hsc kickstart physics workshop - university of .hsc kickstart physics workshop ... light bulb...


Post on 24-Jul-2018




0 download

Embed Size (px)






    1. Hertzs 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 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

    Hertzs Experiments with Radio Waves Heinrich Hertz (18571894) 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 1/10th 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.

  • The University of Sydney School of Physics Ideas to Implementation

    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 (). 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 () by your frequency () in Hertz (Hz) with the formula v = . Show your working:

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

  • The University of Sydney School of Physics Ideas to Implementation

    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

  • The University of Sydney School of Physics Ideas to Implementation

    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

    B = 7.80 104 I (Wb /m2)

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


  • The University of Sydney School of Physics Ideas to Implementation

    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


    electron Current flows in circuit when light shines A

  • The University of Sydney School of Physics Ideas to Implementation

    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 youre 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 phototubes cathode, and to find a value for Plancks 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 dont 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

  • The University of Sydney School of Physics Ideas to Implementation

    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 ______________________________________________________

  • The University of Sydney School of Physics Ideas to Implementation

    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



    Conductor or semiconductor

  • The University of Sydney School of Physics Ideas to Implementation

    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 dont need a huge amount of energy to run them.



    Energy Band gap



    P-Type N-Type






    - - - - Electrons

    E-M radiation emitted

    + -

  • The University of Sydney School of Physics Ideas to Implementation

    Calculate the efficiency of the solar cells Lets 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 com