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## How do you find the magnitude and direction of the electric field and magnetic field?

Gió
Featured 6 months ago

Have a look if it's understandable (AND if it seems correct!).

#### Explanation:

Ok, this is complicated...!

I would try using Lorentz Law:

$\vec{F} = q \vec{E} + q \vec{v} \times \vec{B}$

with the vector product:

I think in both cases you have a proton (with charge $q = 1.6 \times {10}^{-} 19 C$)!

1) at rest: $\vec{v} = 0$, the proton (its charge) is experiencing the $E$ field around it and, as a consequence, the force:
$\vec{F} = q \vec{E}$
($\vec{B} = 0$)
using moduli (say, along the $x$ axis)
$E = \frac{F}{q} = \frac{8.5 \times {10}^{-} 13}{1.6 \times {10}^{-} 19} = 5.3 \times {10}^{6} \frac{V}{m}$
along the $x$ direction and zero magnetic field.

2) Use the same approach with the given value of velocity remembering that now you'll have a magnetic field $\vec{B}$ perpendicular (due to the vector product) to the force (in the $x$ direction) and velocity (in the $y$ direction) and so in the $z$ direction.

We need to consider the decrease in force so we need a negative value in the product: $q \vec{v} \times \vec{B}$ (probably the $\vec{B}$ field will be in the negative $z$ direction giving a negative modulus for the vector product).
We can write using our data into: $\vec{F} = q \vec{E} + q \vec{v} \times \vec{B}$ considering the moduli:

$7.5 \times {10}^{-} 13 = 8.5 \times {10}^{-} 13 + 1.6 \times {10}^{-} 19 \cdot 1.5 \times {10}^{6} \cdot B$
rearranging:
$B = - 0.416 \approx - 0.42 T$

Here it is a possible visual representation:

## What is the repeating universe theory?

George C.
Featured 4 months ago

See explanation for a few thoughts...

#### Explanation:

I think the term "repeating universe theory" could have several different interpretations.

Let us look at an couple of possibilities.

Suppose the nature of the universe is such that it will stop expanding and eventually experience a "big crunch".

Suppose further that such a "big crunch" will be automatically followed by another "big bang" with the same amount of matter/energy, etc. We could call that a "repeating universe theory", but perhaps there's something more...

If such a cycle is inevitable then there are some theories that we might attach to it:

(1) The theory that the following "big bang" will of necessity be identical to that which started this cycle, and will result in exactly the same sequence of events. Such a theory is almost certainly false.

(2) The theory that there are a large but finite number of possible initial configurations for a "big bang" in such a recurring cycle, so if an infinite number of "big bang" - "big crunch" cycles occur then at least one configuration will repeat. OK so far, but does the initial configuration determine all following events? - probably not.

(3) Like (2), but since the length of a "big bang" - "big crunch" cycle is finite, then if spacetime is quantised, there are a finite number of possible states through which the universe goes during a cycle. So there is a ridiculously huge but finite number of possible cycles. So in an infinite number of cycles, at least one possible sequence will repeat.

## How does light affect the universe?

Gió
Featured 4 months ago

I can think of two main effects: Energy and Pressure.

#### Explanation:

Light carries Photon energy proportional to its frequency $f$ in the form given by Einstein's relationship (with a constant $h$):

$E = h f$

This energy will produce effects when interacting, say, with the components of a celestial body (planet, comet, even dust) promoting chemical reactions between elements and the possible formation of new compounds.

Another interesting effect is the Radiation Pressure associated with the propagation of electromagnetic radiation; basically, light can...push...object!
You can see this effect observing the direction of the comets' tail when travelling near the Sun: the plume of particles forming the tail of the comet is pushed away by the Sun's radiation in a radial direction from the Sun!

This pressure will affect clouds of dust and gas (as a current in a river or lake), possibly promoting local condensation of matter to create nuclei for the future formation of stars (the small aggregates of matter will now have a sizeable gravitational force able to further attract matter around them).

Hope it helps; it is the best I could think of.

## How long would it take to travel to another galaxy?

George C.
Featured 3 months ago

It depends...

#### Explanation:

There are some small satellite galaxies of the Milky Way galaxy, but let's consider our largest neighbour in the Local Group, namely the Andromeda Galaxy.

This is situated approximately $2.5$ million light years away.

That means that from the perspective of the people who stay at home, a round trip would take at least $5$ million years.

Note however, that the traveller(s) could experience significantly less time, if they travel with sufficient speed.

For example, if they used a spacecraft that accelerated constantly with an acceleration of $1$g (i.e. $9.8 m {s}^{- 2}$) for about $14$ years (ship time), then decelerated at $1$g for about $14$ years, that would be sufficient to travel from our galaxy to the Andromeda Galaxy.

Returning home the same way, the traveller(s) would experience another $28$ years of time, but arrive home to find that $5$ million years had passed.

## Is it possible to open a black hole?

Phillip E.
Featured 3 months ago

It is not possible to open a black hole.

#### Explanation:

A black hole is a region of spacetime from which nothing, including light, can escape. So, we can't open a black hole in the sense of seeing what lies inside it.

To an external observer time stops at the event horizon and an object approaching it slows down and never appears to reach it. Light from the object gets red shifted and becomes invisible at visible wavelengths.

It is thought that an observer entering a black hole would pass the event horizon without realising it. Though gravitational tide effects would prove destructive.

Nothing can leave a black hole and we can't see inside it. Our laws of physics as we know them can't describe a black hole.

Smaller black holes can emit Hawking Radiation due to one partner of a virtual particle-antiparticle pair entering a black hole and the other partner escapes. This can lead to a black hole evaporating.

Maybe in the future we will discover new physics which will change our understanding of black holes.

## According to Kepler's First Law, planets move in elliptical orbits. Why is that considered accelerated motion? According to Newton, what is the force causing that acceleration?

Parzival S.
Featured 1 month ago

See below:

#### Explanation:

Let's start by talking about Newton's First Law of Motion:

An object at rest will stay at rest unless acted upon by an unbalanced force. An object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

http://teachertech.rice.edu/Participants/louviere/Newton/law1.html

So let's look at the Moon as it orbits around the Earth.

So there's the Moon moving around the Earth (the black dot with the two arrows). Newton's First Law, if there was no other forces acting on the Moon, would follow the Forward Motion arrow - it would fly off happily, in a straight line at a constant speed, forever.

But it doesn't. Why is that?

Because there is another force acting on the Moon - which is the Pull of Gravity arrow. Earth's gravity, if there were no other forces acting on the Moon, would have it plunging down onto (and into) the Earth - resulting in the biggest collision the world has ever experienced.

Thankfully, the balance of the forces, the Moon's inertia and Earth's gravity, act on the Moon to keep it in orbit (one of my professors described it as "the Moon is continually falling towards the Earth and missing").

And so the interaction of the two forces creates accelerated movement - Earth's gravity constantly pulls on the Moon and that is the source of the acceleration (that constant change in direction).

So now to the Kepler portion of the question - it is true that orbits are slightly elliptical and not purely circular. In the case of the Moon, this ellipse is quite elongated compared to the orbit of the Earth revolving around the Sun. However, it is not the elliptical quality of the orbit that makes the orbiting motion accelerated motion.

https://www.ifa.hawaii.edu/~barnes/ASTR110L_S03/lunarorbit.html

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