Transcript Electricity and magnetism

Electricity and magnetism
Nobel prize: graphene
Andre Geim
University of Manchester
Konstantin Novoselov
The unit of electric current is the:
A) Volt
B) Amp
C) Coulomb
D) Ohm
E) Watt
Did you read the assignment>?
A) Yes
B) No
A superconductor is a material that:
A) Has lots of electrons
B) Insulates your home
C) Has zero resistance
D) Nobody has created yet
E) Has extremely high voltage
Heat engines  electricity
The basics of electrostatics
Two equal mass balls are charged, and hang on strings as shown:
What can we conclude about the signs of
Q1 and Q2?
A: One is "+", the other is "-"
B: Both are "+"
C: Both are "-"
D: Both must be the same charge (but we
can't tell if they're both "+", or both "-")
Two uncharged conducting spheres, A and B, are
suspended from insulating threads so that they
touch each other. While a negatively charged
rod is held near, but not touching sphere A, the
two spheres are separated. How will the spheres
be charged, if at all?
Sphere A Sphere B
A)
B)
C)
D)
E)
0
0
+
+
+
0
0
-
A
B
An electron traveling horizontally enters a region
where a uniform electric field is directed upward.
What is the direction of the force exerted on the
electron once it has entered the field?
A) to the left
B) to the right
C) upward
D) downward
E) out of the screen
Pick up here Oct 14
If we put bunch of electrons in a box,
they will
A) clump together.
B) spread out uniformly across box.
C) make a layer on walls.
D) do something else.
Voltage?
Voltage is the energy per charge.
CRT: Cathode ray tube (tv)
vertical capacitor
hot metal "-" cathode
screen
"+" anode (with holes)
horizontal capacitor
Current
Amount of charge
Q
I=
=
time
t
q
Current is the flow of electric charges.
Why isn’t this bird toast?
A)
B)
C)
D)
The bird understands quantum physics
The wire has no current running through it
The bird’s other leg isn’t touching the ground
Wires are insulated for safety – people walk up
to wires and touch them too with no problem
Resistance
IA
IB
Which current is bigger?
A) IA
B) IB
C) They are the same
R
IB
r
R is big resistance
r is small resistance
IA
Which current is bigger?
A) IA
B) IB
C) They are the same
Tues Oct 19 pick up here
Opinions about circuit stuff
Are the conceptual circuit sims (PhE`T) that
we’ve been playing with helpful?
A) Yes
B) No
C) Sort of…
Opinions about circuit stuff
What about the ones I’ve drawn on the
board — helpful?
A) Yes
B) No
C) Sort of…
Opinions about circuit stuff
Are our exercises helping you to see how
electricity ‘works’ in our lives?
A) Yes
B) No
C) Sort of…
Opinions about circuit stuff
Would more math help you to see what’s
happening?
A) Yes
B) No
C) Not sure
Opinions about circuit stuff
How well do you understand voltage, current, and
resistance?
A) I can build a computer from scratch now
B) I can explain all of the examples that we’ve
been through
C) I have a grasp on most of the concepts
D) I might struggle if you asked me a question like
“which lightbulb is brighter” in a circuit with 2 or
3 bulbs and a battery
E) I’m actually kind of lost
Opinions about circuit stuff
How much longer do you want to spend on
this stuff?
A) I want more time to explore and discuss
with other students
B) I want more instruction
C) I want to move on to new stuff
Worksheet on circuits for next week’s hw?
A) Yes please
B) No!
C) Just make it optional
Other feedback?
Resistance
Table of resistivities
Material
Resistivity [Ω·m]
at 20 °C
Temperature
coefficient* [K−1]
Referen
ce
Silver
1.59×10−8
0.0038
[1][2]
Copper
1.68×10−8
0.0039
[2]
Gold
2.44×10−8
0.0034
[1]
Aluminium
2.82×10−8
0.0039
[1]
Calcium
3.36x10−8
0.0041
Tungsten
5.60×10−8
0.0045
[1]
Zinc
5.90×10−8
0.0037
[3]
Nickel
6.99×10−8
0.006
Iron
1.0×10−7
0.005
[1]
Platinum
1.06×10−7
0.00392
[1]
Tin
1.09×10−7
0.0045
Lead
2.2×10−7
0.0039
[1]
Mercury
9.8×10−7
0.0009
[4]
Nichrome[6]
1.10×10−6
0.0004
[1]
Carbon
(amorphous)
5-8×10−4
Carbon
(graphite)[8]
2.5-5.0×10−6 basal
plane
3.0×10−3 // basal
plane
[9]
Carbon
(diamond)[10]
~1012
[11]
Germanium[10]
4.6×10−1
−0.048
seawater
2×10−1
?
Silicon[10]
6.40×102
−0.075
[1]
Glass
1010 to 1014
?
[1][2]
Hard rubber
approx. 1013
?
[1]
Sulfur
1015
?
[1]
Paraffin
1017
?
Quartz (fused)
7.5×1017
?
PET
1020
?
Teflon
1022 to 1024
?
−0.0005
[1][7]
[1][2]
[1]
Surface mount devices
Review…
IA
IB
V
Which current is bigger?
A) IA
B) IB
C) They are the same
Review…
r
R
V
Which voltage drop is bigger?
A) Across r
B) Across R
C) They are the same
R
IB
r
V
R is big resistance
r is small resistance
IA
Which voltage drop is bigger?
A. Across r
B. Across R
C. They are the same
V
R
R
#2
V
R
R
#1
V
R
Which circuit has brighter light bulbs?
A) #1
R
B) #2
C) All light bulbs are the same brightness
R
R
R
V
V
#1
#2
Which circuit has brighter light bulbs?
A) #1
B) #2
C) All light bulbs are the same brightness
Capacitor
Current is what kills
V

R
• Ordinarily skin has a resistance of 5,000,000 ohms.
• When wet, the resistance of skin drops by more than a factor
of a thousand
• The body itself is very conductive (approximately salt water),
typical resistance between appendages is ~ 500 ohms.
• The danger here is that usually we are protected with our dry
skin which can get one accustomed to taking electrical risks.
All it takes is for the skin to be dirty, sweaty, or damp to very
significantly reduce it’s resistance (>1000x) enough to make
current vary, for a given ideal voltage source, from barely
perceptible to heart paralysis.
Review: electric fields from charges
Review: electric fields from charges
Review: which way does a positive
charge move in an electric field?
Electric field
A)
B)
C)
D)
E)
Goes up
Goes down
Goes left
Goes right
Doesn’t move
How about a negative charge (like
an electron)?
Electric field
A)
B)
C)
D)
E)
Goes up
Goes down
Goes left
Goes right
Doesn’t move
What about a magnetic field?
Magnetic field
A)
B)
C)
D)
E)
Goes up
Goes down
Goes left
Goes right
Doesn’t move
Force on a wire
Magnetic field
A)
B)
C)
D)
E)
up
toward you
away from you
right
No force
How can you measure current?
Sample savings for replacing an old 22 c.f. fridge with a newer model
A 22 cf fridge made in this year...
Uses about
this
much
energy...
Replacing with a
2001+ model
saves:
Replacing with a 2008+
Energy Star model
saves:
Replacing with a
CEE Tier 3 model
saves:
$257 / yr.
$266 / yr.
$183 / yr.
$192 / yr.
$101 / yr.
$110 / yr.
$92 / yr.
$101 / yr.
$55 / yr.
$64 / yr.
$18 / yr.
$28 / yr.
-
$9 / yr.
$18 / yr.
525 kWh
-
$5 / yr.
$14 / yr.
2008-2010 Energy Star (20%+ better)
500 kWh
-
-
$9 / yr.
CEE Tier 3 (30%+better)
425 kWh
-
-
-
<1976
2200 kWh
$238 / yr.
1976-86
1700 kWh
$165 / yr.
1987-89
1150 kWh
$83 / yr.
1990-92
1100 kWh
$73 / yr.
1993-00
850 kWh
37 / yr.
2001-2010
600 kWh
-
2001-2004 Energy Star (10%+ better)
550 kWh
2004-2008 Energy Star (15%+ better)
kWh per month
Two bar magnets are brought near
each other as shown. The
magnets...
A) attract
B) repel
C) exert no net force on each other.
A bar magnet is positioned below a horizontal loop of wire with
its North pole pointing toward the loop. Then the magnet is
pulled down, away from the loop. As viewed from above, is the
induced current in the loop clockwise or counterclockwise?
Transformer
You have a transformer with Np=6 primary
windings, and Ns=3 secondary windings, as
shown.
If Vp=120 V AC, what is the voltage in the
secondary circuit?
A) 120 V
B) 60 V
C) 240 V
D) 30 V
Vsecondary
HIGH energy
LOW energy
Circuit Construction Kit
Clicker questions
PICK THROUGH
THESE……
Three activities by Trish Loeblein
phet.colorado.edu
1.Introduction to Electrical circuits
2.Resistors in Series and Parallel Circuits
3.Combo Circuit Lab
Introduction to Electrical
circuitswill be able to
Learning Goals: Students
1.Discuss basic electricity relationships
2.Analyze the differences between real
circuits and the simulated ones
3.Build circuits from schematic
drawings
4.Use a multimeter to take readings in
circuits.
5.Provide reasoning to explain the
measurements and relationships in
circuits.
4. If you build
circuit A and
then add a
resistor as in
circuit B, the
light will
A
A. Look brighter
B. Look less bright
C. There will no change
in brightness
B
Resistors in Series and
Parallel
Circuits
Learning
Goals: Students
will be able to
1.
2. Discuss basic electricity relationships in
series and parallel circuits
3. Analyze the differences between real
circuits and the simulated ones
4. Build circuits from schematic drawings
5. Use a multimeter to take readings in
circuits.
6. Provide reasoning to explain the
measurements in circuits.
2. Which resistor
will have the
greatest current?
50 
B.10 
C.They have
the same
current
A.
3. Which resistor
will have the
greatest current?
A.The top resistor
B.The lower
resistor
C.They have the
same current
4. Which resistor
will have the
greatest voltage?
The top resistor
B.The lower
resistor
C.They have the
same voltage
A.
5. Which resistor
will have the
greatest voltage?
50 
B.10 
C.They have
the same
voltage
A.
6. Which resistor
will have the
greatest voltage?
50 
B.10 
C.They have
the same
voltage
A.
7. Which resistor
will have the
greatest current?
50 
B.10 
C.They have
the same
current
A.
8. Which resistor
will have the
greatest voltage?
The top
resistor
B.The lower
resistor
C.They have the
same voltage
A.
9. Which resistor
will have the
greatest current?
The top
resistor
B.The lower
resistor
C.They have the
same current
A.
happen if the
voltage of the
battery is
increased to 25
A. The voltage
volts?
across the
resistor will
increase
B. The voltage
across the
resistor will
decrease
C. The voltage of the
resistor does not
11. What will happen if the
voltage of the battery is
increased to 25 volts?
The current
through the
resistor will
increase
B. The current
through the
resistor will
decrease
C. The current of the
resistor does not
A.
Combo Circuit Lab
Learning Goals: Students will be able to:
1.Analyze the differences between real
circuits and the ideal ones,
2.Build circuits from schematic drawings,
3.Use a multimeter to take readings in
circuits.
4.Provide reasoning to explain the
measurements in circuits.
12. What is the
total resistance
in this circuit?
A.6.4 
B.21 
C.38 
D.75 
13. What is the
total resistance
in this circuit?
A.6.4 
B.21 
C.38 
D.75 
Faraday’s Electromagnet Lab
by Trish Loeblein May 10, 2010
http://phet.colorado.edu
Learning Goals Activity
1: Students will be able to
1.Predict the direction of the magnet field for different locations around a bar
magnet and electromagnet.
2.Compare and contrast bar magnets and electromagnets
3.Identify the characteristics of electromagnets that are variable and what
effects each variable has on the magnetic field’s strength and direction.
4.Relate magnetic field strength to distance quantitatively and qualitatively
5.Compare and contrast the fields of gravity and magnets qualitatively
Learning Goals Activity 2: Students will be able to:
•Identify equipment and conditions that produce induction
•Compare and contrast how both a light bulb and voltmeter can be used to
show characteristics of the induced current
•Predict how the current will change when the conditions are varied.
1.Which compass shows the correct
direction of the magnet field at point
A?
A.
B.
C.
D.
A
2.Which compass shows the correct
direction of the magnet field at point
A?
A.
B.
C.
D.
A
3.Which compass shows the correct
direction of the magnet field at point
A?
A.
B.
C.
D.
A
4.What will happen if
you switch the battery
so that the positive
end is on the right?
A. The electrons will go faster
B.The electrons will go the slower
C.The compass will switch
directions
D.The electrons will go the other
direction
E.Two of the above.
5.What would you expect the light to
do if you change the coils from 2 to 3
and you move the magnet the same
speed?
A. Show the
same
brightness
B. Show less
brightness
C. Show more
brightness
6.Which would
be a more
strong magnet?
A. A
B. B
C. They would be
the same
D. Not enough
information to
decide
B
A
7.Which would
be a more
strong magnet?
A. A
B. B
C. They would be
the same
D. Not enough
information to
decide
A
B
Magnetic field