A straight wire 15cm long, carrying a current of 6.0A is in a uniform field of 0.40T. What is the force on the wire when it is at right angle to the field

1. Analyze the question

We are given a current-carrying wire placed in a uniform magnetic field. We need to calculate the magnetic force acting on the wire.

• Length ($L$): 15 cm
• Current ($I$): 6.0 A
• Magnetic Flux Density ($B$): 0.40 T
• Angle ($\theta$): 90° (since it is at a "right angle" to the field)

2. Explain the concept

The force ($F$) on a current-carrying conductor in a magnetic field is given by the formula:

Where:

• $B$ is the magnetic flux density (measured in Tesla, T).
• $I$ is the current (measured in Amperes, A).
• $L$ is the length of the wire (must be in meters, m).
• $\theta$ is the angle between the wire and the magnetic field.

Important: Always ensure your length is in SI units (meters) before calculating.

3. Show the solution

Step 1: Convert units The length is given in centimeters. We must convert it to meters:

Step 2: Apply the formula Since $\theta = 90^\circ$, we know that $\sin(90^\circ) = 1$.

Step 3: Calculate

4. Identify the answer

Comparing our result to the given options: A. 0.46N B. 0.35N C. 0.36N D. 0.24N

The calculated force is 0.36N, which matches option C.

5. Exam Tip

In JAMB Physics, always watch out for unit conversions! Many students lose marks because they calculate using centimeters instead of converting to meters. A quick way to remember the formula is "BIL-sin-theta." Also, remember that $\sin(90^\circ)$ is 1, which is the maximum force possible for a wire in a magnetic field.