Magneto statics AP Physics

Magnetism 1.Permanent Magnets – Objects that retain their magnetic properties Ex. the magnet in a compass, bar magnet, horseshoe magnet, speakers, and of course the magnets on the fridge 2.Electromagnets – Objects that exhibit magnetic properties as a result of electric current Ex. electric motors, disk drives, & speakers DWT The properties of attraction possessed by magnets.

Magnets Magnets have two poles… Similar to electric charge…opposites poles

Magnetic Fields (B) & Forces 1.A permanent magnet, a moving charge, or a current creates a magnetic field B at all points in the surrounding space. 2.The magnetic field exerts a force F on any other permanent magnet, moving charge, or current that is present in the field. Magnetic Dipole

Magnetic Field of Earth

Magnetic Forces I The force acting on a moving charge in a B field is: 1.proportional to the magnitude of the 2.proportional to the magnitude of the 3.proportional to the component of the q B v θ

Right Hand Rule (RHR) DWT The direction and the magnitude of the magnetic force is determined by a cross product computation (vector calculus, ugh!). We’ll cheat and use our hands (correction, just right hand) rather than calculus. Right Hand Rule #1 1. Fingers in direction of B Field 2. Thumb in direction of charge’s velocity v 3a. Positive Charge Direction of Force corresponds to direction of the palm 3b. Negative Charge Direction of Force corresponds to direction of back of hand

RHR1 Examples Notation used for Fields x – into surface/page · (dot) – out of surface/page

J.J. Thomson’s e/m Ratio DWT Sir Joseph John “J.J.” Thomson (1856 – 1940) was a British physicist and Nobel laureate, credited for the discovery of the electron (Physics Nobel Prize in 1906), and the invention of the mass spectrometer. Thomson used a cathode ray tube to fire a stream of electrons through magnetic and electric fields (1897). From his experiments he determined: 1.that there must be some small negatively charged particle (known today as the electron, JJ called them corpuscles) 2.the charge to mass ratio (e/m ratio) of this new particle dubbed the electron. J.J.’s Cathode Ray Tube (simplified) Actual Apparatus

Velocity Selector

Mass Spectrometer A scientific device that uses electric and magnetic fields to determine the mass of a substance. How it Works 1.Substance is ionized and vaporized 2.Charged plates accelerate particles 3.Particles pass through magnetic and electric fields (velocity selector) 4.Particles move in a curved path in the presence of a magnetic field

Curved Motion Due to B Field What is the radius R of the circular motion of the particle as a result of the magnetic force?

Mars Rover Scientists want to include a compact mass spectrometer on a future Mars rover. Suppose the instrument is designed to have a magnetic field of 0.01 T, and selects carbon ions that have a speed of 5 x 10 3 m/s and are singly ionized (have a charge of +e). What are the radii R 12 and R 13 of the orbits of 12 C and 13 C ions in this spectrometer?

A particle of mass m and charge –q is accelerated through a potential difference ε. It then passes into a uniform magnetic field of magnitude B directed into the page as shown below. Express your answers in terms of m, q, ε, and B. a)Determine the speed of the electron as it enters the magnetic field. b)Sketch the path of the electron in the magnetic field on the diagram above. c)Determine the radius of the path of the electron. d)An electric field E is now established in the same region as the magnetic field, so that the electron passes through the field undeflected. (i)Determine the magnitude of E. (ii)Indicate the direction of E on the diagram above. Particle in Magnetic Field

Magnetic Forces II The force acting on a segment of conductor with length l, carrying a current I in a uniform B field is: B θ I l

Right Hand Rule Dos (RHR2) Right Hand Rule #2 1. Fingers in direction of B Field 2. Thumb in direction of conductor’s current I (remember, current is direction of positive charges) 3. Direction of Force corresponds to direction of the palm

B Force on Current Loop How do the magnetic forces act on a current carrying loop?

B Force on Current Loop Cont.

Electric Motors Motors operate because of the magnetic force on current carrying conductors.

Force on Conducting Bar Magnetic forces are used to accelerate current carrying conductors.

B Fields From E Currents An electric current creates a magnetic field. Simplest example: Long straight wire

Right Hand Rule Trois (RHR3) Right Hand Rule #3 1. Thumb in direction of current 2. Curl fingers in direction of B field

Current Carrying Wire & Compass

Force b/w Parallel Wires How do current carrying wires affect one another.

Solenoids Almost like the magneto static equivalent of electrostatics’ parallel plate capacitor