Answer:
0.0321 g
Explanation:
Let helium specific heat 
Assuming no energy is lost in the process, by the law of energy conservation we can state that the 20J work done is from the heat transfer to heat it up from 273K to 393K, which is a difference of ΔT = 393 - 273 = 120 K. We have the following heat transfer equation:

where
is the mass of helium, which we are looking for:

Answer:
t = 4.21x10⁻⁷ s
Explanation:
The time (t) can be found using the angular velocity (ω):
<em>Where θ: is the angular displacement = π (since it moves halfway through a complete circle)</em>
We have:
<u>Where</u>:
<em>v: is the tangential speed </em>
<em>r: is the radius</em>
The radius can be found equaling the magnetic force with the centripetal force:

Where:
m: is the mass of the alpha particle = 6.64x10⁻²⁷ kg
q: is the charge of the alpha particle = 2*p (proton) = 2*1.6x10⁻¹⁹C
B: is the magnetic field = 0.155 T
Hence, the time is:

Therefore, the time that takes for an alpha particle to move halfway through a complete circle is 4.21x10⁻⁷ s.
I hope it helps you!
Explanation:
potential energy = mass × gravity force × height
=20 × 10 × 25
= 5000 joules