- Electric Charge and attraction/repulsion- The Photoelectric effect

Atomic Physics

Definitions and units

Charge = Q Unit is Coulombs (C). 1 coulomb is a

huge amount of charge = 6.2 x 1018 electrons.

So the charge on one electron is tinyQe = – 1.6 x 10 -19 C 1 proton is the same but positive

Force between 2 charges

Remember Gravity?

Charge is very similar:

G = Gravitational constant, very small

k = Coulomb’s constant, very large

Differences Electric force much stronger Gravity is always attractive, electric force

can be negative (2 charges attract) or positive (2 charges repel)

eg F = – 100 N

F = +100 N

Q1 Q2

Q1 Q2

Practice

1. Find the Force between a charge of 1.5C and a charge of - 2C that are 1.5m apart.

2. Find the Force between a charge of 80 nC and a charge of 300 nC that are 0.12m apart.

Hint: nC = nanocoulomb = 10 -9 C

Protons and electrons attract

Hydrogenatom:

2 Protons Repel 2 protons in a nucleus are only 1x10 -15 m apart.

Calculate the electrostatic force pushing them apart in N.

A proton has a mass of only 1.7 x 10 -27 kg. So the acceleration it should feel from this force is way faster than the speed of light!

So what sticks the protons together?STRONG NUCLEAR FORCE – much stronger than other forces, but tiny range – only works between a proton or a neutron and its neighbour.

Strong Nuclear Force

Only works between protons and neutrons that are touching

Waves vs. particles Albert Einstein figured out that the Energy of

a particle is proportional to the frequency of it’s wave

E = h . f h = Planck’s constant c = speed of light = 4.1 x 10-15 eV = 3 x 108 m/sYou can put these 2 together to change energy into wavelength:

Energy of an electron is measured in electron Volts (eV)

The Photoelectric effectWhen light is shone on a metal, it

emits (gives out) electrons. Einstein worked out each metal has

a threshold frequency called the work function.

Energy of light photon = E =

Photoelectric effect

This means light below the cut off frequency will not emit a photon

Light of higher f/energy will trigger release of electron

The energy left over is the Kinetic energy of the speeding electron given off.

E = h.f practice

Hydrogen Energy levels The electron in a Hydrogen atom can only have

energy of a certain level. The lowest level E1 is called ‘ground state’.

Ground state

Light wavelengths When the electron drops from one level

to a lower level, a photon of light is emitted (given off)

Example Calculation:

This means you ignore the negative sign

6.56 x 10 -7m = 656 nm

What colour is this wavelength?

Visible light wavelengths So 656 nm wavelength will be a dark red colour.

Higher energy

Lower energy

Example Calculations:Convert answers to nm – are any in the visible range? If so what colour?