KICKSTART PHYSICS IDEAS TO IMPLEMENTATION

1. HERTZ’S EXPERIMENTS

2. CATHODE RAY TUBES

3. THE PHOTOELECTRIC EFFECT

4. CONDUCTORS AND SEMICONDUCTORS

5. SUPERCONDUCTORS

Kickstart would like to acknowledge and pay respect to the traditional owners of the land – the Gadigal people of the Eora Nation. It is upon their ancestral lands that the University of Sydney is built. As we share our knowledge, teaching, learning, and research practices within this University may we also pay respect to the knowledge embedded forever within the Aboriginal Custodianship of Country.

For more information head to http://sydney.edu.au/science/outreach/high-school/kickstart/index.shtml

The University of Sydney School of Physics Ideas to Implementation

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Implement Your Ideas Risk Analysis Conduct a risk analysis by filling out the following table. List 3 risks to do with this investigation in the 2nd year lab. Also, list 3 risks in an industry where physics is used.

− What are the consequences of those risks? Use the risk matrix below to make your judgment. − What precautions would you take to stop those risks from coming about? − What steps would you take to mitigate those risks?

Risk Consequence Precaution/Mitigation

Kickstart

Electricity Radiation Equipment Water LN2

Electrocution (High) Radiation Sickness (High) Tripping and Injury (Medium) Slipping and Injury (Medium) Frostbite (High)

Safety Switch Low Dose/Shielding Don’t Touch No Food/Drink in Lab Personal Protective Equipment

Physics Industry

Industry – Finance/Tech, Games, Solar Power Research – University, Academia Public Service – CSIRO, DSTO, NMI, Questacon, Government, Teachers

See above, jobs might require more specific considerations

See above, jobs will require more specific considerations

What sort of careers do you think you could get if you studied this topic at the University of Sydney?

The University of Sydney School of Physics Ideas to Implementation

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Hertz’s Experiments with Radio Waves Heinrich Hertz (1857-1894) performed experiments with electromagnetic (EM) radiation. These confirmed many of the predictions that Maxwell had made a few years before. Sketch and label a standing wave with the Node, Antinode and Wavelength (λ). Identify some properties of light that Hertz may have used in his experiments

In this experiment you will calculate the length of the microwaves

1. Measure the distance between maxima and multiply this number by 2

2. Multiply your wavelength (λ) by your frequency (ƒ) in Hertz (Hz) with the formula

v = ƒλ

3. Show your working: The measured half-wavelength is generally around 1.5-2cm, corresponding to a total wavelength of 3-4cm. Ideally, the students should come up with the final result:

𝑣 = 1×10!"Hz  ×  0.03m = 3×10!ms-­‐1 More commonly, students will obtain answers slightly larger than the ideal result due to wavelength measurement errors; this is a useful area for discussion of experimental errors to do with design of experiment, process of measurement, measurement tools etc.

− Constant Velocity − Polarisation − Refraction − Diffraction − Reflection − Absorption − Interference

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Cathode Ray Tubes There are two ways you can change the direction of a cathode ray, with an electric field (E), or a magnetic field (B). Quick and dirty derivation of charge to mass ratio for Thompson’s experiment We assume that the electron was accelerated prior to entering the circular path by passage through a potential difference, V. Therefore the kinetic energy of the particle is:

𝑞𝑉 =12𝑚𝑣! which gives 𝑣! =

2𝑞𝑉𝑚

where 𝑣 and 𝑚 are the velocity and mass of the particle. For the path to be circular, the magnetic force must balance the “centrifugal force”:

𝑚𝑣!

𝑟= 𝑞 𝑣×𝐵 rearranging for

𝑞𝑚=

𝑣𝑟𝐵

After squaring this equation and substituting the relation for 𝑣! given above, we have:

𝑞!

𝑚! =2𝑞𝑉𝑚𝑟!𝐵!

You can use the apparatus here to measure the ratio 𝑞/𝑚. The radius of the circular path the electrons take in the tube depends on how fast they are going and how strong the magnetic field is.

The current in the coils (this produces the magnetic field) is: I = ~ 1.2 (A)

You can therefore calculate B (= 7.8x10-4 x I) B = 7.8x10-4 x 1.2 (Wb/m2)

The accelerating voltage is: V = ~ 250 (V)

The radius of the circular path is: r = ~ 0.06 (m)

Use these measurements and the formula to calculate 𝑞/𝑚 in units of Coulombs/kg. 𝑞!

𝑚! =2𝑞𝑉𝑚𝑟!𝐵!

 solve this equation for 𝑞𝑚=

2𝑉𝑟!𝐵!

Using the above equation and numbers, we obtain the following calculation (units omitted for clarity):

2×250(0.05)!×(0.000936)!

=2×250

0.0025×0.000000876096=

5000.00000000219024876096

 

This comes out as: 228  285  484  696   ≅ 2×10!!  Coulombs/kg

Compare this to the actual value:

𝑞𝑚=1.60217662×10!!"  Coulombs

9.10938356×10!!"kg= 1.758×10!!  Coulombs/kg

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The Photoelectric Effect Albert Einstein won his Nobel Prize in Physics for his insight into the photoelectric effect. His understanding of this phenomenon was one of the milestones in the development of quantum theory and introduced the world to the concept of wave-particle duality.

Waves? Particles? Both? Neither? Light definitely does behave like a wave: it diffracts, refracts and interferes. Light also seems to behave like a particle: it can knock electrons off surfaces and transfer energy to other objects in packets of E = hf.

Qualitative – Observing the Photoelectric Effect In this experiment you can observe the photoelectric effect as certain frequencies of light knock electrons out of a metal surface. Now shine the UV lamp on the metal plate. What do you observe?

Quantitative – Measuring the Photoelectric Stopping Voltage Observe and record the amount of energy the ejected electrons receive from the photons. Electrons knocked out from the surface of a material will all have roughly the same amount of kinetic (moving) energy when they leave:

Filter Colour Yellow Green Blue Violet Ultraviolet

Filter Frequency (Hz) 5.19 x 1014 5.49 x 1014 6.88 x 1014 7.41 x 1014 8.20 x 1014

Stopping Voltage (V) ~ 0.7 ~ 0.8 ~ 1.5 ~ 1.6 ~ 1.9

You now have the necessary information to determine the work function of the phototube’s cathode, and to find a value for Planck’s constant. How would you go about doing this? After drawing the graph on the last page of the worksheet, we start with the equations for the energy of the photoelectron and the energy of the electric field opposing the electrons in the phototube:

𝐸! = ℎ𝑓 − ∅,𝐸! = 𝑞𝑉 For stopping voltage, the energy of the photoelectrons are equal to the energy of the electric field opposing the electrons:

𝑞𝑉 = ℎ𝑓 − ∅ This simplifies to:

𝑉 =ℎ𝑞𝑓 −

∅𝑞

The gradient of the graph the students draw will be equivalent to !! which can be rearranged to:

ℎ = 𝑚𝑞,  where  ℎ = 6.626×10!!"  m2kg/s

Remember to fill out the Excel Spreadsheet on the lab computers and the survey attached to the spreadsheet at http://goo.gl/K9zDla

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Conductors and Semiconductors What do you think will happen to the light bulb when we heat up the conductor or semiconductor?

Predict Observe Explain Apply

Conductor Students make their own predictions Light goes out

More thermal energy = more

electrical resistance

Fuses/Circuit Breakers

Semi-conductor Students make their own predictions Light goes on

Energy is given to electrons to move through material

Transistors, Solar Cells

This process of Predict, Observe, Explain, Apply was used by David Unaipon (1872-1967), known as “Australia’s Leonardo Da Vinci”. He was of the Ngarrindjeri people of South Australia and was an Aboriginal inventor and author who made significant contributions to science. David invented many devices, including a sheep shearer that is still in use today. He never received any recognition or monetary benefit from any of his innovative inventions. In a semiconductor, a current can flow through the movement of negative electrons, the movement of positive holes, or both. This property can be used to create devices such as diodes and transistors that can accurately control whether current flows in a circuit.

The Diode Label the diagram for a diode (a p-n junction) with P-type, N-type, direction of electrons and holes, and the battery.

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Superconductors

From the graph, identify the critical temperature of this high-temperature YBa2Cu3O7 superconductor: ~ 90-95 °K

Superconductors – Flux Pinning and Stable Levitation

List some applications of superconductivity Identify 3 limitations of the use of superconductors

− Energy Transmission and Storage − Maglev Trains − SQUID (Superconducting Quantum Interference

Device) − MRI Machines

− Type 2: Fragile and brittle, not ductile − Types 1/2: Cold temperatures required for

superconductivity to occur − Types 1/2: Can only conduct DC current

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Title: Stopping Voltage vs. Frequency