• A microphone is to a speaker as a motor:generator is to a motor:generator
• A motor is to a generator as a microphone:speaker is to a microphone:speaker
• A guitar pickup works the same way as a microphone.
Transformer• Two coils of wire that have a magnetic material
between them. When an A.C. current flows in the primary coil, a changing magnetic field is produced in the magnetic material. This changing magnetic field induces a changing current in the secondary coil. Voltages may be changed:
V2 = [N2 / N1] V1
Where 1 means primary, 2 secondary, N = # of turns of wire
Do Q1-6 page 818 Hons. P722 Q54-57 and 59
• Pole transformer
• 38,000V to 240V
• Why are the 38kV
wires (on top) thinner
than the 240V wires?
• Pad transformer
Transformers (contd.)• Transformers do not work with direct (constant)
current as a changing magnetic field is necessary to induce a voltage in the secondary.
• Alternating current (changes direction 60 times per second in US, 50 /sec outside Americas) is necessary for a transformer to work.
• Depending on the ratio of N2 /N1 the voltage may be stepped up or down by any amount.
• Efficiency can be as high as the high 90’s % with some energy lost as heat in the coils and due to eddy currents producing heat in the magnetic core itself. Large transformers have cooling systems to remove this heat which can lead to failure.
• Thomas Edison built a network of power plants in major cities producing DC current.
• They had to be very close to the power users and the voltage produced was the same as voltage consumed.
• Nicola Tesla emigrated to the US and was asked to solve the problem of supplying power to gold and silver mines in the West. Mines (where power was used) were often miles from fast-running rivers (where power was produced). Wires had to be very thick ($$$) if low voltage used (P=VI). Equipment in mines dangerous if high voltage used in mines.
• Tesla’s Trillion dollar idea:
• Don’t use DC – use AC!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
• Use transformers at power plant to make voltage very high before distribution (thin wires).
• Use transformers at mine to reduce the voltage to safe, practical levels.
• This is what we do today.
• Power is distributed from the power plants at V of the order of 106 Volts over great distances over a grid. Voltage is stepped down successively by transformers until your house receives 240V or 120V.
Answers to P818
• 1) 1.2 X102 V
• 2) 25 Turns
• 3) 156:1
• 4) 3.5 X 104 turns
• 5) 2.6 X10 4 V
• 6) 147 V
Lenz’s Law• The induced voltage in a conductor in a
changing magnetic field, produces a current that creates its own magnetic field which opposes the original magnetic field (the induced field opposes the change that produced it in the first place)
• Induced currents in transformer cores and any conductor in a changing magnetic field flow in circles. The larger the area, the higher these currents are. They are called “eddy currents”
• Sometimes these induced currents are desirable … ex. eddy current braking etc.
• GBSPhysics163 - How does the Giant drop work? (Nick)
• Eddy current brakes are used on
passenger trains a lot. The braking
depends on speed (F=Bqv) so a nice
smooth stop results.
Back e.m.f. and motors• As motor speed increases, a voltage (emf) is
induced in the coil that opposes the original current that made the motor rotate. The net effect is to reduce the original current.
• Motors have a high starting current which tapers off as speed of rotation increases.
Faraday’s first law• Induced V (emf) = -NΔ (AB (CosΘ))/Δt
N is the number of turns of wire, A is area, Θ is angle between B and the circuit or wire. The current direction found by R.H.R. . Could you figure the current direction?
Find current direction
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