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1. Motors use the effect of forces on current-carrying conductors in magnetic fields Jacaranda REVIEW OF MAGNETIC FIELDS P.108-110 Identify that moving charged particles in a magnetic field experience a force This happens because the magnetic field created by the moving charged particle interacts with the existing field Account for the motor effect due to the force acting on a current-carrying conductor in a magnetic field It depends on the field strength, the charge size and the velocity. e.g. Conventional current, alpha particles e.g. Electron flow, beta particles N.B. If the velocity has a component parallel to the magnetic field, that component is not subject to the force and is retained while the other component is accelerated. Record observations, from teacher devised demonstration, of the deflection of an electron stream in a Cathode Ray Tube, by a permanent magnet. Jacaranda Experiment 6.1 Identify data sources, gather, analyse and present information to discuss the Van Allen radiation belts as examples of motion of charged particles in a field Jacaranda PHYSICS FACT P.124 Research material from the internet and other sources on Van Allen radiation belts Discuss the effect, on the magnitude of the force on a current-carrying conductor, of variations in: – the strength of the magnetic field in which it is located – the magnitude of the current in the conductor – the length of the conductor in the external magnetic field Solve problems and analyse information about the force on current-carrying conductors in magnetic fields using F = BI l – the angle between the direction of the external magnetic field and the direction of the length of the conductor F BIl sin B N.B. magnetic field may be created by permanent magnet or electromagnet I M.Edwards 25/4/02 where is the angle between the magnetic field and the conductor So F is directly proportional to B, I, l and sin Perform a first-hand investigation to demonstrate the motor effect Jacaranda Experiment 6.2 Describe qualitatively and quantitatively the force on long parallel currentcarrying conductors in magnetic fields using Use the same approach to show that opposite currents repel F I1 I 2 k l d I2 The right-hand grip rule gives the direction of the magnetic field surrounding conductor 1. B l d M.Edwards 25/4/02 F I1 This shows the field that conductor 2 experiences due to conductor 1 The right-hand push rule gives the direction of the force experienced by conductor 2 F I2 Conductor 2 has the same effect on conductor 1, so they ATTRACT. F F I1 M.Edwards 25/4/02 Describe qualitatively and quantitatively the force on long parallel currentcarrying conductors in magnetic fields using F I1 I 2 k l d k 2.0 10 7 NA2 Conductor 2 experiences a magnetic field due to conductor 1 B Conductor 2 experiences a force due to this magnetic field I1 d F BI 2l l d Substituting for B: M.Edwards 25/4/02 F I1 Bk I2 rearranging gives Force per unit length: I1 I 2 l F k d F I1 I 2 k l d F I1 I 2 k l d k 2.0 10 7 NA2 Two straight current-carrying conductors are placed parallel to each other, 20 cm apart. I1 = 2 A I2 = 5 A 20 cm I1 I2 M.Edwards 25/4/02 (a) Determine the force per unit length of the 2 A wire on the 5 A wire. (b) Describe what will happen to the magnitude of the force as the 2 A wire is rotated 90o until it is perpendicular with the 5 A wire. F I1 I 2 k l d k 2.0 10 7 NA2 20 cm I1 I2 M.Edwards 25/4/02 (a) (b) F/l = kI1I2/d F/l = (2.0 x 10-7 x 2 x 5) / 0.2 F/l = 1 x 10-3 N/m (attraction). 1 mark The force will decrease as the 2 A wire is rotated and will be zero when the two wires are perpendicular to each other. 1 mark (OPTIONAL EXERCISE) Undertake a first hand investigation, devised by the teacher, to observe Oersted’s experiment. Students record the method, their observations, and the significance of the experiment, after teacher led discussion. Jacaranda Experiment 6.3 Record information, after teacher led discussion, to explain the force between the wires as an example of the motor effect D.C. ELECTRIC MOTORS 1. Motors use the effect of forces on current-carrying conductors in magnetic fields Define torque as the turning moment of a force using: Fd M.Edwards 25/4/02 So total Torque, and for n loops l BIld BIl (2d ) Torque from each side BIA I F M.Edwards 25/4/02 d B F Torque for each loop nBIA If the coil is at an angle to a uniform magnetic field F d F BIl Substituting: Increasing F or d increases the turning effect (torque) M.Edwards 25/4/02 Identify the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces Solve problems and analyse information about simple motors using: nBIA cos Describe how the required magnetic fields can be produced either by current-carrying coils or permanent magnets Where are the permanent magnets placed? Why? What is the shape and direction of a magnetic field produced by a current-carrying coil? Can we place an electromagnet in the same circuit as the motor? How? What are the advantages and disadvantages of permanent and electromagnets? Identify data sources, gather and process information to qualitatively Jacaranda describe the application of the motor PHYSICS IN FOCUS P.112 effect in: Student exploration of – the galvanometer large galvanometers – the loudspeaker Student exploration of car stereo speakers E.g. ammeter: The galvanometer can be converted into an ammeter by using a very small resistor in parallel with the galvanometer and changing the scale appropriately. Most of the current will flow through the small resistor, with the remainder through the galvanometer. This allows larger currents to be measured without the needle moving off the scale. voltmeter: Make sure you find the answer to this too! Describe the main features of a DC electric motor magnets (stator) The coil, armature and axle rotate and are known as the rotor. The magnets are stationary and are known as the stator. coil axle F N brush d S armature brush M.Edwards 25/4/02 Source of EMF Split-ring commutator Internet DC motor animation by Walter Fendt HSC Practical Research Task Discuss the importance of the invention of the commutator for developing electric motors Gather and process secondary information to analyse the function of the parts of a commutator side A side B F N S I Conventional current in side B is currently out of the page The right hand push rule tells us this rotor will move anticlockwise. F M.Edwards 25/4/02 A B I When reaches 90 degrees, the split ring will contact the other brush, reverse polarity and current in side B will be into the page M.Edwards 25/4/02 Describe the role of the metal split ring and the brushes in the operation of the commutator So the rotor continues to rotate in an anticlockwise direction B F M.Edwards 25/4/02 A HSC Practical Research Task