hsc kickstart physics workshop - university of...

IDEAS TO IMPLEMENTATIONS

HSC KICKSTART PHYSICS

WORKSHOP

LIST OF EXPERIMENTS

1. Hertz’s Experiments 2. Cathode Ray Tubes 3. The Photoelectric Effect 4. Conductors and Semiconductors 5. Superconductors

Name:_____________________________

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 own 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.

The University of Sydney School of Physics Ideas to Implementation

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.

Sparks ‘On-Air’

To generate the sparks needed for this experiment we use a device called a Wimshurst Generator. This is a really good static electricity generator. For a spark to jump a 1cm gap in air, it needs to have 25,000 Volts. How many volts potential difference is there between the spheres on this machine? __________________ V If Hertz saw sparks 1/10

th mm long in his secondary coil, what voltage was being induced? __________________ V What is the voltage induced in our secondary coil, as measured on the Oscilloscope? __________________ V Identify some properties of light that Hertz may have used in his experiments with radio waves.


A standing wave is a wave that stays in a constant position. We can create standing waves with our rubber rope. Sketch a standing wave and label the Node, Antinode and Wavelength (λ). A standing wave is how Hertz figured out that all parts of the EM spectrum behaved in the same way as light.

C in a microwave A microwave oven uses microwave energy to heat food. Microwaves range from as long as one meter to as short as one millimeter. Due to the fact that they are a part of the electromagnetic spectrum we can use them to calculate the speed of light. Follow the instructions to get a calculation of the speed of light

1. Place fax paper on the foam plate.

2. Dampen fax paper.

3. Place damp fax paper into Microwave oven.

4. Turn Microwave on for 30-45 seconds.

5. Measure distance between hotspots on fax paper and multiply this number by 2

6. Multiply your wavelength (λ) by your frequency (ƒ) in Hertz (Hz) with the formula v = ƒλ. Show your working:

The actual value for the speed of light is 2.99 x 108 m/s. How close were you?


Cathode Ray Tubes

Changing the direction of Cathode Rays

There are two ways you can change the direction of a cathode ray

1. F = qE

An electric field can apply a force to a charged particle: where q is the charge of the particle and E is the strength of the electric field. The force is parallel to the electric field: for a positive charge the force is in the direction of the field, for a negative charge the force is in the opposite direction to the field. Label the diagram with positive and negative charges.

2. F = qvB sin

The force on a particle moving in a magnetic field is in a direction perpendicular to both the field and the direction of motion of the particle. where q is the charge of the particle, v is its velocity, B is the magnetic field strength and is the angle between the direction that the charge is moving and the direction of the magnetic field. The direction of the force is given by the right hand rule. Label the diagram with Force, Velocity, Path of charged particle and Magnetic field.

E field


Measurement of e/m

You can use the apparatus here to measure the ratio e/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. Measure the current and voltage from the front of the power supplies. You measure the electron path radius by seeing where the beam crosses the scale that sits behind the bulb.

The current in the coils is: I = ______________ (A) (This produces the magnetic field) The accelerating voltage is: V = ______________ (V) (This accelerates the electrons out into the bulb) The radius of the circular path is: r = ______________ (m) Use these measurements to calculate e/m in units of Coulombs/kg

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B = 7.80 ×10−4 I (Wb /m2)

Which measurement contributes the most uncertainty to this calculation? How have we adjusted for parallax error in this experiment?

Diameter


The Photoelectric Effect Albert Einstein won his Nobel Prize in Physics for his insight in to 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? Waves and particles?

Light definitely does behave like a wave: it diffracts, refracts and interferes, which are all wave properties. Light also seems to behave like a particle, like a lump: it can knock electrons off surfaces and transfer energy to other objects in packets of E = hf, just like a particle would.

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. When the electroscope is charged, shine dim, ‘white’ light source (the desk lamp on ‘low’) onto the metal plate. Switch the lamp to its brightest setting. Do you observe any change in the charge on the electroscope?

Predict Observe Explain

Photon

electron Current flows in circuit when light shines A


Now shine the UV lamp on the metal plate. What do you observe?

Measuring the photoelectric ‘stopping voltage’

In this experiment you again observe the photoelectric effect, but this time you’re going to measure the amount of energy the ejected electrons receive from the photons. energy E = hf. This means that 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)

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 do you do this? • The electrons had energy K = hf – W when they left the surface. • Electrons passing through the voltage V applied to the phototube will lose an amount of

energy equal to qv • At the stopping voltage, the electrons just don’t make it to the anode, they have lost all

their initial energy — so for this voltage, qV = K. • This means qV = hf – W, which we rewrite as V = (h/q) f – (W/q)

Predict Observe Explain


This is a linear equation, similar to y = mx + c. So you can make a graph of V vs. f. It should be a straight line, with a slope equal to h/q and a y-intercept equal to W/q.

Title ______________________________________________________


Conductors and Semiconductors 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.

Resistivity and heat

The diode Draw a diagram for a diode (a p-n junction) and label the two sides of the junction.

light bulb

battery

switch

Conductor or semiconductor


Light Emitting Diodes (LEDs) An LED schematic diagram shows how the semiconductor and p-n junction is used to produce light

Worlds smallest solar powered cars These little cars convert light energy directly into electric energy that powers the motor. We can use a bright light in this case because the band gap in silicon semiconductors is quite small, so we don’t need a huge amount of energy to run them.

VALENCE

CONDUCTION

Energy Band gap

BAND

BAND

P-Type N-Type

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++

++

++

Holes

- - - -

Electrons

E-M radiation emitted

+ -


Calculate the efficiency of the solar cells Let’s have a look at the efficiency of these little cells. We want to end up with an efficiency that compares energy in to energy out. For solar panels this is generally done with a measurement of Watts per Square meter or W/m2 What is a Watt? A Watt is a measurement of Power, or voltage x current. P=VI For the solar car, I = 40 mA V = __________ Volts, so P = ________________ Watts The efficiency of the cell is the Power/Area A = ________ m2, so Efficiency = __________W/m2

We can now compare this with the efficiency of the sun to arrive at percentage efficiency for the solar cells, or, how good the solar cell is at converting light into electricity. The Suns solar radiation is approximately 1350 W/m2

(Ecell/Esun) x 100 = ________ % How does this compare to the efficiency in a non-renewable power stations?

Image: http://www.mpoweruk.com/energy_efficiency.htm


Superconductors

Superconductors and zero resistance

From the Graph, what is the critical temperature of this YBCO Superconductor? ________________________°K


Risk Analysis A very important part of any experimentation in physics is the identification of any risks, and the subsequent mitigation of those risks. As you can imagine, scientists use some very cool, and technical pieces of apparatus and equipment. Liquid Nitrogen falls into this category.

Before we begin, identify three risks associated with liquid nitrogen. Once these risks have been identified, you must identify what might happen if the risks were to eventuate, and of course, what we can do to mitigate against the risks. These would be our safety rules. Some of the properties of Liquid Nitrogen may help you with identifying the risks. Assign a number to each of your risks using the table to the left.

Risk Consequence Precaution


Superconductors and the Meissner effect

Applications of Superconductivity

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N

S

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Current induced in the superconductor opposes magnetic field that induced it

Magnet is source of field that can induce a current in the superconductor


Implement your ideas If you listen closely throughout the workshop, The demonstrators might let some of this info slip, but if not, just ask them. What does your demonstrator do here at the School of Physics? or random fact from your demonstrator Can you list some safety measures in the lab? Find and list the 4 famous scientists in the lab. Careers with Ideas Your demonstrator might be able to help you with this

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

Industry

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Research

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Public service

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hsc kickstart physics workshop - university of...

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