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Le Fevre High School SACE Stage 2 Physics Practice Test - Magnetic Fields & Motion in Magnetic Fields 1. A small current carrying wire of length 2 cm is placed in a uniform magnetic field as shown. The magnetic field is perpendicular and directed into the plane of the page. (a) Determine the direction of the force on the current carrying wire if the current flows from west to east. (1 mark) (b) The magnitude of the force on the wire is measured as 2 x 10-7 N, when the current flowing through the wire is 1.5 A. Determine the magnitude of the magnetic field. (2 marks) 2. Sketch the magnetic field in each of the following situations. (a) Straight conducting wire. (b) Cross section of a solenoid I Cross sectional cut through centre of the solenoid. I I (3 marks) 3. A particle of mass m, charge +q is initially stationary in an electric field of strength E. E is in the plane of the page as shown. (a) Explain the resulting motion of the positive charge. (2 marks) (b) The electric field is now turned off and a magnetic field of strength B directed at right angles to the plane of the page is now switched on. Explain the resulting motion of the moving charged particle. (2 marks) (c) Show that the period of revolution of the resultant motion is given by E Le Fevre High School 2m Bq (3 marks) 4. (a) Draw a schematic diagram of a moving coil loudspeaker. (4 marks) (b) Briefly explain the action of the moving coil loudspeaker.(3 marks) T 5. (a) What quantities determine the time an ion takes to complete a semi-circle in the dee of a cyclotron. (2 marks) (b) In an experiment a magnetic field of strength 2.5 T was used, and protons were extracted at a radius of 35 cm. What was the period of the protons in the cyclotron? (2 marks) 6. Ions travelling with the same velocity enter a uniform magnetic field initially at right angles to the field. (a) If two of the ions have equal masses but different charges, explain which ion will move in the path with the larger radius? (2 marks) (b) If an ion is travelling with a velocity of 5.0 x 107 m s-1 and the magnetic field is of intensity 3.5 x 10-3 T, determine the magnitude of the force per unit charge acting on the ion. (2 marks) TOTAL MARKS: 28 Le Fevre High School SACE Stage 2 Physics Practice Test - Magnetic Fields & Motion in Magnetic Fields Solution 1. (a) Using the right hand rule I B FB Force will be in the plane of the page and directed up the page. (b) l = 2 cm = 0.02 m, FB = 2 x 10-7 N, I = 1.5 A F = BIlsin at = 90o 2 x 10-7 = B(1.5)(0.02) B = 6.7 x 10-6 T 2. Field lines circular, non uniformly spaced (closer together near the wire) I 3. (a) Since in an electric field and charge experiences a force, then this force on a positive charge is directed the same as that of the field lines. Hence the charge moves in a straight line at a constant acceleration in the direction of the electric field lines. (to the left) (b) Using the right hand rule, with velocity (thumb) to the left, the B field into the page (extended fingers), then the resulting force is in the plane of the page and vertically down. This then changes the direction of the moving particle. However since the force is perpendicular to the velocity, it does not alter its speed. However as the charge changes direction, then the force also changes to remain mutually perpendicular. This results in the particle again changing direction but not speed. This process Le Fevre High School continues resulting in the particle moving in a circular path (or arc of a circle) as the force is always directed perpendicular to the velocity and hence is an centripetal force. (c) Since the centripetal force is supplied by the magnetic force FB = Bqv mv 2 Fc = R 2 mv 2 2 2 As = m R = m R T R 2 R Bqv = Bq T 2 2 2 R m R = Bq T T 2m Re-arranging T = Bq 4. (a) Outer suspension inner suspension Air gap Voice coil Centre cap & dust dome Soft iron pole pieces Ring Magnet 4 (b) two ends of the voice coil are connected to the output terminals of an amplifier. Across these terminals is a changing potential difference that oscillates in proportion to the sound waveform to be reproduced. The changing potential difference produces an oscillating current in the voice coil. 5. (a) the time is affected by the charge & mass of the ion, and the applied magnetic field strength. 2m (b) T = (from # 3) Bq = 2 (1.67 x 10 27 ) (1.6 x 10 19 )(2.5) Le Fevre High School = 2.62 x 10-8 s 6. When they enter the uniform magnetic field then magnetic force supplies the centripetal force. Fc = FB mv 2 = Bqv R mv hence R = Bq since R 1/q and they have the same m, v the particle with the smaller charge will travel in the larger radius path. (b) F = Bqv F/q = Bv = (3.5 x 10-3)(5.0 x 107) = 1.75 x 105 N C-1