Which lead has a higher potential in this electromagnetic flowmeter calculation?

[matt85]

An electromagnetic flowmeter is used to measure blood flow rates during surgery. Blood containing Na+ ions flows due south through an artery with a diameter of 0.40 cm. The artery is in a downward magnetic field of 0.25 T and develops a Hall voltage of 0.35 mV across its diameter. (a) What is the blood speed (in m/s)? (b) What is the flow rate (in m3/s)? (c) The leads of a voltmeter are attached to diametrically opposed points on the artery to measure the Hall voltage. Which of the two leads is at the higher potential?

1. (a) 0.35 m/s; (b) 4.4 times 10-6 m3/s; (c) the east lead
2. (a) 0.43 m/s; (b) 4.4 times 10-6 m3/s; (c) the east lead
3. (a) 0.43 m/s; (b) 5.7 times 10-6 m3/s; (c) the south lead
4. (a) 0.35 m/s; (b) 4.4 times 10-6 m3/s; (c) the south lead
5. (a) 0.43 m/s; (b) 4.4 times 10-6 m3/s; (c) the south lead

I have worked through the problem, and figured out that it is .43 m/s and the flow rate is 4.4E-6 m3/s. I am not sure how to figure out which lead is at the higher potential though. When I apply the right hand rule, it seems like I get a force to the west? So, which is at a higher potential, the east or south lead?

Thanks,
Matt

 

[matt85]

Oops, I meant to say 0.35 m/s.

 

[Chi Meson]

When I apply the right hand rule, it seems like I get a force to the west?

Try again. Are you using the correct right hand?

So, which is at a higher potential, the east or south lead?

On which side will you find the positive ions?

 

[matt85]

To the south?

 

[Chi Meson]

Which way is the force?

 

[matt85]

Force is to the east?

 

[Chi Meson]

Well I guess that's the last choice, but yes. RHR says fingers down, thumb south, palm faces east, that's where the positive ions are, that's where the voltage is high.