Re: [Phys-l] x-ray machine

[John Denker <jsd@av8n.com>]

On 07/08/2009 05:20 AM, Anthony Lapinski wrote:
> I saw a new dentist a few weeks ago. I could easily see the x-ray from the
> chair, and wrote down some information. I often use "real"
> devices/appliances in my classes when discussing electricity, Ohm's Law,
> etc.
>
> x-ray machine:
> 60 kV
> 7 mA
>
> Using Ohm's Law, we get:
>
> R = (60,000 V)/(0.007 A) = 8.57 x 106 ohms
>
> I thought this result was interesting -- either very high or Ohm's Law
> does not apply well in this case. Any ideas?

First of all, a few megohms is not what I would call "very high".  
My own personal resistor supply contains drawers full of such 
critters.  If you don't need a lot of power handling, they're a 
standard article of commerce, and dirt cheap.

Before the days of FET-input op-amps, it was harder to come up
with applications for multi-megohm resistors, but those days
were 25 years ago.

I've designed and built circuits that contained _1000 megohm_
resistors.  That's somewhat beyond the normal range.  For one
thing, you need to be somewhat careful about handling the 
resistor, because fingerprints are conductive and might short 
out the resistor.  It's not a super-big deal;  you just build
the circuit and /then/ clean off the fingerprints with alcohol.


Secondly, there is not the slightest reason to expect the physics
of an Xray tube to be ohmic.  I would expect the current to be an
exponential function of voltage, and also an exponential function
of cathode temperature.

The "7 ma" number is presumably some sort of maximum rated 
current, not the actual current.  In normal standard tubes, the
actual current is not closely controlled.  Neither the voltage
nor the current is steady.

There are /some/ tubes that use a regulated (electronic) voltage
supply and then regulate the current using a grid, but I'm not
sure how widely deployed they are.