# TAMU PHYS 208 - Test 3 review (4 pages)

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## Test 3 review

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## Test 3 review

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School:
Texas A&M University
Course:
Phys 208 - Electricity And Optics
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Ch 28 The force acting on a charged particle moving with velocity through a magnetic field is always perpendicular to v and B If a charged particle moves through a region containing both an electric field and a magnetic field it can be affected by both an electric force and a magnetic force If the fields are perpendicular to each other they are said to be crossed fields If the forces are in opposite directions a particular speed will result in no deflection of the particle When a uniform magnetic field B is applied to a conducting strip carrying current i with the field perpendicular to the direction of the current a Hall effect potential difference V is set up across the strip The electric force F E on the charge carriers is then balanced by the magnetic force F B on them A charged particle with mass m and charge magnitude moving with velocity v perpendicular to a uniform magnetic field B will travel in a circle Various magnetic forces act on the sections of a current carrying coil lying in a uniform external magnetic field but the net force is zero The Hall Effect When a conducting strip carrying a current i is placed in a uniform magnetic field some charge carriers with charge e build up on one side of the conductor creating a potential difference V across the strip The polarities of the sides indicate the sign of the charge carriers Ch 29 Curled straight right hand rule Grasp the element in your right hand with your extended thumb pointing in the direction of the current Your fingers will then naturally curl around in the direction of the magnetic field lines due to that element Parallel wires carrying currents in the same direction attract each other whereas parallel wires carrying currents in opposite directions repel each other To find the force on a current carrying wire due to a second current carrying wire first find the field due to the second wire at the site of the first wire Then find the force on the first wire due to that field Curl your right hand around the Amperian loop with the fingers pointing in the direction of integration A current through the loop in the general direction of your outstretched thumb is assigned a plus sign and a current generally in the opposite direction is assigned a minus sign The magnetic field just outside of a solenoid is zero Ch 30 Faraday s Law 1 A current appears only if there is relative motion between the loop and the magnet one must move relative to the other the current disappears when the relative motion between them ceases 2 Faster motion produces a greater current 3 If moving the magnet s north pole toward the loop causes say clockwise current then moving the north pole away causes counterclockwise current Moving the south pole toward or away from the loop also causes currents but in the reversed directions An emf is induced in the loop at the left in Figs 30 1 and 30 2 when the number of magnetic field lines that pass through the loop is changing 1 weber 1 Wb 1 T m 2 The magnitude of the emf induced in a conducting loop is equal to the rate at which the magnetic flux B through that loop changes with time Here are the general means by which we can change the magnetic flux through a coil 1 Change the magnitude B of the magnetic field within the coil 2 Change either the total area of the coil or the portion of that area that lies within the magnetic field for example by expanding the coil or sliding it into or out of the field 3 Change the angle between the direction of the magnetic field and the plane of the coil for example by rotating the coil so that field is first perpendicular to the plane of the coil and then is along that plane Lenz s Law An induced current has a direction such that the magnetic field due to the current opposes the change in the magnetic flux that induces the current A changing magnetic field produces an electric field Electric potential has meaning only for electric fields that are produced by static charges it has no meaning for electric fields that are produced by induction 1 henry 1 H 1 T m2 A An induced emf L appears in any coil in which the current is changing Initially an inductor acts to oppose changes in the current through it A long time later it acts like ordinary connecting wire Speaking anthromorphically The coil wants to fight the changes so if the current is already going to the right than to produce an induced electric field the current must be decreasing Ch 31 LC Circuit 1 The maximum values of UE and UB are both Q2 2C 2 At any instant the sum of UE and UB is equal to Q2 2C a constant 3 When UE is maximum UB is zero and conversely Whatever the natural angular frequency of a circuit may be forced oscillations of charge current and potential difference in the circuit always occur at the driving angular frequency d RLC Circuit XL XC The circuit is said to be more inductive than capacitive is positive for such a circuit which means that phasor I rotates behind phasor m XC XL The circuit is said to be more capacitive than inductive is negative for such a circuit which means that phasor I rotates ahead of phasor m XC XL The circuit is said to be in resonance a state that is discussed next 0 o for such a circuit which means that phasors m and I rotate together Ch 32 Diamagnetism is exhibited by all common materials but is so feeble that it is masked if the material also exhibits magnetism of either of the other two types In diamagnetism weak magnetic dipole moments are produced in the atoms of the material when the material is placed in an external magnetic field the combination of all those induced dipole moments gives the material as a whole only a feeble net magnetic field The dipole moments and thus their net field disappear when is removed The term diamagnetic material usually refers to materials that exhibit only diamagnetism o A diamagnetic material placed in an external magnetic field develops a magnetic dipole moment directed opposite If the field is nonuniform the diamagnetic material is repelled from a region of greater magnetic field toward a region of lesser field levitating frog Paramagnetism is exhibited by materials containing transition elements rare earth elements and actinide elements see Appendix G Each atom of such a material has a permanent resultant magnetic dipole moment but the moments are randomly oriented in the material and the material as a whole lacks a net magnetic field However an external magnetic field can partially align the atomic magnetic dipole moments

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