Ch20 Magnetism

Metal or Magnet. What is the difference?• Which of the metal bars below is a magnet?• Now, let us take a closer, microscopic look.• Each metal bar has microscopic magnetic domains.• When these domains are randomly aligned, the bar is non-

magnetic.• When these domains are aligned, the bar is a magnetic.

Magnetic Poles• We say that a magnet has poles (ends).

• One pole is named the North pole.

• The other pole is named the South pole.

Fundamental Law of Magnetism• Like poles repel, and unlike poles attract.

• This law is “fundamental” in understanding the operation of many devices that use magnets or electromagnets.

• Some of these devices include speakers, microphones, transformers, electromagnets, motors, generators, and solenoids.

Metal or Magnet. What is the difference?

What happens if a magnet breaks?

Magnetic Field Lines• Around every magnet there are many invisible lines known as

magnetic field lines.

• These lines influence other magnets and charged particles that may be in the vicinity of the magnet.

• Watch what happens to the compass and the blue charged particle as they get near to the magnet.

Magnetic Field Lines – Electric Charges• We observed the path of a moving charged

particle is deflected when it comes close to a magnet.

• This deflection occurs because a charged particle in motion also has a magnetic field.

• A charged particle behaves as a tiny magnet with a north and south pole.

• As a result, a charged particle obeys the fundamental law of magnetism.

• Like poles repel, and unlike poles attract.

Right Hand Rule• The right hand rule is used in order to determine the direction in which the

vector resulting from the cross product would act.

• To find the direction of vector C, you would start by pointing the fingers of your right hand in the direction of the first vector (A) with your palm open in the direction in which vector B points.

• Next, curl your fingers towards the second vector (B).

• Finally, extend your thumb. The direction of C is in the same direction in which your thumb points.

C A B

C

A

B

Right Hand Rule• In what direction would vector Z point?

• Start by pointing the fingers of your right hand in the direction of the first vector (X) with your palm open in the direction in which vector Y points.

• Curl your fingers towards the second vector (Y).

• Finally, extend your thumb. The direction of Z is in the same direction in which your thumb points.

• In which direction would vector W point?

z

x

y

z x y

w y x

Forces on a Charged Particle in a Magnetic Field• You use the cross product in order to determine which direction a

charged particle would move in a magnetic field.

• The equation used to determine the magnitude and direction of this force is

• In this equation, q is the magnitude of the charge, v is the speed of the charge, and B is the magnitude of the magnetic field.

• The cross product is used to determine the direction of the force on a moving charge in the a magnetic field.

• This direction is found using the Right Hand Rule as discussed earlier.

F qv B

Forces on a Charged Particle in a Magnetic Field

• When a charged particle moves through a magnetic field, its magnetic field interacts with the uniform magnetic field and is deflected.

• Use the right hand rule to determine the direction of the force (acceleration) acting on the particle.

• Observe the path of a positive charge moving through a magnetic field.

B

FF qv B

v

Forces on a Charged Particle in a Magnetic Field

• Lets now exam the path for a negatively charged particle moving through the same uniform magnetic field.

• The right hand rule shows us the direction of the cross product is also down.

• However, the negative sign included with the negative charge would reverse the direction of the force applied to the particle.

• As a result, the particle would move up and out of the magnetic field.

B

F

F qv B

v

Interactions Between Electric and Magnetic Fields.• A wire that carries a current (flowing electrons) will bend when it passes through a magnetic field.

• This image is a picture of a magnetic field traveling into the board.

• Right now the current in the wire is off.

• Lets turn off the magnetic field and turn on the current.

• Now turn the magnetic field back on and watch what happens.

• The wire bent. Why?

• The magnetic field produced by moving electric charges interfered with the large magnetic field causing the wire to bend.

Right Hand Rule

Point fingers in the direction of the particles velocity

Curl fingers to point in the direction of the magnetic field

Thumb points in the direction of the force

Motional Electromotive Force

• In the picture below, the “x” represent a uniform magnetic field.

• The gold rods are wires. The horizontal ones are electrically connected to a LED (light emitting diode).

• Watch what happens as the vertical wire rod rolls through the magnetic field.

• The LED lit up. Why do you think it did?

• The magnetic field exerted a force on the electrons (which are “tiny magnets”) in the conductors causing them to move through the wires thereby lighting the LED.

Interactions Between Electric and Magnetic Fields.• The fact that moving electrons (or other charges) generate their

own magnetic fields can be observed by viewing the device below.

• Note that all the compasses are pointed in the same direction (towards the North).

• When electrons flow through the wire, they create a magnetic field.

• This magnetic field interacts with the compass needles (small magnets) causing them to change directions.

Interactions Between Electric and Magnetic Fields.

sinF Il B

Right Hand Rule

•Wrap fingers around the wire with thumb pointing in the direction of current

•Fingers point in the direction of the magnetic field

F Force

I Current

l length

B Magnetic Field

Angle between current and magnetic field

Example Problem WS 11 #7• Two long, parallel wires separated by a known distance carry current as shown.

• What is the magnetic field (magnitude and direction) at point P?

• Begin by using the equations for the long straight wires in order to calculate the magnitudes of the magnetic fields.

• We have already determined the directions

of the magnetic fields at point P.1R 2R

x

P

1B

2B

112

oiBR

222

oiBR

• Two wires carry currents as shown below.

• Calculate the force of attraction or repulsion that exists between these two wires.

• First determine the direction of the magnetic field around these two wires by using our right hand rule.

• Consider the force on wire 2 due to wire 1.

• The magnetic field generated by wire 1 causes the force experienced by wire 2.

• Note the direction of this magnetic field acting on wire 2.

• Applying the cross product, we observe that the force acting on wire 1 is directed towards wire 2.

• Likewise, the force on wire 2 is directed towards wire 1.

Force Between two Wires

F iL B

1i

2i

1B

2B

2F

2F

• What do you suppose would happen if the current in one of the wires changes direction?

• Applying the cross product below and the right hand rule, we find that the force generated by the magnetic fields cause the wires to move away from each other.

• In summary, if the current in both wires moves in the same direction, then the wires will attract each other.

• If the current in the wires moves opposite direction, then the wires will repel each other.

2B

Force Between two Wires

F iL B

1i

2i

1B

2F

1F

Centripetal & Electric forces

x x x x x x x x x x x x

x x x x x x x x x x x x

x x x x x x x x x x x x

x x x x x x x x x x x x

x x x x x x x x x x x x

x x x x x x x x x x x x

v

Interactions Between Electric and Magnetic Fields.

sinF Il B

13-7

A wire carrying a 30A current has a length of 12cm between the pole faces of a magnet at an angle of 60o. The field strength is 0.9T. What is the magnitude and direction of the force.

30 (.12 )(.9 )sin 60F A m T

2.8F N

Magnetic Induction• A coil of wire is wrapped around a nail as shown in the top picture below.• This coil of wire is attached to a power supply in which the current direction may be

reversed.• The bottom nail shows the alignment of the magnetic domains in the top nail.• Watch what happens as the electrons flow through the coil of wire around the nail.• The domains slowly realigned until they were all pointed in the same direction.• The nail/wire coil combination has become and electromagnet (see next slide).• What will happen when we reverse the current in the coil of wire?• In this animation, we saw that moving electrons induced the magnetic dipoles to align.

Magnetic Induction• Realigning magnetic dipoles can also cause electrons to move.• Flowing electrons from a power supply cause the domains in the vicinity of the

first coil to align.• These domains in turn induce the other domains in the nail to realign.• Watch what happens to the electrons in the second coil and watch the light bulb

as the magnetic domains in the vicinity of the second coil realign.• As long as these domains were moving, electrons in the second coil moved

causing the light to go on.• Once the domains stop moving, the electrons stop moving, and the light goes out

even though the electrons are still moving in the first coil.

13-17

Electromagnetic Induction

• Electromagnetic Induction: The process of generating a current through a circuit by relative motion of charged objects through a magnetic field.

• Current can be generated by:– moving a conductor through a magnetic field– a magnetic field can move past a conductor