AAMC FL 4R Question (Cathode/Anode/Electron Flow)

[MedPR]

Which of the following best describes the movement of the electron after being ejected from the cathode?

A. it is stationary
B. it moves with constant speed towards anode
C. it accelerates towards anode
D. It exits the side of the vacuum seal

Which of the following occurs when electrons are ejected from the cathode?

A. voltage across the electrodes reverses polarity
B. voltage difference increases
C. current flows through the circuit
D. total resistance increases.

Both explanations basically say that the electrons move towards the anode because that is the way electron flow ("the definition of current" according to the explanations) is. In other words; according to the answer explanations. electrons flow from cathode to anode AND "current" is electron flow, not proton "flow".

---

Members don't see this ad.

 

[SaintJude]

I don't think the problem has as much to do with the flow of an electron as envisioned in a galvanic/electrolytic cell.

Rather, I think this deals with the electric field setup by the voltage gradient in a circuit. The two electrodes act like a parallel plate capacitor. They give you the positive and negative sign next to the voltmeter to show you that the external electric field within the circuit is moving from right to left! The force due to an electric field on an electron points in the opposite direction of the electric field. So the electron will move in the opposite direction of the electric field lines--towards the anode.

 

[MedPR]

I don't think the problem has as much to do with the flow of an electron as envisioned in a galvanic/electrolytic cell.

Rather, I think this deals with the electric field setup by the voltage gradient in a circuit. The two electrodes act like a parallel plate capacitor. They give you the positive and negative sign next to the voltmeter to show you that the external electric field within the circuit is moving from right to left! The force due to an electric field on an electron points in the opposite direction of the electric field. So the electron will move in the opposite direction of the electric field lines--towards the anode.

Oh I see. So you're saying that current will flow from the battery to the right and pass through the resistor before getting to the vacuum seal, right? This is the conventional way that current (protons) flow.. From cathode (the longer line, or positive end of the battery) through the circuit to the anode (shorter line, or negative end of the battery). So protons hit the resistor before they hit the vacuum sealed region of the circuit.

The thing labeled "anode" in the picture basically builds up a positive charge because the protons can't really pass through the air-space between the "anode" and "cathode". So an electric field is setup pointing from "anode" to "cathode." Now, when an electron is ejected from the "cathode" (by a photon of light, or whatever) it is immediately attracted to the "anode" because of the huge amount of protons built up on the "anode." If that is correct, that makes sense to me.

However, what completes the circuit? What allows current to flow through the vacuum and get back to the battery?

 

[davcro]

However, what completes the circuit? What allows current to flow through the vacuum and get back to the battery?

The two parallel plates are a fully charged capacitor, so no current flows in the circuit. The capacitor loses charge when an electron is displaced from the cathode. To compensate electrons flow from the battery to the cathode until the charge is replenished.

 

[davcro]

One of the confusing points of this problem is it deals with the movement of electrons in an electric field, not in a circuit. In a circuit, electrons flow anode to cathode. However in an electric field, electrons flow cathode to anode. This is because the cathode plate has a negative charge and the anode has a positive charge. This is super important because gel electrophoresis deals with the movement of ions in an electric field. In a gel, a positive ion will be attracted to the cathode.