# Transcript

[Text on screen: Sam Eden, Physicist, The Open University]
SAM EDEN
In this activity, we are going to carry out experiments with the aim of answering two main questions. Firstly, does Coulomb’s law give you the correct electrostatic force that a charged sphere feels due to one other charged sphere? And secondly, does adding vector components give you the correct electrostatic force that a charged sphere feels due to two other charged spheres?
After you’ve watched the experiments, you will analyse the results and compare them with your own calculations using vector components. Now, let’s start with a quick tour of the experiment.
[Text on screen: Anita Dawes, Physicist, The Open University]
ANITA DAWES
OK, so the experiment involves measuring one vector component of the force that a charged sphere feels after we have positioned another charged sphere, or two charged spheres, nearby. So here’s one of the spheres. So it’s a hollow conducting sphere mounted on an insulating plastic rod.
And what we’ve done here is prepared a Perspex system to help us place our spheres in precise positions above a grid. So on this grid, each square is two centimetres by two centimetres. Now, here’s our sphere that we used as a test charge. It’s mounted on an arm with a pressure sensor that’s very sensitive. We can measure the force that it feels between the charges in this direction, perpendicular to the arm.
[In a top-down view of the experiment, sphere A is positioned to the left of the test charge sphere. An arrow labelled ‘Fx’ pointing to the right, away from the test charge, is superimposed on the screen.]
Each time we’re ready to take a measurement of the force, this sensor is connected to this unit here, and we can save a series of values at 20-millisecond intervals. Now let’s talk about the system that we use for charging up the spheres.
SAM EDEN
So this is a power supply with a pin that you can touch onto the spheres to transfer a charge onto them. We’ve put the power supply quite far from the rest of the experiment as part of our effort to minimise external electric fields. The power supply’s been calibrated, so we know that a voltage of 17 kilovolts transfers 30 nanocoulombs onto a sphere. So now we’re ready to start the experiments.
ANITA DAWES
Here’s our first arrangement of the test charge sphere and the other sphere, which we’ll call sphere A. The centres of the spheres are ten centimetres away from each other.
[A distance marker labelled ‘10.0 cm’ is superimposed on a top-down view of the experiment, and is positioned between the test charge sphere and sphere A.]
So first, we’ll record the force on the test charge sphere before we put any charge on the spheres. So I’ll take the readings now.
And you should see the last six force readings on your screen that have been saved by this readout. The readings in millinewtons are minus 0.05, minus 0.05, minus 0.07, minus 0.05, minus 0.07 and minus 0.06. You’ll be able to access all the data from these experiments after you’ve watched this video.
So what we’ll do next is put 30 nanocoulombs of charge on the test charge and on sphere A. Now I’ll record the readings, and you should see them appear on your screen. The values in millinewtons are 0.62, 0.63, 0.65, 0.64, 0.68 and 0.69.
Now we’ll ground these two spheres. So I’ll use an earth rod and touch them to take the charge away. And we’ll add a third sphere into the geometry. And I’ll place it into this position here.
[In a top-down view of the experiment, sphere B is positioned above the test charge sphere. The lines from the test charge sphere to spheres A and B are both labelled as ‘10 cm’ and are at right angles to each other.]
Once again, we’ll check the force without any charge on it. The values in millinewtons are minus 0.08, minus 0.09, minus 0.11, minus 0.12, minus 0.11 and minus 0.10.
And we’ll now put 30 nanocoulombs of charge on all three of the spheres. Once again, you will be able to see the values on the screen. The values in millinewtons are 0.65, 0.64, 0.64, 0.65, 0.67 and 0.68.
Don’t forget that we’re only measuring one component of the force on the test charge. So now we’ll ground the three spheres again. And I will move sphere B into a new position, here.
[In a top-down view of the experiment, sphere B is positioned above sphere A. The lines from sphere A to the test charge sphere and sphere B are both labelled as ‘10 cm’ and are at right angles to each other.]
And now once again, we’ll check the force without any charge applied to the spheres. So I’ll make the measurement.
The values in millinewtons are minus 0.04, minus 0.05, minus 0.06, minus 0.05, minus 0.07 and minus 0.07. And now we’ll put 30 nanocoulombs on all three spheres. We’ll take another measurement. The values in millinewtons are 0.98, 0.99, 0.99, 0.99, 0.98 and 1.00. And that completes our measurements.
SAM EDEN
You will find the data that we’ve just recorded in the next part of the activity. You will then do your own calculations to test if Coulomb’s law combined with vector addition predicts these experimental results with good accuracy.