Question

the motor is used to lift metal boxes. how can the motor be changed to be able to lift a heavier box?
a. decrease the size of the magnets
b. rotate the coil in a counter - clockwise direction
c. add more loops of wire between the magnets
d. change the polarity of the magnet

Explanation:

Step1: Recall motor - principle

The force exerted on a current - carrying conductor in a magnetic field is given by $F = BILN$ (where $B$ is magnetic field strength, $I$ is current, $L$ is length of conductor in the field, and $N$ is number of loops). To lift a heavier box, we need to increase the force.

Step2: Analyze each option

• Option A: Decreasing the size of the magnets will likely decrease the magnetic field strength $B$, reducing the force, so it's incorrect.
• Option B: Changing the direction of rotation of the coil only changes the direction of rotation of the motor, not the force magnitude, so it's incorrect.
• Option C: Adding more loops of wire ($N$ increases) between the magnets will increase the force $F$ according to the formula $F = BILN$, which helps in lifting a heavier box. This is correct.
• Option D: Changing the polarity of the magnet only changes the direction of the magnetic force, not its magnitude, so it's incorrect.

Answer:

C. add more loops of wire between the magnets