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Student : MengZi Guo SPH4U Unit Test #1 Gravity, electric field and magnetic field . Note: There are more than 10 MC questions and more than 2 short answer questions in both tests . Therefore, these tests can be rearranged into more copies to serve students better . The MC questions on both test are put in the same order since there are 20 MC questions, so teachers can take out their best 10 questions to make another copy of the test. Multiple Choice Questions [20 marks] Select the best answer for each of the following questions : 1.Gravity is an _______force. A. Attractive B. Universal C. Mutual D. None of these E. All of above 2.Weight is a measure of_________. A. your mass. B. the gravitational attraction between you and the earth. C. mechanical equilibrium D. all of these E. none of these 3. In which case would you have the largest mass of an object? If the object weighed 1 N on the A. Jupiter B. earth Student : MengZi Guo C. moon 4.The gravitational force between 1kg of iron and earth is ___________ 1kg of marshmallows and earth A. B. C. D. greater than less than the same as None of the above 5. What is static electricity? A. electricity produced by friction B. the energy of motion C. the path along which electrons flows 6. crossing the wires in an electromagnet can reduce its magnetism A. true B. false 7. Electric current passing through a wire produces __________ A. a permanent magnet B. a magnetic field around the wire C. a compass D. geographic poles 8. like charges repel, unlike charge attract and force between two charges is F= is a description of _______. A. B. C. D. Electrostatic forces acting between charges Electromagnetism between charges Potential Energy between charges EM field 9. Magnetic field lines are same as lines of force A. True B. False Student : MengZi Guo 10. Alternating current changes the ______of the current flow. A. direction B. amount C. strength D. voltage 11. Magnetic field lines that curve away from each other show A. where the magnetic field is strongest B. a magnetic domain C. repulsion D. attraction 12. Magnetic field lines _______ A. never cross. B. cross at the north pole. C. cross at the south pole. D. cross halfway between the poles. 13. Magnetic fields are created by ___________ A. magnetic field lines B. potential energy of molecules C. spinning neutrons D. moving electric charges 14. Different electromagnetic waves travel through vacuum at different speed A. .True B. False 15. _________is the measure of the force of gravity on an object. A. mass B. weight C. density D. equation 16. Electric Field lines always come from positive to negative charges A. true Student : MengZi Guo B. false 17. which of the following in the universe is conserved. A. energy B. charge C. linear momentum , p=mv D. angular momentum E. all of above F. none of these 18. If there is no test charge present to measure electric field, then electric field does not exist. A. True B. False 19. The ________around a positive charge points away from the charge, and decreases in magnitude with distance r A. Electricity B. electric field C. electric force D. magnetic field Short Answers 1. a) Find the magnetic forces in following figures, JUSTIFY your answer : [3 marks each] Student : MengZi Guo For the first figure, the F =0 since sin(0)=sin(180)=0, there is no force if v and B are either parallel or antiparallel For the second figure, the force is maximum when v and B are perpendicular to each other since sin(90)=1. b) Three points are arranged in a uniform magnetic field. The magnetic field points into the screen. i. The negative charge moves from point A toward B. what is the direction of magnetic force on the particle? Justify [ 3 marks] ANS : The direction of the force on the particle is to RIGHT. The cross product of the velocity with the magnetic field is to the left and since the charge is negative the force is then to the right ii. A positive charged particle is located at point A and is stationary. what is the direction of magnetic force on the particle? Justify [3 marks] ANS: The answer is zero since velocity is zero , therefore the force is also zero. 2. Find the net force on an electron due to two nearby protons, each at distance r away and 90 degrees apart. [4 marks] ans : Student : MengZi Guo 3. Jack has rubbed a balloon with wool to give it a charge of -1.0 x 10^-6 C. He then acquires a plastic golf tube with a charge of 4.0 x 10^-6C at a given position. He holds the location of charge on the plastic golf tube a distance of 50cm above the balloon. Determine the electrical force of attraction between the golf tube and the balloon. [4 marks] ans : Q1 = -1.0 x 10^-6 C and Q2 = +4.0 x 10-6 C d = 50.0 cm = 0.50 m F = 0.144N 4. The normal hydrogen atom consists of a proton nucleus and an orbital electron, each carrying the fundamental charge of magnitude e= 1.60 x 10^-19 C. Assuming that the orbit is circular and the separation between the particles is 5.3x 10^-11 meters. Find : a) the force of electrical attraction between the particle. [ 2 marks] 8.14 x 10^-8N b) the gravitational force of attraction between them. [ 2 marks] 3.61 X 10 ^-47N c) which force is greater? how many times ?. [ 2 marks] Student : MengZi Guo Electric force is greater, 2.25 x 10 ^-47 times greater. Communication problem When a charged particle is fired into and perpendicular to a uniform magnetic field, it moves along a circular path until it escapes the field. Is this statement true or false, justify your answer in full details. Use a diagram to support your explanation . Find the expression of radius of the orbit and angular speed . [ 6 marks] this statement is true. when a charged particle moves in a magnetic field it experiences a force that is perpendicular to the velocity. since the force is perpendicular to the velocity, the charge d particle experiences an acceleration that is perpendicular to the velocity. The magnitude of the velocity does not change but the direction will change and produce a circular motion. circular motion produces centripetal accerlation, a= V^2/R, R is radius of orbit Newton's second law gives us F=ma=m( v^2/R)=qvB Thus, R=mv/qB Angular speed ω= v/R=qB/m Bonus questions : what is the motion like if the velocity is not perpendicular to B ? we break the velocity into components along the magnetic field and perpendicular to the magnetic field. The perpendicular component will still produce circular motion. the parallel Student : MengZi Guo component produces no force and this motion is unaffected, thus the combination will be a helical type motion. Student : MengZi Guo SPH4U Unit Test #2 Gravity, electric field and magnetic field . * Note: There are more than 10 MC questions and more than 2 short answer questions in both tests . Therefore, these tests can be rearranged into more copies to serve students better. The MC questions on both test are put in the same order since there are 20 MC questions, so teachers can take out their best 10 questions to make another copy of the test. Multiple Choice Questions [20 marks] Select the best answer for each of the following questions : 1.Gravity is an _______force. F. Attractive G. Universal H. Mutual I. None of these J. All of above 2.Weight is a measure of_________. F. your mass. G. the gravitational attraction between you and the earth. H. mechanical equilibrium I. all of these J. none of these 3. In which case would you have the largest mass of an object? If the object weighed 1 N on the Student : MengZi Guo D. Jupiter E. earth F. moon 4.The gravitational force between 1kg of iron and earth is ___________ 1kg of marshmallows and earth E. F. G. H. greater than less than the same as None of the above 5. What is static electricity? D. electricity produced by friction E. the energy of motion F. the path along which electrons flows 6. crossing the wires in an electromagnet can reduce its magnetism C. true D. false 7. Electric current passing through a wire produces __________ E. a permanent magnet F. a magnetic field around the wire G. a compass H. geographic poles 8. like charges repel, unlike charge attract and force between two charges is F= is a description of _______. E. F. G. H. Electrostatic forces acting between charges Electromagnetism between charges Potential Energy between charges EM field 9. Magnetic field lines are same as lines of force C. True D. False Student : MengZi Guo 10. Alternating current changes the ______of the current flow. E. direction F. amount G. strength H. voltage 11. Magnetic field lines that curve away from each other show E. where the magnetic field is strongest F. a magnetic domain G. repulsion H. attraction 12. Magnetic field lines _______ E. never cross. F. cross at the north pole. G. cross at the south pole. H. cross halfway between the poles. 13. Magnetic fields are created by ___________ E. magnetic field lines F. potential energy of molecules G. spinning neutrons H. moving electric charges 14. Different electromagnetic waves travel through vacuum at different speed A. .True B. False 15. _________is the measure of the force of gravity on an object. E. mass F. weight G. density H. equation 16. Electric Field lines always come from positive to negative charges C. true Student : MengZi Guo D. false 17. which of the following in the universe is conserved. G. energy H. charge I. linear momentum , p=mv J. angular momentum K. all of above L. none of these M. 18. If there is no test charge present to measure electric field, then electric field does not exist. C. True D. False 19. The ________around a positive charge points away from the charge, and decreases in magnitude with distance r E. Electricity F. electric field G. electric force H. magnetic field 20. For each of the arrangements of velocity and magnetic field below, determine the direction of the force. Assume the moving particle has a positive charge. A. into, down, right B. down, into, left C. out, down, right D. into, up, left Student : MengZi Guo Short Answers. 1. a) Find the magnetic forces in following figures, JUSTIFY your answer : [ 3 marks each] For the first figure, the F =0 since sin(0)=sin(180)=0, there is no force if v and B are either parallel or antiparallel For the second figure, the force is maximum when v and B are perpendicular to each other since sin(90)=1. b) Three points are arranged in a uniform magnetic field. The magnetic field points into the screen. i. A positive charged particle is located at point A and is stationary. what is the direction of magnetic force on the particle. Justify. [ 3 marks] ANS: The answer is zero since velocity is zero , therefore the force is also zero. Student : MengZi Guo ii. The positive charge moves from point A toward C. what is the direction of magnetic force on the particle? Justify. [3 marks] ANS: Up and left since the cross product is to the upper left and since the charge is positive the force remains to be at upper left. 2. Four point charges, labeled 1 through 4, all with the same magnitude q are placed around the origin as shown. the only negative charge is #2. what is the direction of the electric field at the origin ? [4 marks] ans : 3. At what distance of location must we put two 1.00 micro Coulomb charges in order for the repulsive force between them to be equivalent to the weight of a 1.00kg mass on earth ? [4 marks] Q1 = 1.0 x 10-6 C and Q2 = 1.0 x 10-6 C Felect = Fgrav = mg = 1.0 • 9.8 m/s/s = 9.8 N d= 3.0cm Student : MengZi Guo 4. The normal hydrogen atom consists of a proton nucleus and an orbital electron, each carrying the fundamental charge of magnitude e= 1.60 x 10^-19 C. Assuming that the orbit is circular and the separation between the particles is 5.3x 10^-11 meters. Find : a) the force of electrical attraction between the particle [ 2 marks] 8.14 x 10^-8N b) the gravitational force of attraction between them [ 2 marks] 3.61 X 10 ^-47N c) which force is greater? how many times ? [2 marks] Electric force is greater, 2.25 x 10 ^-47 times greater. communication problem 5.When a charged particle is fired into and perpendicular to a uniform magnetic field, it moves along a circular path until it escapes the field. Is this statement true or false, justify your answer in full details. Use a diagram to support your explanation. Find the expression of radius of the orbit and angular speed . [ 6 marks] Student : MengZi Guo this statement is true. when a charged particle moves in a magnetic field it experiences a force that is perpendicular to the velocity. since the force is perpendicular to the velocity, the charge d particle experiences an acceleration that is perpendicular to the velocity. The magnitude of the velocity does not change but the direction will change and produce a circular motion. circular motion produces centripetal acceleration, a= V^2/R, R is radius of orbit Newton's second law gives us F=ma=m( v^2/R)=qvB Thus, R=mv/qB Angular speed ω= v/R=qB/m Bonus questions : what is the motion like if the velocity is not perpendicular to B ? [3 marks] we break the velocity into components along the magnetic field and perpendicular to the magnetic field. The perpendicular component will still produce circular motion. the parallel component produces no force and this motion is unaffected, thus the combination will be a helical type motion.