Electrons can be “boiled off” of a wire by heating the wire to sufficiently high temperature (typically well over 1000? C). Typically the electrons leave the wire with a fairly low speed which can be approximated as zero. In many devices where this is done the hot wire is also maintained at some voltage which causes the electrons to accelerate away from the wire.
Suppose electrons are boiled off of a wire that is held at an unknown voltage. The electrons accelerate away and eventually hit a metal plate which is grounded. By convention we define the grounded plate as having V = 0.
(a) Is the voltage at the hot wire positive or negative? Explain.
(b) There are various ways that we could measure the speed of the electrons as they impact the grounded plate. Suppose we determine that the electrons have a speed of 2.96 × 107 m/s just before hitting the plate. What is the voltage at the hot wire?