Question

the strength of an electromagnet is primarily proportional to its _______
a. length
b. diameter
c. ampere-turns
d. core resistance

Explanation:

Identify the governing principles

Using the Electromagnetism knowledge point
The magnetic field strength \(B\) of a solenoid electromagnet is directly proportional to the magnetomotive force (MMF) driving the magnetic flux through the magnetic circuit.

Relate magnetomotive force to physical quantities

Using the Ampere-turns and Magnetomotive Force knowledge points

$$ \text{MMF} = N \cdot I $$

where \(N\) is the number of turns of wire and \(I\) is the electric current in amperes. This product is measured in ampere-turns, which directly determines the primary strength of the electromagnet.

Evaluate the given multiple-choice options

Using the Electromagnetism knowledge point

• Option (a) length: Increasing length generally decreases field strength for a fixed number of turns.
• Option (b) diameter: Diameter affects coil volume but is not the primary proportional factor.
• Option (c) ampere-turns: Directly proportional to the magnetomotive force and field strength.
• Option (d) core resistance: Resistance limits current but is not a direct proportional factor for magnetic strength.

Answer:

• a. length
• b. diameter
• c. ampere-turns (Correct answer)
• d. core resistance