Virtual Bubble Chamber

Choose a preset (or set your own charge, mass and energy), switch the magnetic field on, and click Add particle to see the track it leaves.

Add an electron and a positron with the same energy. Compare the direction their tracks curve in. What does this tell you about the sign of a particle's charge?

Keeping the charge and energy fixed, add particles with different relative masses. How does the radius of curvature change as mass increases?

Select the photon preset with the field on. A photon itself leaves no track, but read the question that appears and use it to work out the minimum energy a photon needs to produce an electron-positron pair.

Use the radius of a track you have produced, together with the magnetic field strength and the particle's charge, to calculate its momentum. Enter your calculated radius in the check box and press Check to see how close you are.

Why does a photon leave no track at all, while the electron-positron pair it produces appears as two curved tracks starting from a single point? What does this reveal about conservation of charge and momentum at that point?