In an electron microscope, a cathode emits a beam of electrons into a vacuum, which are then focused onto a sample that is to be “viewed.” Therefore, (A) the electron beam constitutes a current pointing out of the cathode. (B) the electron beam constitutes a current pointing into the cathode. (C) the electron beam is charge in motion but is not a current. (D) there is no current or charge in this example.
The correct answer and explanation is:
The correct answer is (B) the electron beam constitutes a current pointing into the cathode.
In an electron microscope, the electron beam is generated by a cathode that emits electrons due to thermal emission or field emission. These electrons are then accelerated towards the anode, which focuses the electron beam onto the sample. Electrons, being negatively charged particles, move from the cathode (negative electrode) to the anode (positive electrode), creating a current in the process.
A current is defined as the flow of charge, and in this case, the flow of electrons constitutes the current. Since electrons are negative, their movement is opposite to the direction of the conventional current. The conventional current is considered to flow from positive to negative, while the electrons, being negative, actually flow from the cathode (negative) towards the anode (positive). Therefore, the electron beam moves into the cathode, and the direction of conventional current would be into the cathode, as it reflects the flow of positive charge.
This current can be seen as a flow of charge carriers (electrons) within the vacuum, and its movement is indeed an electric current. However, since the electron beam is composed of negatively charged particles, the actual electron flow would be opposite to the conventional current, making the direction of current point into the cathode.
In short, the electron beam is a stream of moving charge, and in the context of conventional current, the beam constitutes a current flowing into the cathode.