if the ammeter used for an experiment cannot return completely to the zero mark when the circuit is off, whats the type of error it has introduced into the experiment​

The magnetic field inside a superconducting solenoid is 4.50 t. the solenoid has an inner diameter of 6.20 cm and a length of 26.0 cm. determine (a) the magnetic energy density in the field and (b) the energy stored in the magnetic field within the solenoid.

Ihave a density of 1.61g/cm^3 and a mass of 28g. find the missing value

What’s #16 asking me and how do i solve it?