# Lect12 - Physics 212 Physics 212 Lecture 12 Lecture 12...

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Unformatted text preview: Physics 212 Physics 212 Lecture 12 Lecture 12 Today's Concept: Today's Concept: Magnetic Force on moving charges Magnetic Force on moving charges = × r r r F qv B 5 10 15 20 25 30 35 Avg Avg = 3.2 = 3.2 Confused Confused Confident Confident Today Today ’ ’ s Plan: s Plan: 1) 1) Review of magnetism Review of magnetism 2) 2) Review of cross product Review of cross product 3) 3) Example problem Example problem Key Concepts: Key Concepts: 1) 1) The force on moving charges due to a The force on moving charges due to a magnetic field. magnetic field. 2) 2) The cross product. The cross product. 05 Magnetic Observations N S • Bar¡Magnets • Compass Needles S N N S N S N S N S N S cut in half 07 N S Magnetic Charge? • Compass needle deflected by electric current I • Magnetic fields created by electric currents • Magnetic fields exert forces on electric currents (charges in motion) V V I I F F V V I I F F Magnetic Observations 12 Magnetism & Moving Charges All observations are explained by two simple equations: = × r r r F qv B Today 2 ˆ 4 μ π × = r r I ds r dB r Next Week 14 What do we know about E & B fields? • E fields are caused by charges. Those charges can be stationary or moving. • B fields are caused by MOVING charges. • E field lines start (go out from ) + charges and end in (go into) – charges • B field lines have no beginning or end • No such thing as a “magnetic charge” has been found (called “magnetic monopole”). Magnets come with N and S poles together and a N-pole or S-pole cannot be isolated. • Notation: “X” denotes into the page, “•” denotes out of the page. 14 Cross Product Review • Cross Product different from Dot Product – A ● B is a scalar; A x B is a vector – A ● B proportional to the component of B parallel to A – A x B proportional to the component of B perpendicular to A • Definition of A x B – Magnitude: ABsin θ – Direction : perpendicular to plane defined by A and B with sense given by right-hand-rule 16 Remembering Directions: The Right Hand Rule F qv B = × r r r x y z B F qv 17 Three points are arranged in a uniform magnetic field. The B field points into the screen. 1) A positively charged particle is located at point A and is stationary. The direction of the magnetic force on the particle is: Preflight 2 F qv B = × r r r The particle The particle ’s velocity is zero. s velocity is zero. There can be no magnetic force. There can be no magnetic force. 10 20 30 40 50 60 18 Three points are arranged in a uniform magnetic field. The B field points into the screen. 1) A positively charged particle is located at point A and is stationary. The direction of the magnetic force on the particle is: Preflight 4 F qv B = × r r r 10 20 30 40 50 60 A B C D E 21 BB BB X qv B F Cross Product Practice • protons (positive charge) coming out of screen • Magnetic field pointing down • What is direction of force on POSITIVE charge?...
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