Two parallel conductors are 0.3m long, and 0.15m apart. They each carry 2.5A of current in the same direction. Calculate the force between them.
I did (2 * 10^-7) (2.5^2) / (0.15) and got the wrong answer. The correct answer is 2.5 * 10^-6.
I used amperes force law.
Ampere's force law gives the force per unit length between parallel conductors as follows:
$$F = 2 k \frac {I^2} {r}$$
where $k = 10^{-7}$ newtons/ampere², $I$ is the current flowing in the two conductors, and $r$ is the distance between them. Although you inserted the correct values into this formula, you're still calculating a force per unit length. Multiply the result by the length of the conductors to obtain a force:
$$2 \times 10^{-7} \times \frac { 2.5^2 } { 0.15 } \times 0.3 = 2.5 \times 10^{-6}$$
(Having said that, the question is slightly flawed. Ampere's equation is only valid when the length of the conductors is much greater than the distance between them.)