# What's the magnetic field?

1. Jul 9, 2009

### SimonZ

1. The problem statement, all variables and given/known data
A straight wire carrying a current of 42 A lies along the axis of a 6.6 cm-diameter solenoid. The solenoid is 70 cm long and has 250 turns carrying a current of 6.0 A.
Estimate the magnitude of the magnetic field 4.2 cm from the wire.

2. Relevant equations
magnetic field due to a straight current I
B = mu_0*I/(2*pi*r)

3. The attempt at a solution
Note 4.2 cm > radius 3.3 cm, so the point is outside the solenoid, the magnetic field is only due to the straight current
use I = 42 A, r = 4.2 cm, get B = 0.20 mT
Anything wrong?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 9, 2009

### rock.freak667

The solenoid carries a current, so it has a magnetic field. For a solenoid

$$B=\mu_0nI \ where \ n=\frac{No. \ of \ turns}{Length \ of \ solenoid}$$

So you'd need to find the resultant mag. field of the solenoid and the straight conductor.

3. Jul 9, 2009

### SimonZ

B = μo(N/l) * I = μonI
is only valid for magnetic field INSIDE the solenoid.
The field OUTSIDE the solenoid is zero.
So the field is only due to the straight current