An electromagnetic actuator moves a lightweight gate in a parcel-sorting machine. A copper bar attached to the gate carries a current through the gap of a permanent magnet.
The only part of the bar inside the magnetic field is 0.15 m long and is at right angles to the field.
During a current pulse, the current in the bar is 3.0 A and the magnetic flux density is 0.40 T0.40\ \text{T}0.40 T.
Calculate the magnetic force exerted on the moving bar during the current pulse.
Use the equation
F=B×I×lF = B \times I \times lF=B×I×l
The gate, bar and other moving parts have a total mass of 0.030 kg.
Calculate their weight.
Use g=10 N/kgg = 10\ \text{N/kg}g=10 N/kg.
Calculate the ratio of the magnetic force to the weight of the moving parts, expressing your answer as a percentage.
The magnetic force initially pushes the gate to the left.
An engineer reverses the current through the bar and also turns the permanent magnet around so that the magnetic field reverses.
Explain why the magnetic force still pushes the gate to the left.