physics chapter 20 & 21
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 causes the particle to gain kinetic energy d. the direction of the force is along the magnetic field e. a magnetic field always exerts a force on a charged particle
(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. Since the force is perpendicular to the motion of the particle, it acts as a centripetal force, changing the particle's direction but not its kinetic energy. That is, since the force is perpendicular to the motion, it does no work on the particle. The direction of the force is always perpendicular to the direction of motion and also perpendicular to the magnetic field
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.
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 piece of iron. (h) A piece of any metal.
(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 pieces of metal do not create magnetic fields.
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 direction. (e) It depends on the B field direction.
(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 changing its kinetic energy.
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) depends on the magnitude of the current.
(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 field, so the current does not need to be changing.
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, so it exerts twice the force on the right wire as the right one exerts on the left one. (b) The wire on the left exerts a smaller force. It creates a magnetic field twice that due to the wire on the right; and therefore has less energy to cause a force on the wire on the right. (c) The two wires exert the same force on each other. (d) Not enough information; we need the length of the wire.
(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.
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 field. (d) It acts at right angles to the direction of the particle's motion. (e) None of the above; all of these statements are valid.
(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. Therefore, all four statements are accurate.
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 current. (c) They point toward the wire. (d) They point away from the wire. (e) They make circles around the wire.
(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.
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
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
Two ways to produce emf
1) move conductor in magnetic field 2) changing magnetic field
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 decrease the current.
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 emf with an internal resistance. This large current to the refrigerator motor from the power source reduces the voltage across the power source because of its internal resistance. Since the power source voltage has decreased, other items (like lights) will have a lower voltage across them and receive less current, so may "dim." As the motor speeds up to its normal operational speed, the back emf increases to its normal level and the current delivered to the motor is now limited to its usual amount. This current is no longer enough to significantly reduce the output voltage of the power source, so the other devices then get their normal voltage. Thus, the lights flicker just when the refrigerator motor first starts up. A heater, on the other hand, draws a large amount of current (it is a very low-resistance device) at all times. (The heat-producing element is not a motor, so has very little induction associated with it.) The source is then continually delivering a large current, which continually reduces the output voltage of the power source. In an ideal situation, the source could provide any amount of current to the whole circuit in either situation. In reality, though, the higher current in the wires causes bigger losses of energy along the way to the devices, so the lights dim.
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 mechanical energy into electric energy, but the input to the laptop is electrical, not mechanical. The cables to and from the charger unit can be considered transmission lines, but they are not the important component inside the charger unit. A transformer can convert a high-voltage input into a low-voltage output without power loss. This is a significant function of the charger unit.
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 mechanical energy into electric energy, but the input to the laptop is electrical, not mechanical. The cables to and from the charger unit can be considered transmission lines, but they are not the important component inside the charger unit. A transformer can convert a high-voltage input into a low-voltage output without power loss. This is a significant function of the charger unit.
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. (d) If no flux is lost, the product of the voltage and the current is the same in the primary and secondary coils.
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 increases, then the voltage must decrease across the transformer. A transformer works due to the induced voltage created by the changing flux. A dc circuit does not have a changing flux, so a transformer does not work with dc current.
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 half; however, this is a dc current. A dc current does not produce a changing flux, so no current or voltage will be induced in the secondary coil.
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). This induced magnetic field pushes against the falling magnet and reduces its acceleration. The speed of the falling magnet increases until the magnetic force on the magnet is the same size as the gravity force. The opposing magnetic field cannot cause the magnet to actually come to a stop, since then the flux would become a constant and the induced current would disappear, as would the opposing magnetic field. Thus, the magnet reaches a state of equilibrium and falls at a constant terminal velocity. The weight of the magnet is balanced by the upward force from the eddy currents.
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) using an electric motor. (e) alternately heating and cooling a wire.
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.
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 in the separation of nonferrous metals (Al, Cu, Pb, brass) from nonmetallic materials?
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 slowly down the incline than the nonmetallic materials. The nonmetallic materials will reach the bottom with larger speeds. The nonmetallic materials can therefore be separated from the metallic, nonferrous materials by placing bins at different distances from the bottom of the incline. The closest bin will catch the metallic materials, since their projectile velocities off the end of the incline will be small. The bin for the nonmetallic materials should be placed farther away to catch the higher-velocity projectiles.
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 slotted bar falls more quickly through the magnetic field.
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 to pull the sheet out.
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 physics magnet, repelling it and so reducing its speed. Then, after the midpoint of the magnet passes through the loop, the induced current will reverse its direction and attract the falling magnet, again reducing its speed.
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 currents are then acted on by the magnetic field, and the resulting force opposes the motion of the swinging metal bar. This opposing force acts on the bar no matter which direction it is swinging through the magnetic field, thus damping the motion during both directions of the swing.
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 field.
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 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 large amount of current could overheat the coils, which are usually wound with many loops of very fine, low-resistance wire, and could melt the insulation and burn out or short out the transformer.
. 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 within the ring? (c) Is a current induced when you withdraw the magnet? For each yes answer, specify the direction. Explain your answers.
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. Using Lenz's law and the right-hand rule, the direction of the induced current when you bring the south pole toward the ring is clockwise. In this case, the number of magnetic field lines coming through the loop and pointing toward you is increasing (remember that magnetic field lines point toward the south pole of the magnet). The induced current in the loop will oppose this change in flux and will attempt to create magnetic field lines through the loop that point away from you. A clockwise induced current will provide this opposing magnetic field. (b) A current is not induced in the ring when the magnet is held steady within the ring. An emf and current are not induced in the ring since the magnetic flux through the ring is not changing while the magnet is held steady. (c) A current is induced in the ring when you withdraw the magnet. An emf and current are induced in the ring due to the changing magnetic flux. As you pull the magnet out of the ring toward you, fewer magnetic field lines are going through the ring. Using Lenz's law and the right-hand rule again, the direction of the induced current when you withdraw the south pole from the ring is counterclockwise. In this case, the number of magnetic field lines coming through the loop and pointing toward you is decreasing. The induced current in the loop will oppose this change in flux and will create more magnetic field lines through the loop that point toward you. A counterclockwise induced current will provide this opposing magnetic field.
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 = 0 after point L. A current is induced in the loop (a) only as it passes line J. (b) only as it passes line K. (c) only as it passes line L. FIGURE 21-52 MisConceptual Question 3. (d) as it passes line J or line L. (e) as it passes all three lines.
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 K, the flux remains constant, as there is no change in field, so no current is induced. As the loop passes line L, the magnetic field flux through the loop decreases and a current is again induced.
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 b, counter c, zero d, AC for d: a, clockwise b, counter c, zero d, ac
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 the loop. For D, the magnetic field from the long wire points into the page in the region of the loop. As the loop moves away from the wire, the magnetic flux decreases, so a clockwise current is induced in the loop to oppose the change in flux.
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 flowing in the second coil (a) whenever a current flows in the first coil. (b) only when a steady current flows in the first coil. (c) only when the current in the first coil changes. (d) only if the second coil is connected to the power supply by rewiring it to be in series with the first coil.
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 the first coil changes, then the flux through the second will also change, inducing a current in the second coil.
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) Empty space.
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 system in the form of heat—it does not provide the energy. In order for the generator to work, an external force is necessary to rotate the generator's axle. This external force does work, which is converted to electric 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 heating which hears the food while the current is large is is produced by a low voltage.
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 acts like a magnet; one end can be considered the north pole and the other the south pole, depending on the direction of the current in the loops
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)
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.
What did oersted say in 1820
Current creates magnetic field (magnetic induction) induced emf. Inducing current
Transformers only work it
Current must be changing. This is why electricity is ac because D.C. Would be a constant current.
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 metal wheels in order to have the braking effect. First, the magnetic field creates eddy currents in the moving metal wheel using the free conduction electrons (the right-hand rule says moving charges in a magnetic field will experience a magnetic force, making them move and creating an eddy current). This eddy current is also in the braking magnetic field. The right-hand rule says these currents will experience a force opposing the original motion of the piece of metal and the eddy current brake will begin to slow the wheel. Good conductors, such as copper and aluminum, have many free conduction electrons and will allow large eddy currents to be created, which in turn will provide good braking results.
Uses of electromagnetic induction
Electric generator, transformer, GFI (ground fault interrupter), microphone, electric guitar pickup, medical uses in measuring brain function
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.
T/F magnetic fields end on charges
FALSE. electric fields end on charges, but magnetic fields are in a loop.
T/F shape of magnet does not make a difference
False, the shape of a magnet makes a difference
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 current. Current induced when the switch was flipped(movement)
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.
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.
who found the charge of the electron?
Millikan. After he did this, we could use Thomsons experiment to find the mass of an electron.
Step up transformer
More turns in secondary coils. Goes to a higher voltage and a lower current
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 change—it will remain at rest. However, you can set a resting electron into motion with an electric field. An electric field will put a force on any charged particle, moving or not. Thus, the electric force can cause the electron to accelerate from rest to a higher speed.
Transformers
Only work in ac Same Power comes out as what came in. Only flipping current and voltage.
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.
When the electric current in two wires is flowing in opposite directions the wires tend to
Repel each other.
Back emf
Running motor creates a current. Current is opposite the current that cause the motor to move. Operating motor is also a generator.
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
what direction does a magnetic field always point?
TO THE SOUTH
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
unit of magnetic field
Tesla: T. 1 tesla=1N/A*m . another unit sometimes used is Gauss (G). 1G=10^-4 T
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 downward. The angle that the Earth's magnetic field makes with the horizontal is called the dip angle.
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 thumb points in the direction of the force (downward). See Fig. 20-11a, copied here.
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.
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 there is no electric field.)
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, and the kinetic energy of the particle will not change. The particle will change direction, but not change speed.
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 the geographic north pole.
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 with ac
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, indicate the direction of the magnetic force due to the mag- netic field produced by the wire. Explain
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, toward the wire b: force to the left, opposite of the direction of the current c: force up, toward the wire d: force to the left, opposite of the direction of the current
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.
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 situation is that there is an electric field with a magnitude and direction (perpendicular to the magnetic field) such that the electric and magnetic forces on the particle cancel each other out. The net force would be zero and the particle would continue moving in a straight line.
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 induced to increase the inward magnetic field, which means that the induced current will be clockwise. (b) If the wire loop is stationary but the current in the wire is decreased, then the inward magnetic field through the loop will again be decreasing, so again the induced current will be clockwise.
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!
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.
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.
electrons travel ______________
clockwise
positive charges travel ________________
counter clockwise
right hand rule: fingers are
direction of the field. North to south
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.
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 is negative work.
motors
interaction between induced magnetic field and perminant magnet in the motor. the current provides an induced magnetic field and it turns
permanent magnets
lodestone- iron ore- magnetite
electric currents produce _______ ___________
magnetic fields
for perpendicular electric and mgnetic filds,
magnetic force and electric force are in opposit direction.
biological uses of magnetic fields
mgnetic bacteria use for orientation. potential mars fossil found. navigation for homing pigeons, robins, bees.
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.
loudspeakers
paper cone. current in coil creates magnetism. permanent magnet interacts with created magnetism. resulting force causes coil to move.
FB=qvB=mv^2/r
particles in a uniform magnetic field go in a circle.
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.
mass spectromter
separate ions by mass. used to measure ion mass and relative numbers. isotope abundances. toxicology, environmental pollutants, csi.
geographic north is magnetic _________
south. geographic north 90 degrees. magnetic north 81 degrees.
ferromagnetic
strongly magnetic ie Fe
suns magnetic field
sun sots occur in pairs because one is a N one is a S. flips every 11 years or so.
galvanometer
takes advantages of the torque on a current loop to measure weak currents
who first used magnets?
the chinese used magnets for navigation over 1000 years ago
the force on the wire depends on:
the current, the length of the wire, the magnetic field, and its orientation. FB=piBsintheta.
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.
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
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.
thomsons eperiment
used electric and magnetic fields to determine charge to mass ratio of electron : cathode ray experiment.
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 surgery
force on charged particles is _________ ___ of electric nd magnetic field
vector sum.
paramagentic
weakly attracted to magnet
diamgnetic
weakly repelled by magnet
when is the path of a charged particle a circle?
when the charged particle is moving perpendicular to a uniform magnetic field.
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.
