Earth's Magnetic Field question

[Suy]

Homework Statement

A high voltage power line carries a current of 110 A of conventional current at a location where the Earth's magnetic field has a magnitude of 5.9*10^-5T points to the north, 72 degree below the horizontal. Find the direction and magnitude of the magnetic force exerted on a 250 m length of wire if the electron flow is (a) horizontally to the east and (b) horizontally to the south.

Homework Equations

The Attempt at a Solution

the Earth's magnetic field has a magnitude of 5.9*10^-5T points to the north, 72 degree below the horizontal.
i don't understand this statement,
any help would be appreciated

 

[Doc Al]

the Earth's magnetic field has a magnitude of 5.9*10^-5T points to the north, 72 degree below the horizontal.
i don't understand this statement,
any help would be appreciated

They're just trying to tell you the direction of the magnetic field, which you need to solve the problem. It's not purely horizontal; it points down at 72 degrees below the horizontal. (90 degrees would mean it points straight down.) Sketch a diagram or two for yourself.

 

[Suy]

it would be like this?
earth magnetic field go from south to north..
http://www.imageshosting.net/out.php/i15222_Untitled1.jpg

 

[willem2]

it would be like this?
earth magnetic field go from south to north..

It's more like this

http://www.physics.sjsu.edu/becker/physics51/images/28_03_Earth_magnetic_field.jpg"

note that close to the poles, the direction of the magnetic field is nearly vertical.

 

[Suy]

Which mean the direction of the magnetic field is pointing downward?
i don't get it..

 

[Doc Al]

it would be like this?
earth magnetic field go from south to north..
http://www.imageshosting.net/out.php/i15222_Untitled1.jpg

[/URL]
No, not exactly. Firstly, the power line goes east-west (for part a). Let's represent east with the x-axis, north with the y-axis, and up with the z-axis. The magnetic field has a positive y-component, a negative z-component, and zero x-component. The power line is purely along the x-axis.

 

[Suy]

http://img28.imageshack.us/img28/4373/43807604.jpg
like this? the red line represent the magnetic field line
going north and downward?
Is the magnetic field surround the Earth like a circle or in side the earth?
it is confusing...

 

[Suy]

Can anyone tell me why the magnetic field is downward?
http://www.physics.sjsu.edu/becker/physics51/images/28_03_Earth_magnetic_field.jpg
This picture is showing that it's go from north (south pole) to south (north pole) and the arrow direction is upward...
or the magnetic field line is in side the earth, south to north?

 

[Doc Al]

Do not get hung up on the general shape and direction of the Earth's magnetic field (at least as far as solving this problem goes). You are given the direction of the field at a specific point and the orientation of the current-carrying wire--use that information to find the force on the wire.

(Note that the Earth's magnetic field is similar to that of a bar magnet, as illustrated in that diagram.)