# Help with textbook question -- Where did the factor of 2 come from?

1. Apr 12, 2016

### Julia Coggins

• moved into h/w help, so template is missing
From "Physics for Scientists and Engineers" by Randall D. Knight page 1006. A 1 m long, 1.0 mm diameter nichrome heater wire is connected to a 12 V battery. Find the magnetic field strength 1 cm away from the wire. This problem is solved in the book, but they calculate the magnetic field with: μ/2π instead of μ/4π that is in the Biot-Savart Law. Everything else in my calculations was correct, so I'm wondering why they used this? The full equation they used is: B=μ/2π(I/d) related from the Biot Savart Law for a current carrying wire B ={μ/4π}{IΔs×r/r^2} the 'r' in the numerator is the direction of r not the magnitude.

2. Apr 12, 2016

### robphy

Did you see the preceding example (continued onto the top of page 1006)
where they used Biot-Savart to obtain the equation for B_wire?

3. Apr 13, 2016

### Julia Coggins

Reading it now. I don't understand how they evaluated that integral?

4. Apr 13, 2016

### robphy

5. Apr 14, 2016

### Julia Coggins

Unfortunately I don't have wolfram alpha pro, I do need to brush up on my calc though. Isn't L'hopitals rules dealing with indefinite integrals like this one?