Metallic Bonding

zn bondImagine metal atoms arranged like chocolates in a box, in neat rows. I cut up a piece of lithium the other day and it was bright and shiny, a usual indicator of neat atomic rows. The electrons are all tightly bound, as usual in their shells with the exception of the outermost ones – perhaps one, two or three having no parent atom, which then becomes an ion. The electrons are free to wander about in the metal lattice or scaffolding, occasionally getting captured for a while and looked after by a metal ion, only to break free and change places. These electrons are sometimes called ‘delocalised’. This ‘sea’ of free electrons is what makes metals good conductors of heat and electricity, helps to explain why they are ductile and malleable – meaning that they can be drawn out into wire like copper or beaten into shapes like gold.

The atoms in metals have a strong attractive force between them and a lot of energy is required to overcome it. Therefore, metals often have high boiling points, for example tungsten used to make filaments in lamps, symbol W melts at 5828 K, extremely high.

You can find out a bit more here

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Rutherford Scattering

In about 1900, this is how we thought atoms might look…plumpud2J J Thomson called this the ‘plum pudding model’.

democDemocritus, the Greek philosopher, is credited with inventing the word ‘atomos’

– Greek for ‘indivisible’. A solid ‘blob’ which couldn’t be cut in half.

Ernest Rutherford was surprised. So surprised, he said about the results that his longsuffering assistants had taken two years to gather..

‘It was as if a fifteen inch howitzer shell had been fired at a piece of tissue paper and had rebounded straight back’.

He had been firing alpha particles at very thin metal films. The positively charged particles would be deflected, being repelled by positively charged atomic nuclei, how much depending on how closely they passed near to them.

Poor Geiger and Marsden, his assistants, had been sitting in the dark for months on end, counting the flashes caused by the interaction of the alpha particles on a phosphor screen which surrounded the evacuated apparatus, thereby giving away their position.

When the results were analysed, they were remarkable. Gone for ever was Thomson’s ‘plum pudding’ model, where the negative bits were scattered randomly within a positively charged atomic ‘soup’; instead, only about one in 8000 alphas were deflected through a significantly large angle, suggesting close approach was as rare as hen’s teeth. Put simply, most of the atom seemed to be empty space.  This full stop to represent the nucleus in the middle of an atom-sized  cinema is about the right proportion.

Firing 10MeV alphas directly at a gold nucleus transfers all their kinetic energy into electric potential energy.

alphascat2

Calculating r gives a value for about 3×10-14m- the approximate nuclear radius

Here’s the simulation link page Fire a few alphas at the gold…

Another famous quote from ER:

‘There is only physics. Everything else is stamp collecting.’

A bit arrogant perhaps. But, this is the guy who really discovered protons (Gk ‘protos’=first)

Easy Electrical Stuff *updated* with new images

If it were possible to connect a voltmetervoltm across the ends of a battery,without any charge passing through either the voltmeter or the battery (it isn’t but nearly), the voltmeter would measure the EMF (electromotive force) of the battery. The voltmeter shown is an analogue voltmeter, where the data measured is continuous, unlike a digital one which samples 0’s and 1’s.

Connecting the battery to a circuit pushes charges round the circuit. Their flow rate in coulombs per second is the current in amps (1A = 1C/s). The ammeter is a charge flowmeter and is connected in series with the components.

The resistance in ohms of a component is the p.d in volts across it divided by the current in amps through it. Resistances add together round a series loop.

It takes energy to drive charge through a component like a light bulb. Easy to see because the light bulb is giving out energy in the form of heat and light which it gets from the battery. The difference in energy between two points in the circuit when 1C of charge flows between them is the potential difference between the two points, measured in volts. So 1V = 1J/C . The voltmeter is an energy comparer so it is connected across a component, in parallel.

If 1A=1C/s and 1V = 1J/C, then volts x amps is measured in J/s or Watts, the rate of energy use of the component. So, a 40W light bulb transfers 40J of electrical energy every second as heat and light.

Imagine a long, thin pipe filled with water. It’s hard to drive water down the pipe. Making the pipe shorter reduces its resistance to flow. It’s like this with current in a wire. If a wire is short, it will have a lower resistance than a long wire. Twice the resistance for twice the length. If the wire is fatter its resistance decreases. Doubling the cross-sectional area, the ‘fatness’, halves the resistance.

When two identical resistors are connected in parallel, the available current is split in half.

res

The one single resistor we could use to replace the pair is HALF the value of either.

This is a picture of a big, old wire wound variable resistor. The longer the wire, the greater the resistance . The red slider is moved  along to include longer or shorter wire in the circuit – more or less resistance.

Electricity For Dummies

shovel.gif

Now, boys and girls. I’m going to tell you a story. Are you sitting comfortably?  Then I’ll begin.

Inside a cell (the thingy you use inside the TV remote to make it go – if there’s two of them then it’s called a battery – )  there’s a guy with one of these.  You don’t believe me? I promise you, it’s true!

Or, if you like – have a look here.  This is an interactive simulation so you can play with it

His job is to keep the + and – charges inside separate He’s a charge shoveller, in other words It takes a lot of energy to keep the charges separate.  It’s written on the cell exactly how much – 1.5V, or 1.5 joules for each coulomb, the EMF of the cell. If there’s a wire connecting the + and -, like a back door, the – charges get to sneak  out through the back door and round to get to the + charges, using up energy as they go.

Mr Coulomb is a backpacker who often visits the cell.

backpacker_cartoon

Here he is, having just passed through the cell, which is his energy shop to fill up his backpack with energy.

He strides off round the circuit. At first, he’s just walking on a nice flat level road, which doesn’t use up much energy. Then he gets to one of these.lightbulbFor him it’s like climbing a very steep hill and he has to use almost all the energy in his backpack to get through the bulb. Meanwhile, the bulb cheerfully converts his electrical energy into heat and light. empty backpackerPoor old Mr Coulomb. He’s worked hard and he needs to get back to the battery to fill up his backpack again. Off he goes, back to the battery for some R&R and a new backpack.