# Please help! Magnetic field physics question?

What speed would a proton need to achieve in order to circle the earth 1000.0 km above the magnetic equator? Assume that Earth's magnetic field is everywhere perpendicular to the path of the proton and that the Earth's magnetic field has an intensity of 4.00 x 10^ -8 T. (Hint: the magnetic force exterted on the proton is equal to the centripetal force, and the speed needed by the proton is its tangentical speed. Remember that the radius of the circular orbit should also include the radius of the earth. Ignore relativistic effects.)

### 2 Answers

- LoganLv 59 years agoFavorite Answer
You should know that centripedal force is F = mv^2/r (mass times centripedal acceleration a = v^2/r). For your problem, m is the mass of the proton, v is the proton's velocity (the unknown you need to solve for), and r is 10^6 m plus the Earth's radius.

But you need the left-side of the equation. How does magnetic force relate to a magnetic field strength? Use the Right-Hand Rule: make your index finger, middle finger, and thumb all perpendicular to each other. Your index finger is the direction of the magnetic field B. Your thumb is the direction of the proton's velocity v. And your middle finger is the direction of the magnetic force F. For your situation, where the charge is moving perpendicular to B, the equation for force is F = qvB. Thus

qvB = mv^2/r

Solve for v...

v = mv^2 / qBr

v / v^2 = m / qBr

1/v = m / qBr

v = qBr / m

You have

q = charge of a proton = 1.6 x 10^-19 C

B = 4 x 10^-8 T

r = r_earth + 10^6 m = 7.37 x 10^6 m

m = mass of a proton = 1.67 x 10^-27 kg

Plugging everything in...

v = (1.6 x 10^-19 C)(4 x 10^-8 T)(7.37 x 10^6 m) / (1.67 x 10^-27 kg)

v = 2.82 x 10^7 m/s

That's almost 9% of the speed of light!

- 3 years ago
When utilising Fb=qv move B, keep in mind q is poor for an electron. For realistic functions on this query, you'll be able to permit for this through treating v as whether it is within the -i path. There are distinctive regulations - for instance you'll be able to you Fleming's Left hand rule for this or a proper hand move-product rule But if A = B move C and you recognize A and (say) B, you'll be able to figure out C if you're cautious. (And you'll be able to regularly examine your reply through trying out that A and B then supply the recognized path of A.)