Rutherford’s Gold Foil Experiment E-practical

A realistic 3D virtual physics experiment

This experiment shows that the structure of an atom is consistent with having a small, positively charged nucleus by reproducing Rutherford's famous investigation.

Realistic experimental technique

E-practicals are 3d, highly realistic emulations based on a blend of theory and the actual practicals, giving an authentic experience of doing the actual practical in a laboratory. Students operate the apparatus, make measurements and obtain their own results. It is designed to reproduce the experience of performing the practical rather than simply demonstrating the underlying physics. Like a real experiment, it can be done badly. Careless setup or measurements can produce poor results, so students have to apply experimental technique and take care with their measurements.

Useful when laboratory access is limited

Ideal for preparation before a practical, revision afterwards, missed practicals, home study, or demonstrating an experiment when setting up the real apparatus isn't practical.

Works directly in a browser

No installation is required. Windows, Mac and suitable mobile devices can run the WebGL e-practical.

Watch it in action

This shows how the e-practical can be used. In particular it shows how the student or teacher interacts with the apparatus and how the interaction can be controlled using the mouse or the alternative on-screen controls.

What you get

The application is supplied as a PowerPoint which provides background, a synopsis of what the student will learn, a description of the Physics involved, a description of the apparatus, the independent and dependent variables, the method, a link to the interactive online e-practical, analysis of the results and further discussion.

School/Department licence £9.99

Valid for 1 year

Introduction

This experiment is one of the most famous as it revolutionised our ideas of the atom. Before Hans Geiger and Ernest Marsden performed this experiment under the direction of Rutherford the atom was known to be made of positively and negatively charged particles but the structure was not known. It was generally accepted to be like J. J. Thomson’s suggestion of a plum pudding with the positive and negative charges like the fruit in the pudding being interspersed throughout the pudding. It was J. J. Thompson who had discovered the electron in 1897.

This is known as Thompson’s plum pudding model. There was some logic behind this. The charges of the same sign could hardly clump together because of the very strong electrical force that repels charges of the same sign.

Ernest Rutherford’s experiment (1911) was very simple in its construction, if rather harder to perform. It consisted of a source of alpha particles aimed at a thin gold foil. A screen made of zinc sulphide which scintillated when a particle hit it. That is, it created a small light flash. The experimenters used a microscope with a screen attached as a viewer.

An alpha particle source in a lead box with a small hole creates a narrow beam of particles which are aimed at the thin gold foil in the centre of the apparatus. The apparatus (except for the detector) is kept within an evacuated chamber to prevent the alpha particles hitting molecules in the air.

The viewer consisting of the microscope and zinc sulphide screen can be placed anywhere in a circle around the apparatus to view the scintillations. The experimenters performed their observations in the dark after a considerable time taken for their eyes to adjust. They then had to count the scintillations which brought its own difficulties – there’s an upper limit to just how fast you can count. So, the apparatus had to be reconfigured for different angles to make the counting possible.

Rutherford deduced from the experiment that the atom had a positively charged centre that contains most of the mass of the atom, this he called the nucleus. Alpha particles are positively charged so must be repelled by a concentration of positive charges, i.e. the proposed nucleus. As relatively few alpha particles were deflected by the gold foil and very few indeed were bounced back, Rutherford deduced that the nucleus must be very small indeed.

What students do

For angles of 1 degree to ten degrees count the number of hits on the detector.
You need to reset the detector and stop the counting between each set.
You can start, stop and reset the counter using the controls on the counter.
Place the mouse over the button (which will change colour to yellow) and click with the left mouse button. The red wheel under the table supporting the evacuated dome turns the detector. To turn it place the mouse over the red wheel (the colour will change to yellow) and use the mouse wheel.
The base of the dome has a scale from which the angle can be read. You may need to zoom in/out using the +/- keys on the number pad to get a clear reading.
Create a table that has a column for the angle, a column for the count, a column for half the angle, a column for this angle in radians (if your sin calculation is going to need radians) a column for sine of halt the angle, a column for cosec of half the angle, and finally one for the cosec raised to the power of 4. You may find it convenient to use a spreadsheet program.
Create a row for the angle 1 through 10. Using the apparatus fill in the Count values for each angle. Then fill in the remaining columns for each row.
To show the relationship we need to plot the second column against the last and verify that this is a straight line. Unfortunately, because the range of data is so large this is hard to do. Create a new table which has columns for the logs as shown on the below.
Now we can plot these and verify that what we get is a straight line which shows that Rutherford’s relationship between the count and the fourth power of the cosec of half the angle is correct.

Want to experience an e-practical yourself?

Try our free Magnetic Field for a Coil e-practical.

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This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.