Electromagnetic forces of a liquid.

[isaacknewtonnes]

Hi All,

 

During a recent university interview i was asked :

 

You are walking with a cup of tea and have to abruptly stop, state which direct your tea would spill in and tell us why?

 

To which i replied :

 

Newtons first law states that an object will remain stationary or in uniform motion until a resultant force is acted upon it. With this is mind whilst walking with the cup full of tea it is in fact you that is in motion not the tea. There fore it will remain stationary until a resultant force is acted upon. As you come to an abrupt stop there is a resultant force present, and due to the molecular arrangement of the cup the cup its self will almost instantaneously stop, the liquid how ever will not.

 

The reasoning for the liquid being displaced more than cup, is this do to the electromagnetic force being much weaker as electrons are not shared ?

 

Thanks

[studiot]

You do not need electromagnetism, witchcraft or alchemy to explain this.

 

Tea, in a cup, is a liquid and liquids possess a property not available to any appreciable extent to solids.

 

A liquid converts a force exerted on it by its container into internal pressure within the liquid.

 

Take a tube of toothpaste or ketchup, point it upwards and squeeze it.

The force of your fingers is converted to increase pressure within the tube and this causes the liquid paste or ketchup to rise vertically.

 

In your teacup, when you stop the front side of the cup exerts a horizontal force on the liquid nearest to it.

This increase the pressure in that part of the tea.

The increased pressure causes the tea to rise in that part of the cup and slop over the edge.

 

You can create more complicated explanations involving momentum, but the explanation is still mechanical in nature.

[isaacknewtonnes]

You do not need electromagnetism, witchcraft or alchemy to explain this.

 

And if you were to explain through the use "witchcraft and alchemy" (aka Newtonian physics) ?

[Strange]

 

 

 

There seem to be two completely separate issues in your post. Firstly, Newton's laws of motion, to which you seem to have given an accurate, if overly complex, answer.

 

Then ...

 

The reasoning for the liquid being displaced more than cup, is this do to the electromagnetic force being much weaker as electrons are not shared ?

 

Is this just asking why some things are liquid and others are solid?

 

Ultimately, this does come down to electromagnetic forces, and how strong they are between the molecules of the material.

 

The reasons for this are fairly complicated. It is not just a matter (I don't think) of whether electrons are shared (a covalent bond) or not (an ionic bond). Although most ionic molecules tend to have higher melting points and so are usually solid at room temperature.

Edited April 14, 2016 by Strange

[isaacknewtonnes]

 

There seem to be two completely separate issues in your post. Firstly, Newton's laws of motion, to which you seem to have given an accurate, if overly complex, answer.

 

Then ...

 

 

Is this just asking why some things are liquid and others are solid?

Apologies for the confusion.

 

The question being why does the coffee spill out of a mug when given a sudden resultant force?

 

My initial thoughts generally speaking was down to the covalent bonding in the cup allowing for minimal movement of the cup, where as the lack of bonding in the liquid allows for more movement of the liquid. The bonds also link to the EM forces exerted.

 

But yet you say the reasoning is further than this ?

[Strange]

I would say there is less reasoning than that required!

 

Newton's first law (i.e. inertia) says that things will continue in their state of motion. You only need to say that the cup is solid and the tea is liquid, and therefore free to move, to explain why the tea continues moving but the cup (which you, hopefully, have a form grip on) doesn't.

 

Yes, electromagnetic forces are the reason for things being solid or liquid, but not directly relevant to the question asked.

 

I'm afraid I would mark someone down in a test (or a job interview) if they went into too much irrelevant detail. On the other hand, I would like them to be able to give that detail if asked.

[studiot]

If you stop the cup moving forwards how can the tea continue moving, except upwards?

 

The tea is tightly packed into the cup.

Would a tightly packed solid do the same?

Edited April 14, 2016 by studiot

[Strange]

If you stop the cup moving forwards how can the tea continue moving, except upwards?

 

Excellent point. Kudos for pointing that out. You've got the job!

[DrP]

Upwards? Depends on the angle of the sides of the cup... up and out the top and forwards, unless the sides are exactly vertical. They are usually sloped though.

[John Cuthber]

If I had been the interviewer, I'd have been more impressed if you told me that you carry your tea with somethinglike this so it doesn't spill so easily.

http://www.qwerkity.com/aegean-tray-prod13063/

[studiot]

 

And if you were to explain through the use "witchcraft and alchemy" (aka Newtonian physics) ?

 

 

If you really want witchcraft and alchemy, how about this:

 

There is a non zero, although vanishingly small, probability that the tea will quantum tunnel through the cup wall and keep going.

 

However I would point out that every time I go scuba diving I bet my life on the quantum probability that the air in my tank will not quantum tunnel out of my air tank.

I also rely on my dive computer, which uses quantum tunneling to operate in the sub atomic realm, not to run out of air in the conventional manner.

 

Edited April 14, 2016 by studiot