who first used magnets?
the chinese used magnets for navigation over 1000 years ago
what is the difference between magnetism and charge?
+ and - charges can be separated magnets cannot be separated. they always become dipoles even if cut in half. MONOPOLES DO NOT EXIST
ferromagnetic
strongly magnetic ie Fe
paramagentic
weakly attracted to magnet
diamgnetic
weakly repelled by magnet
magnetic fields
are closed loops. pont from north to south, strong field = more lines, electric fields end on charges. not so with magnetic fields, because theyre in a loop!
T/F magnetic fields end on charges
FALSE. electric fields end on charges, but magnetic fields are in a loop.
earths magnetic field
further out, it is distorted by the solar wind. moves about 10 Km per year. has reversed about 400 X in the last 330 million years. biological organism use this ot navigate. proves plate techtonics.
suns magnetic field
sun sots occur in pairs because one is a N one is a S. flips every 11 years or so.
biological uses of magnetic fields
mgnetic bacteria use for orientation. potential mars fossil found. navigation for homing pigeons, robins, bees.
permanent magnets
lodestone- iron ore- magnetite
T/F shape of magnet does not make a difference
False, the shape of a magnet makes a difference
geographic north is magnetic _________
south. geographic north 90 degrees. magnetic north 81 degrees.
electric currents produce _______ ___________
magnetic fields
right hand rule: fingers are
direction of the field. North to south
right hand rule the thumb is
the direction the charges move. if a charge is positive, its the palm that is the direction of teh force. if the charge is negative, the backhand is the direction of the force.
unit of magnetic field
Tesla: T. 1 tesla=1N/A*m . another unit sometimes used is Gauss (G). 1G=10^-4 T
the force on the wire depends on:
the current, the length of the wire, the magnetic field, and its orientation. FB=piBsintheta.
FB=Pi
B
sin(theta)
this equation defines the magnetic field B when current is perpendicular to the field, (90 degrees) sin theta = 1 so FB=I
L
B . if the current is parallel to the force, sintheta=0, so F is 0
FB=qvB
q is the charge, v is the speed, B is the magnetic field. when theta = 0 the particle moving along with the magnetic field, so there is no force. if the particle is not moving, v is 0, so there is no force.
what direction does a magnetic field always point?
TO THE SOUTH
does a magnetic field do work?
if there is a magnetic force, no. this is because the force is perpendicular to the motion, so it does no work on the charge. however, if the force and the motion are in the same direction, there is positive work. if they move in opposite dircetion there
when is the path of a charged particle a circle?
when the charged particle is moving perpendicular to a uniform magnetic field.
FB=qvB=mv^2/r
particles in a uniform magnetic field go in a circle.
electrons travel ______________
clockwise
positive charges travel ________________
counter clockwise
if a charged particle travels at an angle, there is ________
work. because the charge spirals around. parallel component is zero F so it has a constant v. perpendicular component has a F that causes a circular path. together, these velocity and F components create a helix, creating aurora.
aurora
charged particles striking atoms in atmosphere excite electrons.
solenoid
A long coil of wire consisting of many loops (or turns) of wire. The current in each loop produces a magnetic field, The magnetic field within solenoids can be fairly large because it is the sum of the fields due to the current in each loop. solenoids act
how to strengthen a solenoid
more coil (more turns in the coil), iron coil, more voltage (current)
uses of solenoid
old style telephones: closing circuit creates magnetic field in coil. iron rod is attracted and strikes the bell. also used as starter in car.
galvanometer
takes advantages of the torque on a current loop to measure weak currents
motors
interaction between induced magnetic field and perminant magnet in the motor. the current provides an induced magnetic field and it turns
loudspeakers
paper cone. current in coil creates magnetism. permanent magnet interacts with created magnetism. resulting force causes coil to move.
particle detector: bubble chamber
chamber of low temperature liquid sits in magnetic field. charged particle traveling through leaves trail of bubles. using right hand rule, we could tell the charge of the particle.
mass spectromter
separate ions by mass. used to measure ion mass and relative numbers. isotope abundances. toxicology, environmental pollutants, csi.
cyclotrons
used to make charged particles accelerate. (charged particles moving in magnetic field- travel in a circle. given a speed boost while moving between the 'dees') used to make short half-life radioisotopes for medical purposes. used in proton beam cancer su
force on charged particles is _________ ___ of electric nd magnetic field
vector sum.
for perpendicular electric and mgnetic filds,
magnetic force and electric force are in opposit direction.
thomsons eperiment
used electric and magnetic fields to determine charge to mass ratio of electron : cathode ray experiment.
who found the charge of the electron?
Millikan. After he did this, we could use Thomsons experiment to find the mass of an electron.
T/F: groups of atoms (domains) pointing the same direction creates a magnetic effect.
true. individual iron atoms are magnetic, and the magnetism is much greater if they are aligned in the same direction. a magnet, if undisturbed, will tend to retain its magnetism. if you strike it, youll disrupt magnetism.
1: A compass needle is not always balanced parallel to the Earth's surface, but one end may dip downward. Explain
The Earth's magnetic field is not always parallel to the surface of the Earth�it may have a component perpendicular to the Earth's surface. The compass will tend to line up with the local direction of the magnetic field, so one end of the compass will dip
2. Explain why the Earth's "north pole" is really a magnetic south pole. Indicate how north and south magnetic poles were defined and how we can tell experimentally that the north pole is really a south magnetic pole.
The pole on a magnetic compass needle that points geographically northward is defined at the north pole of the compass. This north pole is magnetically attracted to the south pole of other magnets, so the Earth's magnetic field must have a south pole at t
4. A horseshoe magnet is held vertically with the north pole on the left and south pole on the right. A wire passing between the poles, equidistant from them, carries a current directly away from you. In what direction is the force on the wire? Explain.
The force is downward. The field lines point from the north pole to the south pole, or left to right. Use the right-hand rule. Your fingers point in the direction of the current (away from you). Curl them in the direction of the field (to the right). Your
8. If a negatively charged particle enters a region of uniform
magnetic field which is perpendicular to the particle's velocity, will the kinetic energy of the particle increase, decrease, or stay the same? Explain your answer. (Neglect gravity and assume
The magnetic force will be exactly perpendicular to the velocity, which means that the force is perpendicular to the direction of motion. Since there is no component of force in the direction of motion, the work done by the magnetic force will be zero, an
9. In Fig. 20-47, charged particles move in the vicinity of a current-carrying wire. For each charged particle, the arrow indicates the initial direction of motion of the particle, and the � or - indicates the sign of the charge. For each of the particles
Use the right-hand rule to determine the direction of the force on each particle. In the plane of the diagram, the magnetic field is coming out of the page for points above the wire and is going into the page for points below the wire.
a: force down, towa
14. Suppose you have three iron rods, two of which are magne- tized but the third is not. How would you determine which two are the magnets without using any additional objects?
Put one end of one rod close to one end of another rod. The ends will either attract or repel. Continue trying all combinations of rods and ends until two ends repel each other. Then the two rods used in that case are the magnets.
15. Can you set a resting electron into motion with a magnetic field? With an electric field? Explain.
No, you cannot set a resting electron into motion with a magnetic field (no matter how big the field is).
A magnetic field can only put a force on a moving charge. Thus, with no force (which means no acceleration), the velocity of the electron will not ch
17. A charged particle moves in a straight line through a partic- ular region of space. Could there be a nonzero magnetic field in this region? If so, give two possible situations.
Yes. One possible situation is that the magnetic field is parallel or antiparallel to the velocity of the charged particle. In this case, the magnetic force would be zero, and the particle would continue moving in a straight line. Another possible situati
1. Indicate which of the following will produce a magnetic field:
(a) A magnet.
(b) The Earth.
(c) An electric charge at rest.
(d) A moving electric charge.
(e) An electric current.
(f ) The voltage of a battery not connected to anything. (g) An ordinary
(a, b, d, e) A common misconception is that only permanent magnets (such as a magnet and the Earth) create magnetic fields. However, moving charges and electric currents also produce magnetic fields. Stationary charges, ordinary pieces of iron, and other
4. When a charged particle moves parallel to the direction of a magnetic field, the particle travels in a
(a) straight line. (c) helical path.
(b) circular path. (d) hysteresis loop.
(a) The charged particle only experiences a force when it has a component of velocity perpendicular to the magnetic field. When it moves parallel to the field, it follows a straight line at constant speed.
5. As a proton moves through space, it creates
(a) an electric field only.
(b) a magnetic field only.
(c) both an electric field and magnetic field.
(d) nothing; the electric field and magnetic fields cancel
each other out.
c) Electric fields are created by charged objects whether the charges are moving or not. Magnetic fields are created by moving charged objects. Since the proton is charged and moving, it creates both an electric field and a magnetic field.
6: which statements about the force on a charged particle placed in a magnetic field are true?
a: a magentic force is exerted only if the particle is moving
b. the force is a maximum if the particle is moving in the direction of the field
c. the force cau
(a) A stationary charged particle does not experience a force in a magnetic field. Therefore, the particle must be moving to experience a force. The force is a maximum when the particle is moving perpendicular to the field, not parallel to the field. Sinc
7. Which of the following statements is false? The magnetic field of a current-carrying wire
(a) is directed circularly around the wire.
(b) decreases inversely with the distance from the wire. (c) exists only if the current in the wire is changing.
(d) d
(c) Section 20-5 shows that the magnetic field from a current is directed circularly around the wire, is proportional to the current flowing in the wire, and is inversely proportional to the distance from the wire. A constant current produces a magnetic f
8. A wire carries a current directly away from you. Which way do the magnetic field lines produced by this wire point?
(a) They point parallel to the wire in the direction of the
current.
(b) They point parallel to the wire opposite the direction
of the c
(e) It is common to confuse the direction of electric fields (which point toward or away from the charges) with magnetic fields, which always make circles around the current.
9. A proton enters a uniform magnetic field that is perpen- dicular to the proton's velocity (Fig. 20-51). What happens to the kinetic energy of the proton?
(a) It increases.
(b) It decreases.
(c) It stays the same.
(d) It depends on the velocity directio
(c) A common misconception is that a force always does work on the object. Since the magnetic force is perpendicular to the velocity of the proton, the force acts as a centripetal force, changing the proton's direction, but not doing any work and thus not
11. Which of the following statements about the force on a charged particle due to a magnetic field are not valid?
(a) It depends on the particle's charge.
(b) It depends on the particle's velocity.
(c) It depends on the strength of the external magnetic
(e) Equation 20-3 shows that the magnetic force depends upon the particle's charge, its velocity, and the strength of the external magnetic field. The direction of the force is always perpendicular to the magnetic field and the velocity of the particle. T
12. Two parallel wires are vertical. The one on the left carries a 10-A current upward. The other carries 5-A current down- ward. Compare the magnitude of the force that each wire exerts on the other.
(a) The wire on the left carries twice as much current
(c) This question requires a consideration of Newton's third law. The force that one wire exerts on a second must be equal in magnitude, but opposite in direction, to the force that the second exerts on the first.
What did oersted say in 1820
Current creates magnetic field (magnetic induction) induced emf. Inducing current
Faraday, would magnetic field induce an electric current?
He found that a changing magnetic field creates current (induced current) He had a battery hooked up to a switch wrapped around an iron ring which on the other side was connected to a galvanometer. The magnetic field of x interacted with y creating a curr
Two ways to produce emf
1) move conductor in magnetic field
2) changing magnetic field
Faradays law to increase induced current
1) magnitude of induced emf is related to how fast we change the magnetic field
2) to create more current need more coils
3) stronger magnet
Uses of electromagnetic induction
Electric generator, transformer, GFI (ground fault interrupter), microphone, electric guitar pickup, medical uses in measuring brain function
Lenzs law
The induced current in a loop is in a direction that creates a magnetic field that opposes the change in magnetic flux that created it. Magnetic field of magnet and wire must be opposite since magnet is not pulled in. Conservation of energy.
Induction cooker
A coil of copper wire is placed under a cooking pot. An alternating electric current flows through the coil which produces and oscillating magnetic field. This induce and electric current in the pot. Current flowing in the metal pot produces resistive hea
Eddy currents
We do not need a circuit. A flat piece of metal will have circulating currents induced by magnets. By lenzs law it can be used to stop motion.
Ac generator
Slip ring, two separate rings, one positive one negative. Voltage like a sin wave
D.C. Generator
Split ring. Only one ring. Voltage like hills
Back emf
Running motor creates a current. Current is opposite the current that cause the motor to move. Operating motor is also a generator.
Brown out
Large motor with low resistance. Draws more current. Causes rest of circuit to have reduced voltage. Once the motor is at full speed, back emf is created and brown out is over.
If there is a power brown out
Large motor with low resistance do not spin fast enough. Do not create much back emf. P=i^2r so large current heats circuit and the motor burns out.
Why did we choose ac
The power plants would need to be very close together because the power loss is so great for D.C.. If we were able to run enough voltage for ac, this would not be a problem. So we use transformers to lower voltages to each house. Transformers only work wi
Step down transformer
Takes high voltage from power lines and lowers it to enter your house. Lowers voltage, increases current. Primary would be hooked up to power lines, and secondary hooked up to house. Count the number of coils
Step up transformer
More turns in secondary coils. Goes to a higher voltage and a lower current
Transformers only work it
Current must be changing. This is why electricity is ac because D.C. Would be a constant current.
Transformers
Only work in ac
Same Power comes out as what came in. Only flipping current and voltage.
Bulb dimming problem
Bulb dims after iron is shoved into coil bc current decreases with the addition of a magnetic field (its opposing the other current)
Tf: the North Pole of a magnet points towards the earths geographic South Pole
FALSE!! It points toward the earths geographic North Pole, but to the South Pole of the earths magnetic field.
When the electric current in two wires is flowing in opposite directions the wires tend to
Repel each other.
A particle travels in a bubble chamber that is held in a strong magnetic field. Looking at the tracks the particle makes in the chamber, we can use the right hand rule to find the
Charge of the particle.
. Suppose you are holding a circular ring of wire in front of you and (a) suddenly thrust a magnet, south pole first, away from you toward the center of the circle. Is a current induced in the wire? (b) Is a current induced when the magnet is held steady
3. (a) A current is induced in the ring when you move the south pole toward the ring. An emf and current are induced in the ring due to the changing magnetic flux. As the magnet gets closer to the ring, more magnetic field lines are going through the ring
a) A wire loop is pulled away from a current-carrying wire (Fig. 21-47). What is the direction of the induced current in the loop: clockwise or counterclockwise?
b) what if the wire loop stays fixed as the current I decreases?
a) The magnetic field through the loop due to the current-carrying wire will be into the page. As the wire loop is pulled away, the flux will decrease since the magnetic field is inversely proportional to the distance from the wire. Current will be induce
Explain why, exactly, the lights may dim briefly when a refrigerator motor starts up. When an electric heater is turned on, the lights may stay dimmed as long as the heater is on. Explain the difference.
11. When the motor first starts up, there is only a small back emf in the circuit (back emf is proportional to the rotation speed of the motor). This allows a large current to flow to the refrigerator. The power source for the house can be treated as an e
13. Will an eddy current brake (Fig. 21-20) work on a copper or aluminum wheel, or must the wheel be ferromagnetic? Explain.
Eddy current brakes will work on metallic wheels, such as copper and aluminum. Eddy current brakes do not need to act on ferromagnetic wheels. The external magnetic field of the eddy brake just needs to interact with the "free" conduction electrons in the
A bar magnet falling inside a vertical metal tube reaches a terminal velocity even if the tube is evacuated so that there is no air resistance. Explain.
14. As a magnet falls through a metal tube, an increase in the magnetic flux is created in the areas ahead of it in the tube. This flux change induces a current to flow around the tube walls to create an opposing magnetic field in the tube (Lenz's law). T
It has been proposed that eddy currents be used to help sort solid waste for recycling. The waste is first ground into tiny pieces and iron removed with a magnet. The waste then is allowed to slide down an incline over permanent magnets. How will this aid
15. The nonferrous materials are not magnetic, but they are conducting. As they pass by the permanent magnets, eddy currents will be induced in them. The eddy currents provide a "braking" mechanism which will cause the metallic materials to slide more slo
The pivoted metal bar with slots in Fig. 21-50 falls much more quickly through
a magnetic field than
does a solid bar.
Explain
16. The slots in the metal bar prevent the formation of large eddy currents, which would slow the bar's fall through the region of magnetic field. The smaller eddy currents then experience a smaller opposing force to the motion of the metal bar. Thus, the
If an aluminum sheet is held between the poles of a large bar magnet, it requires some force to pull it out of the mag- netic field even though the sheet is not ferromagnetic and does not touch the pole faces. Explain.
17. This is similar to the situation accompanying Fig. 21-20. As the aluminum sheet is moved through the magnetic field, eddy currents are created in the sheet. The magnetic force on these induced currents opposes the motion. Thus it requires some force t
A bar magnet is held above the floor and dropped (Fig. 21-51). In case (a), the
magnet falls through a wire
loop. In case (b), there is nothing betwen the magnet and the floor. how do the speeds of the magnets compare?
18. The speed of the magnet in case (b) will be larger than that in case (a). As the bar magnet falls through the loop, it sets up an induced current in the loop, which opposes the change in flux. This current acts like a magnet that is opposing the physi
A metal bar, pivoted at one end, oscillates freely in the absence of a magnetic field; but in a magnetic field, its oscil- lations are quickly damped out. Explain. (This magnetic damping is used in a number of practical devices.)
19. As the metal bar enters (or leaves) the magnetic field during the swinging motion, areas of the metal bar experience a change in magnetic flux. This changing flux induces eddy currents with the "free" conduction electrons in the metal bar. These eddy
A transformer designed for a 120-V ac input will often "burn out" if connected to a 120-V dc source. Explain. [Hint: The resistance of the primary coil is usually very low.]
22. When 120 V dc is applied to the transformer, there is no induced back emf that would usually occur with 120 V ac. This means that the 120 V dc encounters much less resistance than the 120 V ac, resulting in too much current in the primary coils. This
A wire loop moves at constant velocity without rotation through a constant magnetic field. The induced current in the loop will be
(a) clockwise. (b) counterclockwise. (c) zero.
(d) We need to know the orientation of the loop relative to the magnetic fiel
2. (c) A common misconception is that a moving loop would experience a change in flux. However, if the loop is moving through a constant field without rotation, then the flux through the loop will remain constant and no current will be induced.
A square loop moves to the right from an area where B
For D:
(a) clockwise.
(b) counterclockwise. (c) zero.
(d) alternating (ac).
B = 0, completely through a region containing a uniform magnetic field directed into the page (Fig. 21-52), and then out to B
3. (d) A current is induced in the loop when the flux through the loop is changing. As the loop passes through line J it enters a region with a magnetic field, so the flux through the loop increases and a current will be induced. When the loop passes line
Two loops of wire are moving in the vicinity of a ery long straight wire carrying a steady current. find the direction of the induced current in each. C is moving parallel to the current, d is moving away from the current to the right.
for C:
a, clockwise
4. (c, a) The magnetic field near a long straight wire is inversely proportional to the distance from
the wire. For C, the loop remains at the same distance from the wire, so the magnetic flux through the wire remains constant and no current is induced in
8. Twoseparatebutnearbycoilsaremountedalongthesame axis. A power supply controls the flow of current in the first coil, and thus the magnetic field it produces. The second coil is connected only to an ammeter. The ammeter will indicate that a current is f
8. (c) When a steady current flows in the first coil, it creates a constant magnetic field and therefore a constant magnetic flux through the second coil. Since the flux is not changing, a current will not be induced in the second coil. If the current in
When a generator is used to produce electric current, the resulting electric energy originates from which source?
(a) The generator's magnetic field.
(b) Whatever rotates the generator's axle.
(c) The resistance of the generator's coil. (d) Back emf.
(e)
9. (b) A generator converts mechanical energy into electric energy. The generator's magnetic field remains unchanged as the generator operates, so energy is not being pulled from the magnetic field. Resistance in the coils removes electric energy from the
which of the following can a transformer accomplish?
a, changing the voltage but not he current
b, changing the current but not the voltage
c, changing power
d, changing both the current and the voltage
11. (d) A common misconception is that a transformer only changes voltage. However, power is conserved across a transformer, where power is the product of the voltage and current. When a transformer increases the voltage, it must also proportionately decr
12. (d) If the charger unit had a battery, then it could run the laptop without being plugged in. A motor converts electrical energy into mechanical energy, but the laptop charger output is electrical energy, not mechanical energy. A generator converts me
12. (d) If the charger unit had a battery, then it could run the laptop without being plugged in. A motor converts electrical energy into mechanical energy, but the laptop charger output is electrical energy, not mechanical energy. A generator converts me
Which of the following statements about transformers is false?
(a) Transformers work using ac current or dc current.
(b) If the current in the secondary is higher, the voltage is
lower.
(c) If the voltage in the secondary is higher, the current is
lower.
13. (a) In a transformer with no lost flux, the power across the transformer (product of current and voltage) is constant across the transformer. Therefore, if the voltage increases, then the current must decrease across the transformer. If the current in
A 10-V, 1.0-A dc current is run through a step-up trans- former that has 10 turns on the input side and 20 turns on the output side. What is the output?
(a) 10 V, 0.5 A.
(b) 20 V, 0.5 A. (c) 20 V, 1 A. (d) 10 V, 1 A. (e) 0 V, 0 A.
14. (e) It may appear that (b) is the correct answer if the problem is interpreted as an ac step-up transformer with twice as many loops in the secondary coil as in the primary. It is true that an ac current would double the voltage and cut the current in
The alternating electric current at a wall outlet is most commonly produced by
(a) a connection to rechargeable batteries.
(b) a rotating coil that is immersed in a magnetic field.
(c) accelerating electrons between oppositely charged capacitor plates.
(d
15. (b) Many people may not realize that generators (rotating coils in a magnetic field) are the heart of most electric power plants that produce the alternating current in wall outlets.