Difference between revisions of "PhET/C3/Photoelectric-Effect/English"

Title slide

Learning objectives

How to use Interactive PhET simulation, Photoelectric Effect.

Pre-requisites

Learners should be familiar with topics in high school science.

System Requirements

Ubuntu Linux OS version 14.04

Java version 1.7.0

Firefox Web Browser version 53.02.2.

Learning Goals

Study Photoelectric Effect.

Determine Threshold frequency.

Find Stopping potential and Work function.

Study the factors affecting current and energy of electrons.

Photoelectric Effect

Ejected electrons are counted by a detector that measures their kinetic energy.

Link for PhET simulation

http://phet.colorado.edu

http://phet.colorado.edu

Press Ctrl+Alt+T keys.

Type cd Downloads and press enter.

Type java space hyphen jar photoelectric_en.jar and press enter.

Point to interface.

At the prompt type cd Downloads and press enter.

Then type, java space hyphen jar space photoelectric_en.jar and press enter.

Photoelectric Effect simulation opens.

Point to the terminal.

Click Cancel.

Click Cancel to continue.

Click on Options menu.

Point to Show photons and

Control photon number instead of intensity.

File, Options and Help.

Options menu has two options,

Show photons and

Control photon number instead of intensity.

Point to Intensity and wavelength texboxes.

We can also input Intensity and wavelength values in their respective boxes.

point to metal surface.

point to Detector.

A metal surface and a detector to measure kinetic energy of electrons.

Point to voltage slider.

Battery is provided with a Voltage slider.

Point to Sodium Target.

By default Sodium is selected as Target metal.

Click on Show photons option.

Point to the photons.

Uncheck Show photons option.

In Options menu click on Show photons option.

Observe that light is shined in the form of photons.

Uncheck Show photons option.

Point to electrons.

Point to detector.

Electrons are ejected as soon as light falls on Sodium metal surface.

There is no time lag between incident radiation and electrons emission.

These electrons flow towards a detector.

Current Vs battery voltage

Current Vs light intensity

Electron energy Vs light frequency.

Point to graph.

Point to the red dot.

We see a graph of current vs battery voltage.

Notice a red dot on the graph.

Point to Graph.

Point to red line.

Point to electrons.

Notice that current remains constant as we increase the voltage.

This is indicated by the red line.

As we increase the voltage, speed of photoelectrons increases.

Drag the Intensity slider upto 90%.

Click on Current vs light intensity check box.

Drag the Intensity slider upto 90%.

Point to green line.

This is indicated by the green line.

Point to graphs.

Point to blue line

Point to current value.

Observe the graphs.

Notice that energy increases linearly with the increase in Frequency.

This is indicated by the blue line.

Observe the change in current.

Increase in frequency, increases the energy of photoelectrons.

This results in increase in the kinetic energy of ejected electrons.

Point to snapshot window.

Point to Graphs.

Click on close button.

A snapshot window opens.

It gives information about Graphs and Experimental Parameters.

Using this snapshot, we can compare graphs with different settings.

Close the snapshot window.

Threshold frequency

This frequency is, Threshold Frequency, denoted by ʋ₀.

Below the Threshold Frequency, Photoelectric Effect is not observed.

Observe the wavelength at which electron ejection stops.

point to Current meter.

point to electrons.

Point to 539 nm.

Point to Current meter.

At 539 nm electrons start ejecting from sodium metal surface.

It means that, 539 nm is threshold wavelength for Sodium.

Here, value of current is 0.00.

Threshold frequency calculation for sodium.

539 nm = 539 x 10^-9 m.

Frequency can be calculated using following formula.

c= ʋλ, ʋ = c/λ

ʋ = (3 x 10^8m/s)/ (539 x 10^-9m)

ʋ= 0.56 x 10¹⁵Hz.

I will convert it to metres by multiplying with 10^-9.

Threshold frequency can be calculated using following formula.

Threshold frequency of sodium is 0.56 x 10¹⁵ Hz.

Click on drop down arrow and select Platinum.

Drag the slider to UV region.

Drag the slider to UV region untill electron ejection starts.

Point to the ejected electrons.

Notice that a large number of electrons with different energies are ejected.

Uncheck the box.

Uncheck the box to show electrons with different energies.

Assignment

Calculate the Threshold Frequency of Platinum.

Work Function

Different metals have different values of work function.

It is denoted by ϕ₀.

Work function

Elements with low Ionization enthalpy values have low work function.

Example: Li, Na, K, Rb, and Cs.

Work function for Sodium

For Sodium,

Ʋ_{0 =} 0.56 x 10¹⁵ hZ which we have calculated earlier.

w₀ = hʋ₀

w₀ = 6.626 x 10^-34x0.56x10¹⁵

w₀ = 6.626 x 0.56 x 10^-19

w₀ = 3.711 x 10^{-19 J}

w₀ =3.711 x 10^-19 / 1.60218 x 10^-19

w_{0 =2.31 eV}

Work function is calculated by using following formula.

w_{0 =} hʋ₀

Work function for Sodium is 2.31eV.(electron volts)

Work function for Calcium

Formula for work function is w_{0 =} hʋ₀

For Calcium,

Ʋ₀ =0.703 x 10¹⁵ hZ

w₀ = hʋ₀

w₀ = 6.626 x 10^-34 x 0.703 x 10¹⁵

w₀ = 6.626 x 0.703 x 10^-19

w₀ = 4.66 x 10^-19/1.6021 X 10^-19

wo=2.9 eV

Stopping potential

It is a negative voltage required to stop electrons from reaching the other side.

At Stopping Potential, photoelectric current becomes zero.

Drag wavelength slider to 539 nm.

Change Sodium as target metal.

Drag the wavelength slider to threshold wavelength of sodium.

That is 539 nm.

Point to electrons.

Point to -0.04 V and electrons.

At which voltage, electrons will bounce off from detector?

At -0.01 V(volts), electrons start to bounce off from detector.

Observe At -0.04 V, no electrons are ejected from sodium.

Assignment

Calculate the work function for Zinc, Copper and Calcium.

Determine the stopping potential for the same metals.

Summary

In this tutorial we have learnt about,

Photoelectric Effect, PhET simulation.

Summary

1. About Photoelectric Effect.
2. To determine Threshold Frequency.
3. To find Stopping Potential and Work Function and
4. Study the factors affecting current and energy of electrons.

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