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PHYS132 Spring 2012 FIELDS (continued) February 13, 2012
PHYS132 Spring 2012 Organization Chapter 9 Qualitative description (pictures); chapter 9 Chapter 10 Relating forces and fields (magnitude) Relating forces and fields (direction) Chapter 11 Predicting fields (magnitude)
PHYS132 Spring 2012 The concept of a “field” • Something “around” the object that impacts any object that comes near. • Each object has its own field; the field “belongs” to the object • The strength of an object’s field at any particular location depends how far one is from the object and some property of the object (like its mass, charge or magnetic moment).
PHYS132 Spring 2012 Using arrows to indicate the field Q: What’s the difference between a force and a field? From http://www.vias.org/physics/bk4_06_03.html
From National High Magnetic Field Laboratory PHYS132 Spring 2012 Using lines to indicate the field
PHYS132 Spring 2012 Question • Does an electromagnet have an electric field? • What does the magnetic field look like for a straight wire with current? • Why is the magnetic field strongest inside the solenoid and weak outside?
PHYS132 Spring 2012 Relating forces and fields • The impact on 1 due to 2 is called a force. Depends on interaction of 1 with 2. • Mathematically, we split the interaction into two parts, each containing an intrinsic property of one of the two objects.
PHYS132 Spring 2012 Typical electric field values Measured at a distance of 30 cm(From: Federal Office for Radiation Safety, Germany 1999)
PHYS132 Spring 2012 Typical magnetic field values • The magnetic field is measured in teslas (T) • Portable radio (30 cm away) is ~0.000001 T (1 µT) • Earth’s magnetic field is ~0.00005 T (50 µT) • Refrigerator magnet: 0.005 T (5 mT) • MRI: 1.5-3 T • Used to levitate a frog: 16 T • Strongest obtained in a non-destructive way: 97.4 T
PHYS132 Spring 2012 Force vs. field directions • Gravitational force on object 1 is in the same direction of object 2’s gravitational field at that location. • Electric force on object 1 is in the same direction of object 2’s electric field at that location if object 1 is positively charged. Otherwise, the force is in the opposite direction. breakdown of insulators
PHYS132 Spring 2012 Force vs. field direction • Magnetic torque on loop 1 is to align the loop so that its magnetic field is aligned with object 2’s magnetic field at that location. • Magnetic force on wire 1 is perpendicular to both the direction of the current through the wire 1 and object 2’s magnetic field at that location. • Magnetic force on moving charge 1 is perpendicular to both the velocity of object 1 and object 2’s magnetic field at that location.
PHYS132 Spring 2012 Demonstrations • Torque on a loop with current • Force on a straight wire with current (and RHR)
PHYS132 Spring 2012 Chapter 11 (Predicting Fields) • Object 2’s field = force on 1 / intrinsic property of 1 • To predict the strength of the field, first predict the force exerted on another object, then divide by the quantity of that object.
PHYS132 Spring 2012 Gravitational field • Gravitational force on object 1 due to 2 • Fg = Gm1m2 / r2 • Divide by m1 to get object 2’s gravitational field • g = Gm2 / r2
PHYS132 Spring 2012 Electric field • Electric force on object 1 due to 2 • Fe = kq1q2 / r2 • Divide by q1 to get object 2’s electric field • E = kq2 / r2
PHYS132 Spring 2012 Magnetic field
PHYS132 Spring 2012 For Wednesday • Read chapter 11 • Submit “reading review” question on chapter 11 • Quiz C • Second chance at quiz B • Part C checkpoint homework will be due Friday for chance at quiz C retake