Question

An X-ray tube operates at a potential of 100kV and 10mA tube current. If only 1% of the electrical power is converted into X-ray, the rate at which the target heated is:
A) 99W
B) 990W
C) 100W
D) 1000W

Answer 1

As the question mentioned only 1% of the electrical power is converted into X-ray. So, 99% is used to heat up. First, to find total power from that we have to calculate heating rate or heating power, and Power is measured in watt.
Formula used:
$Power = VI$

Complete answer:
Focus on the line given in the question is only 1% of the electrical power is converted into X-ray. The remaining 99% is used to heat up. So, we have to find the rate of heating power.
We know that formula $Power = VI$
V is the potential or voltage.
I is the current.
First, we have to write given values in the question.
V = 100kV, I = 10mA
These values substitute in the mentioned formula.
$\Rightarrow Power = 100kV \times 10mA$
In the above equation, we have to put k and m values.
$\Rightarrow Power = 100 \times {10^3} \times 10 \times {10^{ - 3}}$ W
Here we have to remember 1% of power is converted into X-rays, the remaining 99% goes for heating. So, we have to find a heating rate which equals 99% of 1000 watts.
Hence we can write
Heating rate ( H ) = 99% of 1000 W
$\Rightarrow H = \dfrac{{99}}{{100}} \times 1000$
$\therefore H = 990$W
Hence the heating rate is 990 W.

So, the correct answer is option B.

Additional information:
Power is nothing but the rate at which work is done. The unit of work done is the watt, which is equal to joule per sec.

Note:
In these types of questions sometimes they may ask the heating rate in the unit of cal/sec. There we have to remember some conversation terms and then finally we chose the correct answer. In this question, we have to find 99% of 1000W not 1% of this, so be careful while reading the question.