How much more thumb pressure must a nurse use to administer an injection with a hypodermic needle of inside diameter 0.30 mm compared to one with inside diameter 0.60 mm. Assume that the two needles have the same length and that the volume flow rate is the same in both cases?
- 16 times as much
- Twice as much
- 8 times as much
- None of these
Explanation
Flow of liquid through a needle depends a lot on the needle’s radius. Very small changes make a big difference.
The rule is: pressure needed is inversely proportional to the fourth power of the radius.
Needle 1: radius = 0.15 mm (0.30 mm diameter)
Needle 2: radius = 0.30 mm (0.60 mm diameter)
Compare the pressures:
Pressure ratio = (0.30/0.15)⁴ = 2⁴ = 16
So the nurse must press 16 times harder for the smaller needle to get the same flow.
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ریڈیو کس نے ایجاد کیا؟
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