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Alpha emission is the least penetrating, easily be blocked by a few cm of air, skin or thin paper.
Beta emission can be blocked by a few cm of aluminium.
Gamma emission can be blocked by a few cm of lead or metres of concrete.
Refer to the video to understand how to apply the concepts.
With first glance, we may think of using kinematics to solve. It can, but the process will be longer. Refer to the video towards the end.
For such questions, it will be much easier to use conservation of energy (COE).
Besides the usually equating KE to GPE, we must not forget that we can also equate KE to WD. All are energy.
Emf is the work done (energy needed) to carry a unit charge (1C) through the whole complete circuit.
By definition, it is with respect to a unit charge (1C).
In general, it can also mean what is the emf when a certain amount of energy is carry certain amount of charge round the complete circuit.
Using V = E/Q, as long as the V is 5V, it is correct.
Note that 1C is NOT 1 electrons. The amount of charge on an electron is about 1.6 x 10^-19 C. So there are many electrons in 1 C.