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Magnetism 19 & 20 Properties of Magnets Magnets have 2 ends (polarized) North-seeking (north pole) South-seeking (south pole) Magnets always have two opposite magnetic poles. If you break a magnetic in half you get 2 smaller magnets with 2 poles. You do not get separate poles. Like poles repel Unlike poles attract Orientation Magnets always orient themselves in a north-south direction Earth is a giant magnet The north pole of a compass points north so the south magnetic pole of the Earth is near the Earth’s geographic north pole Temporary Magnets Either end of a magnet will attract either end of a piece of metal If you touch a magnet to a piece of metal the piece of metal will become a magnet If you pull away the magnet, the piece of metal’s magnetism disappears Permanent Magnets Many permanent magnets are made of ALNICO V, an iron alloy Contains 8% Aluminum, 14% Nickel, and 3% Cobalt Magnetic Field Lines Imaginary Field lines are closed loops that leave the north pole and enter the south pole Magnetic field around a magnetic does exist Electromagnetism 1820- Hans Christian Oersted was experimenting with electric current in wires. By placing a wire on top of a compass he demonstrated a connection between magnetism and electricity Magnetic field and electric field are perpendicular to each other Magnetic Field near Wire When there is an electric current in a coil of wire, the coil has a field like that of a permanent magnet The current carrying coil has a north and south pole and is itself a magnet Called an electromagnet Strength of the field is proportional to the current in the coil Solenoids An electric current in a single circular loop of wire forms a magnetic field all around the loop A long coil of wire consisting of many loops is a solenoid. The field from each loop in a solenoid adds to the electric field of the other loops. Solenoids When a wire is looped several times to form a coil and a current is allowed to flow through the coil, the field around all the loops is always in the same direction. Electromagnets The strength of the field of an electromagnet is proportional to the current in the coil. Increasing the number of loops in an electromagnet increases the strength of the magnetic field. Placing an iron rod inside the coil of an electromagnet will also increase the strength of the field Electrons Each electron in an atom acts like a tiny electromagnet The magnetic fields of electrons in a group of neighboring atoms can combine together The direction of the magnetic field between two magnets is from the N-pole of one magnet to the S-pole of a second magnet Force on a Current in a Magnetic Field By placing a wire between magnets you have an effect on the current. The strength of some typical magnetic fields is in the book. F = BIL F = force, B = magnetic field measured in teslas (T), I = current (amps), L = length of the wire that lies in the magnetic field Uses of a wire in a magnetic field Loudspeakers Galvanometers A device used to measure small currents Electric motors Converts electrical energy to kinetic energy Force on a single charged particle Charged particles do not have to be confined to a wire, but can move as long as there is a vacuum. F = Bqv Electromagnetic Induction Superconductors and permanent magnets can cause objects to float If a current passes through the coil of wire around a central rod, it produces a continually changing magnetic field that can affect magnetic substances Producing electric current Michael Faraday and Joseph Henry showed that a changing magnetic filed could produce electric current Generating Current Either the conductor can move through a magnetic field or the magnetic field can move past a conductor It is the relative motion between the wire and the magnetic field that produces the current EMF The potential difference, or voltage, given to charges by a battery is called electromagnetic force (EMF) This is not a true force and is measured in volts EMF = BLv Self- inductance A microphone depends on induced EMF Self-inductance produces an EMF when current changes in a single coil A transformer has two coils wound around the same iron core. One coil is the primary and the other is the secondary coil The secondary coil The EMF induced in the secondary coil, called the secondary voltage, is proportional to the primary voltage Secondary voltage = # of turns on secondary coil Primary voltage # of turns on primary coil Electric Generator When a generator is connected in a closed circuit, the induced EMF produces an electric current The current is greatest when the motion of the loop is perpendicular to the magnetic field (horizontal) Generators / Motors Almost identical in construction, but they convert energy in opposite directions A generator converts mechanical energy to electric energy, while a motor converts electrical energy to mechanical energy Alternating-Current The power produced by a generator is the product of the current and the voltage Average power: Pac = ½ Pac max P = I2R Effective current: Ieff = 0.707 Imax Average AC power: Pac = Ieff2R Effective Voltage: Veff = 0.707 Vmax Lenz’s Law The direction of the induced current is such that the magnetic field resulting from the induced current is moved toward the left end of a coil Also applies to motors. When a currentcarrying wire moves in a magnetic field, an EMF is generated. This is in a direction that opposes the current and is called the backEMF Transformer Induction between coils is the basis for the operation of a transformer. Transformers are widely used because they change voltages with little loss of energy Electromagnetism The source of most radio and TV waves is accelerating electrons It is the electron’s charge that results in electric fields, and the electron’s motion that produces magnetic fields Electric & Magnetic Force The force exerted by a magnetic field is perpendicular to the field and to the direction of motion of the electrons The forces due to the electric and magnetic field are equal and opposite in direction Electric & Magnetic Force The forces are balanced only for electrons that have a specific velocity The magnetic force is perpendicular to the direction of motion of the electrons causing a centripetal acceleration of the electrons Elements and ions Thomson showed that atoms of the same element could have the same chemical properties but different masses, called isotopes The masses of positive ions can be measured precisely by using a mass spectrometer Electric & Magnetic Fields A changing magnetic field produces a changing electric field and a changing electric field produces a changing magnetic field Either accelerating charges or changing magnetic fields produce electric and magnetic fields that move through space These combined fields are called an electromagnetic wave Antenna A wire called an antenna is used to form an electromagnetic wave EM waves can be generated over a wide range of frequencies. Antenna An AC generator is one method of creating the oscillating fields in the antenna To generate waves at higher frequencies is to use a coil and capacitor connected in a series circuit Oscillation To increase the oscillation frequency the size of the coil and capacitor must be made smaller Quartz crystals have a property called piezoelectricity which also allows them to generate electromagnetic waves Reception of EM waves A simple wire antenna can detect EM waves, several wires can be used to increase the detected EMF To select waves of a particular frequency and reject others, a coil and capacitor circuit is connected to the antenna The combination of antenna, coil & capacitor circuit, and amplifier is a receiver X rays High-frequency electromagnetic waves They are produced when electrons are accelerated to high speeds by means of potential differences of 20000+ volts When the electrons crash into matter, their kinetic energies are converted into the very high-frequency EM waves called X rays