Physics Question about Magnetic Force & parallel wires

[tnbstudent]

I have a homework question that I am having some trouble with. This is my first post - so be easy on me.

Question:
A weight-lifter is able to hold a barbell with a maximum total mass of 205 kg above his head. A physicist uses him to conduct a classroom demonstration, asking him to hold one insulated conducting rod (without any weights on it) above his head, while a second, very long rod is placed under his feet, parallel to the first rod. The rod that he is "lifting" has a length of 2.00 m and, together with its electrical leads, a mass of 10.0 kg. The rods are separated by a distance of 2.15 m. What is the maximum current that can be passed through the rods, in the same direction, before he is forced to yield? The same amount of current flows through both rods.

I started by using the basic force equation: F=m*a and then the formula for Force of parallel wires: F=μI1I2/2∏r

My answer:
I tried to set the force of what the lifter could lift equal to the force of the wire.
I got 2009N as the force he could lift (2009N=205kg*9.8m/s^2)
Then I found the force of the wire to be 98N (98N=10kg*9.8m/s^2)
Then I subtracted the 98 from 2009 to get 1911N as the force created by the current in the wire
Since I1 and I2 are the same I used 2I and my equation looks like:
1911N = (4∏X10^-7)*(2I)*(2.0m)/(2∏*2.15m)
When I solve I get I=5.14 e ^9

 

[gneill]

I have a homework question that I am having some trouble with. This is my first post - so be easy on me.

Question:
A weight-lifter is able to hold a barbell with a maximum total mass of 205 kg above his head. A physicist uses him to conduct a classroom demonstration, asking him to hold one insulated conducting rod (without any weights on it) above his head, while a second, very long rod is placed under his feet, parallel to the first rod. The rod that he is "lifting" has a length of 2.00 m and, together with its electrical leads, a mass of 10.0 kg. The rods are separated by a distance of 2.15 m. What is the maximum current that can be passed through the rods, in the same direction, before he is forced to yield? The same amount of current flows through both rods.

I started by using the basic force equation: F=m*a and then the formula for Force of parallel wires: F=μI1I2/2∏r

My answer:
I tried to set the force of what the lifter could lift equal to the force of the wire.
I got 2009N as the force he could lift (2009N=205kg*9.8m/s^2)
Then I found the force of the wire to be 98N (98N=10kg*9.8m/s^2)
Then I subtracted the 98 from 2009 to get 1911N as the force created by the current in the wire
Since I1 and I2 are the same I used 2I and my equation looks like:
1911N = (4∏X10^-7)*(2I)*(2.0m)/(2∏*2.15m)
When I solve I get I=5.14 e ^9

Your expression: μI1I2/2∏r gives the force per unit length of the parallel wires, not the total force. Check the units --- they should work out to N/m.

Also, since when is I*I = 2*I?

 

[tnbstudent]

Thanks - I had a typo. I actually have my formula as
F=μ*I1*I2*l/2∏r where l is the length of the wire
Once, I clear up my brain lapse and use I^2 instead of 2I I get 101,349A.
I only have one try at this one, so I'd like to make sure this is right before I submit it

Thanks again for the help

 

[gneill]

Thanks - I had a typo. I actually have my formula as
F=μ*I1*I2*l/2∏r where l is the length of the wire
Once, I clear up my brain lapse and use I^2 instead of 2I I get 101,349A.
I only have one try at this one, so I'd like to make sure this is right before I submit it

Thanks again for the help

That result looks good. You might want to consider expressing your answer using the appropriate significant figures.

 

[asteriatic]

which is a very large number


Hello and welcome to the community! It's great to see you taking an interest in physics and seeking help with your homework question. I can see that you have made a good attempt at solving the problem using the force equation and the formula for magnetic force between parallel wires. However, there are a few things to consider in your approach.

Firstly, the maximum current that can be passed through the rods before the lifter is forced to yield is not directly related to the weight he can lift. The force he can lift is dependent on his own strength and the force of gravity, while the maximum current is dependent on the magnetic force between the two parallel wires.

Secondly, when using the formula for magnetic force between parallel wires, it is important to use the distance between the wires, not the length of the wire itself. In this case, the distance between the wires is 2.15m, not 2.0m.

Lastly, when solving for the current, it is important to consider the units. The units for magnetic force are Newtons (N), while the units for current are Amperes (A). Therefore, your final answer should have units of Amperes, not just a number.

I would recommend reviewing the formula for magnetic force between parallel wires and making sure to use the correct values for distance and units in your calculation. I hope this helps and good luck with your homework!