# 2 problems conserning magnetic fields and units.

• Kruum
In summary, the conversation discusses a physics problem involving the calculation of kinetic energy, force, and magnetic field. The difficulty lies in converting units and determining the direction of the magnetic field. The conversation also delves into the use of resources during an exam and the need for a data sheet.
Kruum

## Homework Statement

http://www.aijaa.com/img/b/00125/3627010.jpg

http://www.aijaa.com/img/b/00758/3627011.jpg

## Homework Equations

$$E_k=\frac{1}{2}mv^2$$
$$\vec{F}=m\frac{\vec{v}^2}{r}$$
$\vec{F}=q\vec{v} \times \vec{B}$
$$\frac{v}{r}=\frac{2\pi}{T}=\omega$$

## The Attempt at a Solution

The only difficulty in problem 2 is the unit of the mass of the proton. I can't use any info that's not given in the problem or I don't know by heart. I know the mass of the proton is about 1u, but I don't know how to convert it to kg. First I solve $$E_k$$ for $$v=\sqrt{\frac{2E_k}{m}$$ and then by setting the forces equal I get $$r=\frac{mv}{qB}$$. Now here's where I need kg instead of u - or am I missing something.

The thing that gets me in number 5, is the way the magnetic field is given. If we set the Earth's surface along the xy-plane so that the positive y-axis points towards north and the positive x-axis towards east. Then $$\vec{B_h}=18\mu T\vec{j}$$. But is $$\vec{B_v}=-54\mu T\vec{j}$$? I think in that case they would have given the magnetic field as "$$B=36\mu T$$ to south". So they only option left is $$\vec{B_v}=-54\mu T\vec{k}$$. Which one is it and why? From here the problem is pretty straight forward: I just need to find the cross product of the vectors and calculate its length.

I'm sorry for the non-native English, but ask if you can't understand something I've written.

Edit: Some typos...

Last edited by a moderator:
The mass of a proton is 1.67 x 10^-27 kg.
You can find things like that in Wikipedia or in the back of a physics text.

Many people do not realize that there is an "S" pole up in northern Canada and an "N" in Antarctica! You can check - a compass that points north will also point toward the S end of a magnet. I expect you will keep the full vertical component, but multiply the horizontal component of B by cos(45) to get the part that is perpendicular to the wire.

Delphi51 said:
The mass of a proton is 1.67 x 10^-27 kg.
You can find things like that in Wikipedia or in the back of a physics text.

But that's the whole point, if this kind of a question is in an exam, I can't use Wikipedia or any other source of information, other than the question or my memory. So I guess these kind of things has to be known by heart.

I expect you will keep the full vertical component, but multiply the horizontal component of B by cos(45) to get the part that is perpendicular to the wire.

Why should B be perpendicular to the wire? If I can just figure out $$\vec{B_v}$$, I can create $$\vec{B}$$ by adding the vertical and horizontal components together. And then calculate the cross product $$\vec{l} \times \vec{B}$$

Many people do not realize that there is an "S" pole up in northern Canada and an "N" in Antarctica! You can check - a compass that points north will also point toward the S end of a magnet.

Thanks for the reminder, that made things clear! Since Earth has curvature, so does the magnetic field. One of the fields components then must be towards the center of the Earth and that makes the vertical component align with $$-\vec{k}$$.

Surely they put constants on the exam - e, mass of electron and proton, c, G, h, etc.
Most people have them in their calculators - do they clear calculators for exams?
Ideally, there would be a data sheet for the course that you use through the term and on the exam.

Delphi51 said:
Surely they put constants on the exam - e, mass of electron and proton, c, G, h, etc.

Well, I really hope that!

Most people have them in their calculators - do they clear calculators for exams?
Ideally, there would be a data sheet for the course that you use through the term and on the exam.

We aren't allowed to use calculators in exams. Only pens, eraser and rulers are allowed. Oh, and a slide rule, if you happen to have one.

## 1. What are magnetic fields and how are they created?

Magnetic fields are invisible areas of force surrounding magnets or electrically charged objects. They are created by moving electric charges, such as electrons, and are also produced by the Earth's core and other celestial bodies.

## 2. What are the units of measurement for magnetic fields?

The most commonly used unit for magnetic fields is the tesla (T). Other units include gauss (G) and the smaller unit of microtesla (μT). Some older units, such as the oersted (Oe), are still used in certain fields of study.

## 3. How are magnetic fields measured?

Magnetic fields can be measured using a device called a magnetometer. This instrument detects the strength and direction of the magnetic field, and can be used to create maps of magnetic fields in a given area.

## 4. What are some applications of magnetic fields?

Magnetic fields have many practical applications, such as in magnetic levitation trains, electric motors, and generators. They are also used in medical imaging, such as MRI machines, and in compasses for navigation.

## 5. Can exposure to magnetic fields be harmful to humans?

There is currently no evidence that exposure to the magnetic fields found in everyday life, such as those produced by household appliances, has any negative effects on human health. However, prolonged exposure to extremely strong magnetic fields, such as those found in some industrial settings, may cause health issues and should be avoided.

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