Use Ampere's law to find the magnetic field inside a long ideal solenoid carrying a current i. The solenoid has n turns per unit length. $ B . d ( = 4 , J B. L = Figure 29-20 Application of Ampere's law to B= MO N . 1 a section of a long ideal solenoid carrying a current i. The Amperian loop is the rec- Ni = number of tunen tangle abcda. loop SB.d L = S Bal cosho- 0 By dL = BL to to to . P length Figure 29-19 Magnetic field lines for a real solenoid of finite length, The field is strong and uniform at interior points such as P but relatively weak at external points such as Py