Atomic Physics and photoelectric effect

1. - Electric Charge and
attraction/repulsion
- The Photoelectric effect
Atomic Physics

2. 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 tiny
Qe = – 1.6 x 10 -19 C
 1 proton is the same but positive

3. Force between 2 charges
 Remember Gravity?
 Charge is very similar:
G = Gravitational
constant, very small
k = Coulomb’s constant,
very large

4. 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

5. 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

6. Protons and electrons attract
Hydrogen
atom:

7. 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.

8. Strong Nuclear Force
Only works between protons and
neutrons that are touching

9. 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/s
You can put these 2 together to
change energy into wavelength:
Energy of an electron is
measured in electron Volts
(eV)

10. The Photoelectric effect
 When 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 =

11. 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.

12. E = h.f practice

13. 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

14. Light wavelengths
 When the electron drops from one level to a
lower level, a photon of light is emitted (given
off)

16. Example Calculation:
This means you ignore
the negative sign
6.56 x 10 -7m = 656 nm
What colour is this
wavelength?

17. Visible light wavelengths
 So 656 nm wavelength will be a dark red colour.
Higher energy Lower energy

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