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Magnetism. Properties of Magnets. All magnets have two areas of strongest force, called poles. Each magnet has one north pole and one south pole . Like poles repel, and opposite poles attract. The magnetic region where you can “feel the force” is called a magnetic field. Magnetic Poles.
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Properties of Magnets • All magnets have two areas of strongest force, called poles. • Each magnet has one north pole and one south pole. • Like poles repel, and opposite poles attract. • The magnetic region where you can “feel the force” is called a magnetic field.
Magnetic Poles If you break a bar magnet in half, each half still behaves as a complete magnet. Break the pieces in half again, and you have four complete magnets. Even when your piece is one atom thick, there are two poles. This suggests that atoms themselves are magnets.
What makes some things magnetic, while other things can’t be magnetized? Spinning electrons cause small magnetic fields around each atom. Magnetic materials have atoms whose magnetic fields can be lined up in the same direction. Areas where atoms’ magnetic fields line up are called magnetic domains. magnetic domain Randomly arranged domains =No magnet! Magnetic domains lined up =Magnet! Magnetic Materials
Magnetic Fields Iron filings sprinkled on a sheet of paper over a bar magnet will tend to trace out a pattern of lines that surround the magnet. The space around a magnet, in which a magnetic force is exerted, is filled with a magnetic field. The shape of the field is revealed by magnetic field lines.
Magnetic Fields Magnetic field lines spread out from one pole, curve around the magnet, and return to the other pole.
Magnetic Fields • The direction of the magnetic field outside a magnet is from the north to the south pole. • Where the lines are closer together, the field strength is greater. • The magnetic field strength is greater at the poles. • If we place another magnet or a small compass anywhere in the field, its poles will tend to line up with the magnetic field.
Electric Fields • Electric fields arise from voltage. • Are a vector. • Their strength is measured in Volts per meter (V/m) • An electric field can be present even when a device is switched off • Can do work. • Particles may change speed • Particles may change direction
Electric Fields • Field strength decreases with distance from the source. • Most building materials shield electric fields to some extent • Begin on positive charges and end of negative charges. • Electric force acts along the direction of the electric field. • Acts on a charged particle regardless of whether the particle is moving.
Magnetic Fields • Magnetic fields arise from current flows. • Are a vector. • Their strength is measured in Tesla (T). • Magnetic fields exist as soon as a device is switched on and current flows. • CANNOT do work • Particle speed is constant • Particle direction can change.
Magnetic Fields • Field strength decreases with distance from the source. • Magnetic fields are not attenuated by most materials. • Have no beginning and no end, form continuous circles. • Magnetic force acts perpendicular to the magnetic field • Acts on a charged particle ONLY when the particle is in motion.
Electric Currents & Magnetic Fields • A moving charge produces a magnetic field. • An electric current passing through a conductor produces a magnetic field because it has many charges in motion. • The magnetic field surrounding a current-carrying conductor can be shown by arranging magnetic compasses around the wire. • The compasses line up with the magnetic field produced by the current, a pattern of concentric circles about the wire. • When the current reverses direction, the compasses turn around, showing that the direction of the magnetic field changes also.
Electric Currents & Magnetic Fields • When there is no current in the wire, the compasses align with Earth’s magnetic field.
Electric Currents & Magnetic Fields • When there is no current in the wire, the compasses align with Earth’s magnetic field. • When there is a current in the wire, the compasses align with the stronger magnetic field near the wire.
Electric Currents & Magnetic Fields A current-carrying coil of wire is an electromagnet.
magnetic north pole geographic north pole magnetic south pole geographic south pole Magnetic lines of force around the earth are like the field lines around a giant bar magnet. The magnetic north pole and the geographic north pole are not located in the same place! The north pole of a compass points to the earth’s magnetic north pole. The Earth is a magnet!
In 1820, H.C. Oersted discovered that an electric current flowing through a wire had a magnetic field around it. Electricity can cause magnetism! Electromagnets are powerful magnets that can be turned on and off. You can make an electromagnet stronger by (1) putting more turns of wire in the coil or (2) making a larger soft iron core, or (3) increasing the current through the wire. Electricity to Magnetism
A simple DC electric motor contains a permanent magnet, an electromagnet, and a commutator. When current flows through the electromagnet, it turns within the magnetic field of the permanent magnet, changing electricity to mechanical energy. Current meters also use permanent magnets and electromagnets. When current flows through a wire, it makes an electromagnet. The force between the electromagnet and the permanent magnet makes a needle move on the meter. Uses for electromagnets
Joseph Henry and Michael Faraday discovered that magnetism could also produce electric current. This is called electromagnetic induction. If a magnet is moved back and forth through a coil of wire, current can be made to flow through the wire. This is the idea behind electric generators and transformers. Magnetism to Electricity Current moves right in wire. Current moves left in wire.
Electromagnetic Induction • No battery or other voltage source was needed to produce a current—only the motion of a magnet in a coil or wire loop. • Voltage was induced by the relative motion of a wire with respect to a magnetic field.
Electromagnetic Induction • The production of voltage depends only on the relative motion of the conductor with respect to the magnetic field. • Voltage is induced whether the magnetic field moves past a conductor, or the conductor moves through a magnetic field. • The results are the same for the same relative motion.
Electromagnetic Induction • The amount of voltage induced depends on how quickly the magnetic field lines are traversed by the wire. • Very slow motion produces hardly any voltage at all. • Quick motion induces a greater voltage. • Increasing the number of loops of wire that move in a magnetic field increases the induced voltage and the current in the wire. • Pushing a magnet into twice as many loops will induce twice as much voltage.
Electromagnetic Induction Twice as many loops as another means twice as much voltage is induced. For a coil with three times as many loops, three times as much voltage is induced.
Electromagnetic Induction • We don’t get something (energy) for nothing by simply increasing the number of loops in a coil of wire. • Work is done because the induced current in the loop creates a magnetic field that repels the approaching magnet. • If you try to push a magnet into a coil with more loops, it requires even more work.
Electromagnetic Induction Work must be done to move the magnet. • Current induced in the loop produces a magnetic field (the imaginary yellow bar magnet), which repels the bar magnet.
Electromagnetic Induction Work must be done to move the magnet. • Current induced in the loop produces a magnetic field (the imaginary yellow bar magnet), which repels the bar magnet. • When the bar magnet is pulled away, the induced current is in the opposite direction and a magnetic field attracts the bar magnet.
Electromagnetic Induction • The law of energy conservation applies here. • The force that you exert on the magnet multiplied by the distance that you move the magnet is your input work. • This work is equal to the energy expended (or possibly stored) in the circuit to which the coil is connected.
Electromagnetic Induction • If the coil is connected to a resistor, more induced voltage in the coil means more current through the resistor. • That means more energy expenditure. • Inducing voltage by changing the magnetic field around a conductor is electromagnetic induction.
Generators produce AC current for home and industrial use. Water, wind, or steam are used to move large electromagnets through the coils of wire to produce current. Transformers are used to step up voltage of electricity that must travel long distances through wires. Other transformers then step down the voltage before it enters our homes. Uses for Electromagnetic Induction
Faraday’s Law • Faraday’s law describes the relationship between induced voltage and rate of change of a magnetic field: • The induced voltage in a coil is proportional to the product of the number of loops, the cross-sectional area of each loop, and the rate at which the magnetic field changes within those loops.
Faraday’s Law • The current produced by electromagnetic induction depends upon • the induced voltage, • the resistance of the coil, and the circuit to which it is connected. • For example, you can plunge a magnet in and out of a closed rubber loop and in and out of a closed loop of copper. • The voltage induced in each is the same but the current is quite different—a lot in the copper but almost none in the rubber.
Fleming’s Hand Rules • If a current carrying conductor placed in a magnetic field, it experiences a force due to the magnetic field. On the other hand, if a conductor moved in a magnetic field, an emf gets induced across the conductor (Faraday's law of electromagnetic induction). • John Ambros Fleming originated two rules to determine the direction of motion (in electric motors) or the direction of induced current (in electric generators). The rules are called as, Fleming's left hand rule (for motors) and Fleming's right hand rule (for generators).
Left Hand Rule • Fleming's left hand rule is applicable for electric motors. Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a force. According to Fleming's left hand rule, if the thumb, fore-finger and middle finger of left hand are stretched perpendicular to each other as shown the figure, and if fore finger represents the direction of magnetic field, the middle finger represents the direction of current, then the thumb represents the direction of force.
Right Hand Rule • Fleming's right hand rule is applicable for electrical generators. As per Faraday's law of electromagnetic induction, whenever a conductor is moved in an electromagnetic field, and closed path is provided to the conductor, current gets induced in it.
Right Hand Rule • According to Fleming's right hand rule, the thumb, fore finger and middle finger of right hand are stretched perpendicular to each other as shown in the figure at right, and if thumb represents the direction of the movement of conductor, fore-finger represents direction of the magnetic field, then the middle finger represents direction of the induced current.
