A. When the current increases.
B. We assume that removing the core will reduce the permeability, thus causing the field strength to be reduced.
C. Increasing the size of the wire could have an effect either way, depending on the way the coil is wound and what you mean by wire size.
If you increase the cross section, and keep the same number of turns, the number of ampere-turns could increase with the same applied voltage, thus increasing the magnetic field. If increasing the cross-section decreases the number of turns, the total number of ampere turns may not increase by that action.
If you increase the wire gauge, thus reducing its cross section, you could wind more turns. If the applied current stays the same, you have more ampere-turns and thus increased field strength.
There is an optimum number of turns for a given core dimension and wire insulation thickness. Depending on where you are in relation to that optimum, changes could increase or decrease field strength.
D. See the discussion of answer C. The effect could go either way.
Dropping the ambient temperature will generally reduce the resistance of the wire, so increase the current and therefore field strength. Again, the effect depends on what you hold constant. If you hold applied current constant, decreasing the temperature may or may not have an effect, depending on the temperature coefficient of permeability.
B. We assume that removing the core will reduce the permeability, thus causing the field strength to be reduced.
C. Increasing the size of the wire could have an effect either way, depending on the way the coil is wound and what you mean by wire size.
If you increase the cross section, and keep the same number of turns, the number of ampere-turns could increase with the same applied voltage, thus increasing the magnetic field. If increasing the cross-section decreases the number of turns, the total number of ampere turns may not increase by that action.
If you increase the wire gauge, thus reducing its cross section, you could wind more turns. If the applied current stays the same, you have more ampere-turns and thus increased field strength.
There is an optimum number of turns for a given core dimension and wire insulation thickness. Depending on where you are in relation to that optimum, changes could increase or decrease field strength.
D. See the discussion of answer C. The effect could go either way.
Dropping the ambient temperature will generally reduce the resistance of the wire, so increase the current and therefore field strength. Again, the effect depends on what you hold constant. If you hold applied current constant, decreasing the temperature may or may not have an effect, depending on the temperature coefficient of permeability.
