High School Physics : Understanding the Relationship with Current

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #51 : Electricity And Magnetism

A current of \(\displaystyle 8.5*10^8A\) runs through a straight wire. If the resulting magnetic field has a radius of \(\displaystyle 1.2m\), how strong is the magnetic field?

\(\displaystyle \small \mu_0=4\pi*10^{-7}\frac{T\cdot m}{A}\)

Possible Answers:

\(\displaystyle 0.01T\)

\(\displaystyle 141.67T\)

\(\displaystyle 340T\)

\(\displaystyle 30.22T\)

\(\displaystyle 337.6T\)

Correct answer:

\(\displaystyle 141.67T\)

Explanation:

Ampere's law states: 

\(\displaystyle B=\frac{\mu_0 I}{2\pi r}\).

In other words, the magnetic field (\(\displaystyle B\)), is equal to a constant (\(\displaystyle \mu_0\)) times the current (\(\displaystyle I\)) divided by the circumference of the magnetic field it is creating.

We are given the current, the constant, and the radius. Using these values, we can solve for the magnetic field strength.

\(\displaystyle B=\frac{\mu_0 I}{2\pi r}\)

\(\displaystyle B=\frac{(4\pi*10^{-7}\frac{T\cdot m}{A})( 8.5*10^8A)}{2\pi 1.2m}\)

Notice that the \(\displaystyle \pi\) cancels out.

\(\displaystyle B=\frac{(4*10^{-7}\frac{T\cdot m}{A}) (8.5*10^8A)}{2* 1.2m}\)

\(\displaystyle B=\frac{340{T\cdot m}}{2.4m}\)

\(\displaystyle B=141.67T\)

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