# Homework Help: Magnetic Force of sodium ions

1. Sep 23, 2009

### balling12

1. The problem statement, all variables and given/known data
Sodium ions (Na+) move at 0.844 m/s through a bloodstream in the arm of a person standing near a large magnet. The magnetic field has a strength of 0.259 T and makes an angle of 59.0° with the motion of the sodium ions. The arm contains 95.0 cm^3 of blood with a concentration of 2.50 X 10^20 Na+ ions per cubic centimeter. If no other ions were present in the arm, what would be the magnetic force on the arm?

2. Relevant equations
F=qvBsin(theta)

3. The attempt at a solution
F= q(.844m/s)(.259T)(sin59)
Thats as far as I have gotten. I am guessing that I need to use the volume of blood and concentration of ions to find the charge but I am not sure how to do so and have looked all through my text book for help.

2. Sep 23, 2009

### kuruman

If there are 2.5x1020 ions in 1 cm3, can you find how many ions are in 95 cm3?

If the force on one ion is (as you say) F= q(.844m/s)(.259T)(sin59), can you the total force on all the ions in the 95 cm3?

3. Sep 24, 2009

### balling12

do i just multiply 95 by 2.5X10^20?

4. Sep 24, 2009

### kuruman

What do you think?

5. Sep 24, 2009

### balling12

well i tried that and once i worked it out the solution was wrong so im guessing that was incorrect.

6. Sep 24, 2009

### kuruman

Multiplying these two numbers is the correct thing to do. Whether you carried out the actual calculation correctly by pushing the right buttons on your calculator is another issue. If you show your calculation and the numbers you got, maybe someone will be able to figure out where you went wrong.

7. Sep 24, 2009

### balling12

ok well i multplied the two numbers and got: 2.375X10^22. however once i plugged this into the equation to deteremine the magnetic force i got 4.45X10^21 which is incorrect so im still unsure where i went wrong?

8. Sep 24, 2009

### noblegas

dq=density*volume; That also helps

9. Sep 24, 2009

### kuruman

You are not showing your calculation, just the result. Show how you multiplied, added, divided these numbers.

10. Sep 24, 2009

### balling12

(95)(2.5X10^20)=2.375X10^22

(2.375X10^22)(.844)(.259)(sin59)= 4.4X10^21

11. Sep 24, 2009

### kuruman

You need to redo your calculation. First off, the calculation as you show it above does not give 4.4x1021. Secondly, you did not include the charge on the ion.

12. Sep 24, 2009

### balling12

well i re-did my calculation and every way i work it i still get that same answer. and i am not sure what you mean by calculating the charge on the ion.

13. Sep 25, 2009

### kuruman

Look at the equation you posted:

F= q(.844m/s)(.259T)(sin59)

What does q stand for? Answer: The charge on the ion. Uncharged particles experience no force in a magnetic field whether they are moving or not.

14. Sep 25, 2009

### balling12

So how do I find q?

15. Sep 25, 2009

### kuruman

What is the charge on a singly ionized atom (Na+)?

16. Sep 26, 2009

### balling12

im clueless

17. Sep 26, 2009

### kuruman

18. Sep 26, 2009

### Bill Foster

The total number of atoms/particles/whatever times the charge of each one.

The charge of an electron is http://lmgtfy.com/?q=charge+of+electron

A "singly ionized atom" means that a single electron has been lost. A sodium atom has 11 electrons. A singly ionized sodium atom has 10 electrons. Since it has 11 protons, it has a net charge of +e, where e is given by the charge of the electron in the link above.

19. Sep 26, 2009

### willem2

q is in coulombs. How many Na+ ions are there in 1 coulomb?

20. Sep 26, 2009

### kuruman

6.25x1018.

21. Sep 26, 2009

### balling12

so what do i do with the 6.25X10^18?

22. Sep 26, 2009

### kuruman

** Edit **

That's the answer to how many sodium ions in 1 coulomb. You can use that number or you can use what Bill Foster suggested in post #18

Last edited: Sep 26, 2009
23. Sep 27, 2009

### balling12

so do i need to multiply 1.6X10^-19 by 6.25X10^18 to get the charge(q)?

24. Sep 27, 2009

### kuruman

A. 1.6x10-19 Coulombs is the charge on one electron.

B. 2.375x1022 is the number Na+ ions you have in 95 cm3

C. q is the total charge on all the sodium atoms you have, it is measured in Coulombs.

D. Do what Bill Foster suggested.