Electricity and magnetism

Nobel prize: grapheneAndre 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

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 equal mass balls are charged, and hang on strings as shown:

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) 0 + B) - + C) 0 0 D) - 0 E) + -

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 leftB) 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)

Current

I = =Qt

Current is the flow of electric charges.

Amount of charge

time

Why isn’t this bird toast?

A) The bird understands quantum physics

B) The wire has no current running through it

C) The bird’s other leg isn’t touching the ground

D) 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

IA

IB

Which current is bigger?A) IA

B) IB

C) They are the same

R

r

R is big resistance

r is small resistance

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 nowB) I can explain all of the examples that we’ve

been throughC) I have a grasp on most of the conceptsD) 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]  

Reference  

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 −0.0005 [1][7]

Carbon (graphite)[8]

2.5-5.0×10−6 ⊥basal plane3.0×10−3 // basal plane

[9]

Carbon (diamond)[10]

~1012 [11]

Germanium[10] 4.6×10−1 −0.048 [1][2]

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 ? [1]

PET 1020 ?

Teflon 1022 to 1024 ?

Material  

Resistivity [Ω·m] at 20 °C  

Temperature coefficient* [K−1]  

Reference  

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 −0.0005 [1][7]

Carbon (diamond)[10]

~1012 [11]

Germanium[10] 4.6×10−1 −0.048 [1][2]

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 ? [1]

PET 1020 ?

Teflon 1022 to 1024 ?

Surface mount devices

Review…

IA IB

Which current is bigger?A) IA

B) IB

C) They are the same

V

Review…

r R

Which voltage dropvoltage drop is bigger?A) Across rB) Across RC) They are the same

V

IA

IB

Which voltage drop is bigger?A. Across rB. Across RC. They are the same

R

r

R is big resistance

r is small resistance

V

R RV

R R

#2

R R

Which circuit has brighter light bulbs?A) #1B) #2C) All light bulbs are the same brightness

#1

V

V

Which circuit has brighter light bulbs?A) #1B) #2C) All light bulbs are the same brightness

R R R

#1 #2

V V

Capacitor

Current is what kills

• 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.

V

R

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) Goes upB) Goes downC) Goes leftD) Goes rightE) Doesn’t move

How about a negative charge (like an electron)?

Electric field

A) Goes upB) Goes downC) Goes leftD) Goes rightE) Doesn’t move

What about a magnetic field?

Magnetic field

A) Goes upB) Goes downC) Goes leftD) Goes rightE) Doesn’t move

Force on a wire

Magnetic field

A) upB) toward youC) away from youD) rightE) 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 a2001+ model

saves:

Replacing with a 2008+Energy Star model

saves:

Replacing with aCEE Tier 3 model

saves:

<1976 2200 kWh $238 / yr.$257 / yr. $266 / yr.

1976-86 1700 kWh $165 / yr.$183 / yr. $192 / yr.

1987-89 1150 kWh $83 / yr.$101 / yr. $110 / yr.

1990-92 1100 kWh $73 / yr.$92 / yr. $101 / yr.

1993-00 850 kWh 37 / yr.$55 / yr. $64 / yr.

2001-2010 600 kWh -$18 / yr. $28 / yr.

2001-2004 Energy Star (10%+ better) 550 kWh - $9 / yr. $18 / yr.

2004-2008 Energy Star (15%+ better) 525 kWh - $5 / yr. $14 / yr.

2008-2010 Energy Star (20%+ better) 500 kWh - - $9 / yr.

CEE Tier 3 (30%+better) 425 kWh - - -

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.edu1.Introduction to Electrical circuits2.Resistors in Series and Parallel Circuits 3.Combo Circuit Lab

Introduction to Electrical circuits Learning Goals: Students will be able to

1.Discuss basic electricity relationships2.Analyze the differences between real circuits and the simulated ones3.Build circuits from schematic drawings4.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.Look brighter

B.Look less bright

C.There will no change in brightness

A B

Resistors in Series and Parallel Circuits 1. Learning Goals: Students will be able to

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 drawings5. 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?

A. 50 B.10 C.They have the

same current

3. Which resistor will have the greatest current?

A.The top resistorB.The lower resistor C.They have the

same current

4. Which resistor will have the greatest voltage?

A. The top resistor

B.The lower resistor

C.They have the same voltage

5. Which resistor will have the greatest voltage?

A. 50 B.10 C.They have the

same voltage

6. Which resistor will have the greatest voltage?

A. 50 B.10 C.They have the

same voltage

7. Which resistor will have the greatest current?

A. 50 B.10 C.They have the

same current

8. Which resistor will have the greatest voltage?

A. The top resistor

B.The lower resistor

C.They have the same voltage

9. Which resistor will have the greatest current?

A. The top resistor

B.The lower resistor

C.They have the same current

10. What will happen if the voltage of the battery is increased to 25 volts?A. The voltage

across the resistor will increase

B. The voltage across the resistor will decrease

C. The voltage of the resistor does not change

11. What will happen if the voltage of the battery is increased to 25 volts?

A. The current through the resistor will increase

B. The current through the resistor will decrease

C. The current of the resistor does not change

Combo Circuit LabLearning 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.eduLearning 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 fasterB.The electrons will go the slowerC.The compass will switch

directionsD.The electrons will go the other

directionE.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 A

B

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