Magnetic Fields: Question 1

Worked solution

Step 1: Set up Fleming’s left-hand rule

Fleming’s left-hand rule applies to the motor effect, the force on a current-carrying conductor in a magnetic field. Hold the left hand so that the thumb, first finger and second finger are all mutually at right angles to one another:

• First finger → Field
• Second finger → Current
• Thumb → resulting force (Motion)

Step 2: Apply it to this wire

Here the field points into the page, so the first finger points away from you, into the page. The current flows from $X$ to $Y$, horizontally to the right, so the second finger points right.

With the first finger into the page and the second finger to the right, the thumb, held perpendicular to both, points vertically upward, in the plane of the page.

Step 3: Why the other options are wrong

• B (downward): this is the reverse of the correct thumb direction; it would result from mixing up which finger represents the field and which represents the current.
• C (out of the page): the force must be perpendicular to both the current (horizontal, in the plane of the page) and the field (into the page). The only directions perpendicular to both of these are vertically up or vertically down, in the plane of the page, not out of the page.
• D (into the page): the force cannot be parallel to the field itself; a force parallel to $B$ would require the current to be perpendicular to the page, which it is not here.

Step 4: Checking the magnitude (for completeness)

Although only the direction is asked for, the magnitude follows from $F=BIL\sin\theta$ with $\theta=90°$ (wire perpendicular to field): $F=0.45\times3.2\times0.25\times\sin90°=0.36\text{ N}$

Recomputing as a check: $0.45\times0.25=0.1125$, and $0.1125\times3.2=0.36\text{ N}$, the same result by a different grouping, confirming $F=0.36\text{ N}$.

Final answer

• The magnetic force on the wire is $\boxed{\text{vertically upward}}$, in the plane of the page (magnitude $0.36\text{ N}$), option A.