Electricity and Magnetism Magnets Magnetism Magnetic fields Two

Electricity and Magnetism

Magnets, Magnetism, Magnetic fields • Two poles - North and South • Like repels and opposite attracts • Sounds a lot like electricity. What’s different?

Magnetic field similar to E field Field lines point from N pole to S pole outside of the magnet

Where does a magnetic field come from? • In general moving charged particles create their own magnetic fields • Faraday explored this, as shown in the movie (based on Oersted’s work) So current-carrying wire creates a magnetic field. What about regular magnets?

Magnetic fields in a magnet • All matter has spinning electrons ( moving charged particles) • Electrons revolve around the nucleus and also rotate (quantum spin number, s) • The rotation (more so than the revolution) produces a magnetic field around each electron. • A lot of the magnetic fields cancel each other out (opposite directions, same magnitude) • But in iron, fields do not cancel entirely • Each atom is a tiny magnet.

Atoms with unpaired electrons are paramagnetic

Iron, Nickel, Cobalt—Magnetic Metals

Nuclear Magnetic Resonance

Which is basically the same as MRI

What causes the earth’s magnetic field? • Our theory is: moving charged particles in the liquid part of the earth’s core create a magnetic field • Geographic North pole is actually magnetic South • What about a flip?

Northern Lights-charged particles from sun enter Earth’s magnetic field

Typical strength of magnets and magnetic fields • Units are TESLA for mks combination of units; GAUSS for cgs combination • Basically it’s a Newton/amp-meter (weird!)

Magnetic field strength • • • Typical Values Here is a list of how strong some magnetic fields can be: In a magnetically shielded room 10^-14 Tesla Interstellar space 10^-10 Tesla Earth's magnetic field 0. 00005 Tesla Small bar magnet 0. 01 Tesla Within a sunspot 0. 15 Tesla Small NIB magnet 0. 2 Tesla Big electromagnet 1. 5 Tesla Strong lab magnet 10 Tesla Surface of neutron star 100, 000 Tesla Magnetar 100, 000, 000 Tesla • (At a distance halfway to the moon, a magnetar could strip information from the magnetic stripes of all credit cards on Earth. )

What determines the strength of a magnetic field? Well, that depends… • In a straight current-carrying wire, B = µ 0 I/2πr where r is distance from wire • In a circular loop, at the center, B = µ 0 IN/2 a where a is radius of loop. • Inside a long solenoid, B = µ 0 IN/L where N is the number of loops per meter • µ 0 is the permittivity of free space, an electrical constant that conceptually represents how much a field influences particles • General Law:

And to further complicate the issue: • There are two magnetic fields, H and B. In a vacuum they are indistinguishable, differing only by a multiplicative constant that depends on the physical units. Inside a material they are different. The term magnetic field is historically reserved for H while using other terms for B. Informally, though, and formally for some recent textbooks mostly in physics, the term 'magnetic field' is used to describe B as well as or in place of H. There are many alternative names for both.

And there’s magnetic flux… • Sort of how much of the field passes through the surface.

For charged particles, the force “felt” due to a magnetic field depends upon: • • Velocity of the charged particles , v Charge of the charged particles, q Distance away from the charged particles, r Force a magnetic field exerts on a charged particle, F = qv. B sinϴ, where B is the magnetic field strength (force has max value when v and B are perpendicular).

Getting back to Faraday…. Electricity and magnetism are related in three major ways: 1. Moving charged particles create magnetic fields around themselves 2. External magnetic fields exert a force on a current carrying wire or stream of charged particles. 3. Moving magnetic fields induce current.

1. A moving charge induces a magnetic field. This means a currentcarrying wire produces a magnetic field. The compass needle deflects in directions tangent to the circle – The compass needle points in the direction of the magnetic field produced by the current

The Right Hand Rule

Different configurations from in-class Faraday lab • • • Wire/nail…. Pink loop Big loop Small loop Multiple loops

External magnetic fields exert a force on a current carrying wire (or a stream of charged particles) The blue x’s indicate the magnetic field is directed into the page – The x represents the tail of the arrow Blue dots would be used to represent the field directed out of the page – The • represents the head of the arrow In this case, there is no current, so there is no force

Second (or third) right hand rule

Demo in class • Make a circuit of a voltage source, ammeter and wires • Hold section of wire vertically • Voltage source is OFF • Bring strong magnet close to vertical wire • Turn voltage source ON • Observe……. • Reverse the current and redo…observe…

B is into the page The current is up the page The force is to the left B is into the page The current is down the page The force is to the right

Electro-magnet Superhero! • Charged-up superhero at rest has an electric field. • Charged-up superhero in motion has E field AND a magnetic field!! (#1) • Unfortunately, villains can use powerful magnets to exert force and control the moving Superhero! (#2) • Or Evil Genius

What if the superhero was like a beam of electrons? • What if it was a stream of moving charged particles? like a CRT display in an older tv or computer screen or oscilloscope?

Let’s look at an oscilloscope • Link to paer. rutgers. edu/pt 3 • http: //paer. rutgers. edu/pt 3/experimentindex. php? topicid=10&cycleid=46 • Observe how the electron beam is affected by the magnet and its magnetic field

Let’s look at the inside of a CRT tube • Link to ‘how stuff works’ • http: //electronics. howstuffworks. com/tv 3. ht m • What would happen if your tv was on and you touched the screen with a magnet?

So far… • 1. Current carrying wires have a magnetic field • Shape of field depends on configuration of wire • 2. EXTERNAL magnetic fields exert a force on a current carrying wires • CONCLUSION: magnetic fields interact… – Can use this info for other purposes…

Finally #3) To induce a voltage, current in a conducting wire…. • There must be relative motion between the magnet and the coil of wire • Faster motion…. more voltage, current • More coils, larger cross-sectional area…more voltage, current

Actually…the real reason is… • The relative motion causes a…… • CHANGING MAGNETIC FIELD THROUGH THE AREA OF THE COIL !! • Do a demo in class… • That’s the idea behind a generator!!

Back to #2, If the current carrying wire is forced to move, doesn’t the wire then have kinetic energy? • YESSSSS! That kinetic energy can then be used to do work!!!! • THAT is the idea behind…………. . • THE ELECTRIC MOTOR!!

Electric Motor An electric motor converts electrical energy to mechanical energy – The mechanical energy is in the form of rotational kinetic energy An electric motor consists of a rigid current-carrying loop that rotates when placed in a magnetic field

Generator vs motor • Motor: electrical energy to rotational kinetic energy • Generator: rotational kinetic energy to electrical energy

To wrap up…. • Principles of electromagnetism can be used to…. – Design devices to do mechanical work using electricity – Design devices to generate electricity by doing mechanical work – Many other devices …….

Other cool stuff that uses electricity and magnetic fields • A relatively weak magnet can be made stronger by superimposing the magnetic field from a coil of current carrying wire… AN ELECTROMAGNET! http: //hila. webcentre. ca/projects/electromagnet/

A solenoid • Uses the magnetic field inside a coil of current carrying wire to force a cylinder of metal inside it to slide along the inside of the coil • http: //www. youtube. com/watch? v=e 5434 d. DB -7 w • http: //www. solenoidcity. com/solenoid/manu al/construction. htm

• Unlike x-rays and computed tomographic (CT) scans, which use radiation, MRI uses powerful magnets and radio waves. The MRI scanner contains the magnet. The magnetic field produced by an MRI is about 10 thousand times greater than the earth's.

• The magnetic field forces hydrogen atoms in the body to line up in a certain way (similar to how the needle on a compass moves when you hold it near a magnet). When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals. For example, healthy tissue sends back a slightly different signal than cancerous tissue.

• Single MRI images are called slices. The images can be stored on a computer or printed on film. • MRIs can be done with or without contrast dye.

• MRI can easily be performed through clothing. However, because the magnet is very, very strong, certain types of metal can cause significant errors, called artifacts, in the images. • It can also attract other metal objects that aren’t tied down

Why are these machines noisy? • The noise is due to the rising electrical current in the wires of the gradient magnets being opposed by the main magnetic field. The stronger the main field, the louder the gradient noise.

Other devices • Pick up coil in electric guitar • Speakers, Headphones http: //en. wikipedia. org/wiki/Headphones http: //www. crutchfield. com/learningcent er/home/headphones-glossary. html#driver

Magnetic Resonance Imaging • You or your body part lies in the bore of a magnetic field • Strength of MRI magnets ~ 0. 5 to 2 Tesla up to 60 T or more for research • All metal objects must be removed from MRI room or secured: can be violently attracted to the machine once it’s turned on • Some magnetic fields created by winding of current carrying wire • Some fields created by permanent magnets • Some created by superconducting magnets – like first situation except resistance of wire is minimized by supercooling the wire

• The magnetic field aligns your hydrogen atoms along the direction of the magnetic field (hydrogen has a strong inclination to do this) • The machine applies a radio frequency pulse specific to hydrogen to the part of the body of interest • This causes those hydrogen atoms to move in the opposite direction; this is the ‘resonance’ part of the system. • In addition, smaller magnets are used to create changes in the overall field • These smaller magnets are turned on and off in a specific manner • And as the RF signal is turned on and off, the hydrogen atoms return to their original motion and the machine is designed to detect this and uses computer programs to create a digital image