Conceptual Physics

What is the acceleration of the

electron? Its mass is 9.1×10í^31

kg.

a = F/m

m = me = 9.1×10í^31 kg

a = (í3.2×10í^25 N)/(9.1×10í^31 kg)

a = í3.5×10^5 m/s^2 (opposite to field)

23.9 - Interactive checkpoint: work on a charge

A uniform electric field of strength

3.00×10^5 N/C is oriented in the íx

direction. How much work is done by

the field to move an electron 2.60 m

in the +x direction?

Answer:

W = J

23.10 - Physics at work: spacecraft powered by electric fields

In 1998, NASA began testing in space a new type of rocket drive: the ion propulsion system. A conceptual diagram (not drawn to scale) of this drive is shown in Concept 1. Traditional rocket engines use chemical reactions to expel rocket fuel exhaust from the engine at high speed. If you have seen video footage of the rockets that carried astronauts to the Moon, you have seen a traditional rocket engine. In contrast, the ion drive uses an electric field to accelerate charged gas particles, or ions. It accelerates these particles to extremely high velocities, velocities much greater than those achieved by chemical rocket exhausts. Why are higher velocities of interest? Recall the law of conservation of momentum. In this case, the “forward” momentum of the rest of the rocket must match the “backward” momentum of the rocket exhaust. Momentum is the product of mass and velocity. Since the ion drive accelerates particles to greater velocities than those of chemical drives, less fuel mass is necessary than with chemical systems to produce the same amount of momentum. Less massive space probes are important because they require less energy to launch into space. In practice, a space probe is still launched by a powerful chemical rocket engine, but once it is relatively free of the Earth’s gravity, the probe relies on a less massive ion propulsion system to maneuver. (Ion systems supply less than one newton of force, a small fraction of the force required to launch a rocket from the Earth’s surface.) The system’s lesser mass also means it accelerates more for a given force. The first step in the operation of this engine system is to bombard xenon gas with electrons. These fast electrons “knock off” an electron from each xenon atom, turning it into a positive ion. The ions are then inserted into an electric field created by two charged plates. The plates are less than a meter in diameter and are placed close together. The electric field between the plates accelerates the ions to a velocity of about

NASA's Deep Space 1 ion drive undergoing testing at the Jet Propulsion Laboratory.