1Circuits revision• Here’s the circuit for the flashing neon bulb.• What is the period of the flash in seconds ?2Magnetic Fields revisionPlease try problem 15 in Ch 24 on page 825.“What is the magnetic field at the center of the loop….”3Magnetic Fields and Forces• Magnetism• Magnetic field shapes and direction• Fields near electric currents• Magnetic forces• Moving charges and magnetism• Magnetic machines• Magnetic materials4Magnetism• Fundamental force of nature• Related to electricity, but not the same5Experimental Observations• Magnetism does not move an electroscope, it does not act on stationary charges• Long range force (action over a distance)• There are 2 poles, north and south, and they come in pairs• Like poles repel, unlike poles attract• Poles attract magnetic materials6Magnetic Field lines• Magnetic Fields around a bar magnet• Similar to an electric dipole• Start at north pole, terminate at south pole7Like and unlike polesMagnetic field lines between poles8Electric Currents and Magnetic FieldsOersted found that a current can move a magnetic compass9Direction of Magnetic fieldWe use the right handed rule to find which way a magnetic compass would point10Magnetic field near a loop• Bend the wire into a loop.• Dots - field is coming out of the page. • Crosses - field is going in to the page11Field near a solenoid• Many loops will concentrate the field inside the coil• Called a solenoid – contains a uniform magnetic field12Magnetic field due to a currentExperimentally, the field strength, B, is proportional to current, I, and inversely proportional to distance, r.Units of Tesla, where μ0is the permeability constant – 1.257x10-6TmA-1rIB2013Tesla is a large unit• Magnets in the lab – 0.1 to 1 T• Kitchen magnets – 5x10-3T• Earths magnetic field – 5x10-5T• Superconducting magnets – in accelerators and maglev trains – 10 T14Magnetic Field at the center of a current loopInside a loop radius R:RIB2015Magnetic Field at the center of a current loop with N turnsIf the loop has N turns, but its not yet a solenoid we have:RNIB2016Magnetic field inside a solenoidThe uniform field in a solenoid isFor a solenoid with N turns, Length L and current I.Note: independent of the coil radius. Field is uniform.LNIB017Magnetic Forces• The magnetic fields around two wires will attract or repel, just like bar magnets.• A magnetic field exerts a force on a current, or moving charge• Currents in the same direction attract• Opposite currents repel18Direction of Magnetic Force• The force on a wire with a current is perpendicular to both the magnetic field the direction of the current.• We use another right hand rule19Magnitude of the Magnetic ForceThe force between a magnetic field and a current along a wire length L perpendicular to the field is:ILBF 20Magnitude of the Magnetic ForceThe force between a magnetic field and a current along a wire length L at an angle, α to the field is:If the current and B field are parallel – there is no force.sinILBF 21Force on a moving charge• A current, I, is a moving charge.• The charge q moves along the wire length L in time Δt• The velocity will be L/Δt• We find that qv=ILqvILLqvtqItLv22Magnitude of the Magnetic ForceThe force between a magnetic field and a charge, q, moving with a velocity, v perpendicular to the field is:qvBF 23Magnitude of the Magnetic ForceThe force between a magnetic field and a charge, q, moving at velocity, v, at an angle, α to the field is:If the moving charge and B field are parallel – there is no force.sinqvBF 24Direction of Magnetic Force• The force on a moving charge is perpendicular to both the magnetic field the direction of the charge.• Note the thumb is now the direction of the +ve charge, instead of the current I.25Path of charges in a magnetic field• The force on a charged particle in a magnetic field is perpendicular to its direction of motion.• We always get circular or spiral paths of charged paths in a magnetic field26Path of charges in a magnetic field• Centripetal force of an object in a circlemRqBvqvBrmvF227Path of charges in a magnetic field• If we accelerated the ions in an electric field V, the charge to mass ratio can be measured,222221RBVmqmvqVE28Mass spectrometer• First measurement of e/m for the electron• Used to distinguish different types of atoms and isotopes29Aurora Borealis• Solar wind from the sun (protons & electrons) gets deflected by Earth’s magnetic field.• Portion of velocity perpendicular to the field lines, curves the ionizing particles into spirals• Ionize O2and N2in the ionosphere30Magnetic forces between currents• Consider two wires carrying currents I1and I2.• The field at the top wire isdILIFLIBFdIB2221012121220231Magnetic forces between currentsFrom the field from the single wire, we can deduce the force between 2 wires carrying currents I1and I2isdILIFwiresparallel221032Torques and Magnetic Moments• Torque was defined in chapter 7• Quantity to measure the force applied near a pivot• Useful for calculating rotational motion33TorqueTorque, τ, measures the effectiveness of a force at causing an object to rotate about a pivotsinrF34Torque on a current loop in a B field• Current loop in a uniform field• The forces on the top and bottom wires will rotate the loop35Torque on a current loop in a B field• The total torque, τ, will be the sum of the torques on the top and bottom wires.• Loop height L, wire length Wsinsin22BIWLLF36Torque on a current loop in a B field•In general, the torque on a loop area A will be:The loop is forced to align with the magnetic fieldsinIAB37Using torque - MRIs• Magnetic Resonance Imaging (MRI) uses the protons magnetic moment in hydrogen atoms in high 1T fields.• The rate of the emitted radio waves from the excited states are detected38Using Torque – Electric motorUsing commutators, the loop can be made to spin, to produce rotational movement39Permanent Magnets -Ferromagnetism• Ferromagnetism is a property of certain elements – the ability to maintain a permanent magnetic field• Depends on the crystalline structure of the metal• Found in alloys of iron, cobalt, nickel, gadolinium, dysprosium, europium• Half full electron shells, the magnetic dipole of the
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