 These processes that we have postulated can in fact occur, and are known as beta decays. To change potassium-40 into calcium-40 requires a neutron changing into a proton. To conserve electric charge, some other particle must be emitted, and in this case an electron is emitted. Although we won't discuss it in detail here, it turns out that another particle, an electron antinutrino, is also emitted. You will learn more about this when you study particle physics. The total process involves a neutron turning into a proton, plus an electron and an electron antinutrino. The proton remains in the nucleus, so that in this case the potassium-40 turns into calcium-40, while the electron and the antinutrino are emitted. We call the emitted particles the decay radiation. The emitted electron is called a beta-minus particle, and this process is a beta-minus decay. The electron and antinutrino have mass, hence they have energy, using equals mc2, and they also carry kinetic energy. In this way the total energy is conserved, despite the mass energy of calcium-40 being less than the mass energy of potassium-40. The change from scandium-40 to calcium-40 occurs by an analogous process, in this case changing a proton into a neutron, an antinelectron, and an electron-nutrino. An antinelectron is usually called a positron, and in the context of this nuclear decay it is also called a beta-plus particle, and this is a beta-plus decay. Similar to the ball on the hill, a force must be involved to drive these physical processes. In fact, beta-minus and beta-plus decays are a consequence of the weak nuclear force.