Unit 8 Fields

1. Unit 8 Fields Honors Physics May 2014 Barton

2. FIELDS - general Answer the following question in the space provided in your packet: Using your own prior knowledge and life experience, how do you know gravitational, electric, and magnetic fields exist? (give specific examples if you can think of any)

3. Real World Reading • Read the article at this site: http://oceanservice.noaa.gov/education/kits/tides/tides06_variations.html • Answer the following question in a 5-6 sentence paragraph: (answer in the space provided in the packet) • What is the role of gravitation in tidal patterns?

4. Gravitational Fields • Gravity is a non-contact force • Gravity is always attractive • Newton’s Law of Universal Gravitation: • Fg – force of gravity (N) • G – universal gravitation constant G=6.67*10-11 Nkg2/m2 • m1 & m2 – masses of objects exerting the gravitational force on one another (kg) • d – distance between the centers of the two masses (m) • Sometimes an r is used instead of a d in this equation • Additional information: http://www.physicsclassroom.com/class/circles/u6l3c.cfm

5. Gravitational Field Drawings - Earth Gravitational field around the earth – the length of the arrows indicate the strength of the field at that distance from the earth

6. Gravitational Field Drawings – Earth & Moon Interaction – 2 different examples Example 1: Moon Earth Example 2: Moon Earth

7. Answer to gravitational field question from packet • 3. In which diagram do the field lines best represent the gravitational field around Earth?

8. Answers to gravitational potential energyquestions from packet • 8. As a ball falls freely toward the ground, its total mechanical energy • A. Decreases • B. Increases • C. Remains the same • 7. A block slides along the frictionless path shown in the figure below. From greatest to least rank the amount of gravitational potential energy the block has at the different points labeled on the track. • A, B, C, D • C, D, B, A • D, C, B, A • A, B, D, C

9. Mechanical Energy – Gravitational Potential Energy and Kinetic Energy • Mechanical Energy – energy acquired/lost by object when work is done ME = ET = KE + PE = K + U • Kinetic Energy (KE or K) – energy of motion K = ½mv2 • Gravitational Potential Energy(PE or U) – stored energy of position relative to surface U = mgh • Law of Conservation of Energy – energy cannot be created or destroyed • Energy can be… • Transferred – given from one object to another • Transformed – changed from one type of energy into another

10. Answers to gravitational potential energyquestions from packet 9. A ball is dropped from the top of a cliff. Which graph best represents the relationship between the ball’s total energy and elapsed time as the ball falls to the ground? [Neglect friction.] • 10. A child, starting from rest at the top of a playground slide, reaches a speed of 7.0 meters per second at the bottom of the slide. What is the vertical height of the slide? (ignore friction) [hint: remember, the total initial energy = total final energy] • 0.71 m • 1.4 m • 2.5 m • 3.5 m

11. Magnets – Drawing Magnetic Fields

12. Magnets – Magnetic Fields • Magnetic force can be attractive or repulsive • Field lines are vectors • Produce CLOSED LOOPS • Point from NORTH to SOUTH outside the magnet • Point from SOUTH to NORTH inside the magnet • **Magnets are POLARIZED, which means they have TWO DISTINCT, OPPOSITE ENDS** • Magnetic fields are produced by MAGNETS & MOVING CHARGES • MOVING CHARGES experience magnetic fields • like ELECTRIC FIELDS

13. Magnets – Magnetic Fields

14. Earth’s Magnetic Field • The earth’s MAGNETIC SOUTH pole is very near to the earth’s GEOGRAPHIC NORTH pole • The earth’s MAGNETIC NORTH pole is very near to the earth’s GEOGRAPHIC SOUTH pole

15. Magnets • Natural Magnets: • domains inside have ELECTRONS aligned in their SPINS • magnets cause other materials to become POLARIZED • they become temporary magnets; ie: nail or paperclip • Permanent Magnets: • domains are “PERMANENTLY” ALIGNED • Non-magnets: • domains inside have ELECTRONS with UNALIGNED SPINS

16. Aligned (magnetic) Not aligned (non-magnetic)

17. Answers to magnetic field questions from packet 2. In which diagram do the field lines best represent the magnetic field around Earth? • The diagram below shows the magnetic field lines between two magnetic poles, A and B. Which statement describes the polarity of magnetic poles A and B? • Ais a north pole and B is a south pole. • Ais a south pole and B is a north pole. • Both A and B are north poles. • Both A and B are south poles

18. Electric Fields • charges change the space around them, this change is the electric field produced by that charge • charges interact w/each other and exert forces on each other through their electric fields

19. Electric Fields Created by Charges & Charged Plates

20. Properties of electric fields • E-fields are produced by charges and we represent them with arrows(vectors) •  the lines NEVER cross but they do bend •  the closer the lines, the stronger the E-field (magnitude) •  E-field lines point in the direction that a positive charge feels a force (direction) •  E-field is zero inside a conductor • E-field lines are perpendicular to metallic surfaces • Supplemental electric field information: http://physics.bu.edu/~duffy/py106/Electricfield.html

21. Electrostatic constant = 8.99*109 Nm2/C2 Equations Electric Force units: N Charge creating the field units: C Distance units: m Charge in the field units: C Electrostatic constant = 8.99*109 Nm2/C2 Charges units: C Electric Field strength units: N/C or V/m Electric Force units: N Distance units: m

22. Answers to electric field questions in packet 15. The center of a -0.0035 C charge is 0.44 m to the left of the center of an 0.008 C charge. Determine the magnitude and direction of the electrostatic force on the -0.0035 C charge. F = (kq1q2)/r2 = [(8.99*109)(0.0035)(0.008)]/(0.442) = 1.3*106 N to the right 16. A positive charge is inside an electric field and is experiencing an electric force to the right. Use words and a diagram to show the direction in which the field points. Field points to the right (see slide 19) 17. A negative charge is inside a leftward pointing electric field. Use words and a diagram to show the direction in which the charge feels a force. Negative charge feels a force to the right (see slide 19) 18. An electric field is measured using a positive charge of 3.0*10-6 C. This charge experiences a force of 0.12 N to the right. What are the magnitude and direction of the electric field strength at the location of the charge? E = F/q = (0.12N)/(3*10-6C) = 4*104 N/C to the right 19. A negative charge of 4.5*10-6 C produces an electric field. What is the electric field strength and direction at a distance of 25 cm? 25 cm = 0.25 m E = (kq)/r2 = [(8.99*109)(4.5*10-6C)]/(.25m)2 = 6.5*105 V/m 20. The distance between an electron and a proton is varied. Which pair of graphs best represents the relationship between gravitational force, Fg, and distance, r, and the relationship between electrostatic force, Fe, and distance, r, for these particles?

23. Conductors & Insulators • Conductors – materials that allow electrons to move freely from atom to atom • Insulators – impede the flow of electrons from atom to atom

24. Charging by Conduction • a neutral object is touched by a charged object • Neutral object either gains or loses electrons to become charged (protons are NOT transferred) Neutral object becomes negative  gains e- Neutral object becomes positive  loses e-

25. Charging by Induction • Charged object is brought near a neutral object causing polarization • Polarization – separating opposite charges within an object

26. Answers to charging question from packet • 21. Two metal spheres that are initially uncharged are mounted on insulating stands, as shown below. A negatively charged rubber rod is brought close to, but does not make contact with, sphere X. Sphere Y is then brought close to X on the side opposite to the rubber rod. Y is allowed to touch X and then is removed some distance away. The rubber rod is then moved far away from X and Y. What are the final charges on the spheres? • Sphere X Sphere Y • A. Zero Zero • B. Negative Negative • C. Negative Positive • D. Positive Negative • E. Positive Positive

27. MAGNETIC FIELD FORCES on moving charges • magnetic fields apply a force to moving charges & current-carrying wires • Force on a charged, moving particle in a magnetic field: F = qvB • q = particle’s charge (units: coulombs, C) • v = particle’s velocity (units: m/s) • B = magnetic field strength(units: T, Tesla) • **How do we determine the direction of the force?** • Right Hand Rule: Relationship between force, magnetic field, and velocity of a POSITIVE PARTICLE • fingers: DIRECTION OF MAGNETIC FIELD (B) • thumb: DIRECTION OF VELOCITY (v) • palm: DIRECTION OF FORCE (F) ***For negative particle, opposite of right hand rule*** *** a particle must be moving PERPENDICULAR to the magnetic field *** *** a particle moving PARALLEL to mag. field will feel NO FORCE (F =0)***

28. MAGNETIC FIELD FORCES – moving charges Determine the direction of the unknown variable for a proton moving in the field using the given information: F is out of the page F is down F is left

29. MAGNETIC FIELD FORCES – current-carrying wire • Force on a current-carrying wire in a magnetic field: F = ILB • F = FORCE (units: N) • I = CURRENT (units: A) • L = WIRE LENGTH (units: m) • B = MAGNETIC FIELD STRENGTH (units: T, teslas) • **How do we determine direction?** APPLY RIGHT HAND RULE • THUMB IS CURRENT (I)(instead of velocity)