Particles
Atomic structure, ions, nuclide notation and isotopes.
Structure of the atom
- Most of the atom is empty space.
- The nucleus contains protons and neutrons.
- Protons have charge +1 and neutrons have charge 0.
- Electrons have charge -1 and a very small mass.
- A neutral atom has the same number of protons and electrons.
Nuclide notation
- Proton number Z is the number of protons.
- Nucleon number A is the total number of protons and neutrons.
- Number of neutrons = A - Z.
- For a neutral atom, electrons = protons.
nucleon number = protons + neutrons
Ions and specific charge
- Positive ions form when electrons are lost.
- Negative ions form when electrons are gained.
- Specific charge means charge divided by mass.
- Electrons have a much larger specific charge than protons because their mass is much smaller.
specific charge = Q / m
Isotopes
- Isotopes are atoms of the same element.
- They have the same number of protons.
- They have different numbers of neutrons.
- They have similar chemical properties but different masses.
nucleus
proton number
nucleon number
ion
specific charge
isotope
Quick Quiz
Question: What is the difference between isotopes?
They have the same number of protons but different numbers of neutrons.
EM & Quantum
Photoelectric effect, photons, work function and energy levels.
Photoelectric effect
- Light can behave as packets of energy called photons.
- One photon transfers energy to one electron.
- Electrons are emitted only if the photon energy is large enough.
- This supports the particle model of light.
Work function and threshold frequency
- The work function is the minimum energy needed to remove an electron from a metal surface.
- The threshold frequency is the minimum frequency needed for electron emission.
- Below threshold frequency, no electrons are emitted.
- Above threshold frequency, higher frequency gives greater maximum kinetic energy.
hf = Φ + Eₖ(max)
What affects what?
- Increasing intensity increases the number of emitted electrons per second.
- Increasing frequency increases the maximum kinetic energy of electrons.
- Emission is immediate if the frequency is above threshold.
Energy levels
- Electrons in atoms occupy discrete energy levels.
- Excitation moves an electron to a higher level.
- De-excitation causes photon emission.
- The photon energy equals the difference between the two levels.
ΔE = hf = hc / λ
photon
photoelectric effect
work function
threshold frequency
excitation
de-excitation
Quick Quiz
Question: What changes when light intensity increases in the photoelectric effect?
The number of emitted photoelectrons per second increases, as long as the frequency is already above threshold.
Electricity
Resistance, resistivity, superconductors and potential dividers.
Resistance of a wire
- Resistance increases with length.
- Resistance decreases with larger cross-sectional area.
- Resistance depends on the material.
- For a metal wire, resistance usually increases with temperature.
Resistivity
- Resistivity is a property of the material.
- It is measured in Ωm.
- Lower resistivity means better conduction.
R = ρL / A
Superconductors
- Below a critical temperature, resistivity falls to zero.
- They are useful in MRI scanners and maglev trains.
- The main difficulty is the cooling needed.
Potential dividers
- A potential divider uses resistors in series.
- The output voltage depends on resistance values.
- They are used with LDRs, thermistors and variable resistors.
Vout = (R₂ / (R₁ + R₂)) × Vin
resistance
resistivity
superconductor
critical temperature
potential divider
thermistor
Quick Quiz
Question: What happens to resistance if a wire’s cross-sectional area becomes 4 times bigger?
The resistance becomes 4 times smaller, assuming the material and length stay the same.
Mechanics
Motion graphs, vectors, projectiles, moments, Newton’s laws and energy.
Motion graphs
- Gradient of displacement-time graph = velocity.
- Gradient of velocity-time graph = acceleration.
- Area under velocity-time graph = displacement.
Scalars and vectors
- Scalars have magnitude only.
- Vectors have magnitude and direction.
- Force, velocity and acceleration are vectors.
Projectile motion
- Horizontal and vertical motion are independent.
- Horizontal velocity stays constant if air resistance is ignored.
- Vertical acceleration is always downward due to gravity.
Moments
- A moment is the turning effect of a force.
- Moment depends on force and perpendicular distance from the pivot.
- In equilibrium, clockwise moments equal anticlockwise moments.
moment = force × perpendicular distance
Newton’s laws
- If resultant force is zero, velocity stays constant.
- Resultant force causes acceleration.
- Force equals mass times acceleration.
- Interaction pairs act on different objects.
F = ma
Work, power and efficiency
- Work done is energy transferred.
- Power is the rate of energy transfer.
- Efficiency compares useful output with total input.
W = Fs cosθ
P = W / t
velocity
acceleration
vector
projectile
moment
Newton's laws
power
efficiency
Quick Quiz
Question: Why is horizontal acceleration zero in projectile motion when air resistance is ignored?
Because there is no horizontal resultant force acting on the projectile. Only weight acts, and that is vertical.