A positive charge with very small charge compared to the charge source of the field
Let's consider an electric field produced by a source charge Q.
In order to test the direction and magnitude of the electric field, we use a positive test charge q. The reason for chosing a positive charge is that a positive charge will move following the directions of the electric field lines (in fact, the positive charge will be repelled from charge Q if charge Q is positive, so it will follow the electric field lines, which point away from the charge).
Moreover, the test charge q must be small compared to charge Q. This is because we don't want to modify significantly the configuration of the electric field, so the test charge q must be negligible compared to charge Q.
answer: the correct answer is 1.
explanation: there are various process in which a radioactive nuclei decays:
1) alpha decay: in this process, a heavier nuclei decays into lighter nuclei by releasing alpha particle. the mass number is reduced by 4 units.
2) electron capture: in this decay process, a parent nuclei absorbs an electron and gets converted into a neutron. simply, a proton and an electron combines together to form a neutron. mass number does not change in this process.
3) gamma ray emission: it is a decay process in which an unstable nuclei gives excess energy by a spontaneous electromagnetic process. this decay releases . this process does not change the mass number.
4) positron emission: it is a type of decay process, in which a proton gets converted to neutron and an electron neutrino. this is also known as -decay. in this the mass number remains same.
from the above information, we see that only alpha-decay is producing a change in the mass number of the nucleus.