OCR GCSE Physics: Matter (J249)

This guide covers Topic 1 Matter, the first of eight content topics (P1-P8) in OCR GCSE (9-1) Physics A (Gateway Science) (J249), first teaching September 2016. A ninth topic, P9 (practical skills), is assessed with Topics P1-P4 on Paper 1/3 (and assumed on Paper 2/4), and provides the skills for the 15% practical content in the written examinations. The qualification is tiered, with Foundation and Higher tier papers.

Where this fits in J249

Matter establishes the particle-model view of solids, liquids and gases, and how pressure arises from particle behaviour – foundational physical concepts that recur throughout later topics, including Energy and the thermal physics woven through several other topic areas.

Syllabus coverage

OCR GCSE PHYSICS (J249) — TOPIC 1 MATTER

• P1.1 The particle model — how the arrangement and movement of particles differs between solids, liquids and gases, the development of the atomic model from Dalton through Thomson, Rutherford (with Geiger and Marsden), and Bohr, the nuclear description of the atom, and the order-of-magnitude size of an atom
• P1.2 Changes of state — how energy transfer causes changes of state, and how mass is conserved during these changes
• P1.3 Pressure — how pressure arises in gases and how it can be calculated, and how pressure in liquids and the atmosphere varies with depth, height, and density, including floating, sinking and upthrust

How to approach it

The particle model (P1.1) is the conceptual foundation for the whole topic, so be confident describing and comparing particle arrangement, movement and energy across all three states before moving to changes of state. Changes of state (P1.2) is often tested through energy-transfer diagrams and calculations, so practise linking a named change of state (melting, boiling, condensing) to the direction of energy transfer involved. Pressure (P1.3) is the most calculation-heavy sub-topic here – practise applying the relevant pressure equations and interpreting what a calculated value means physically, since exam questions frequently pair calculation with explanation. Because Foundation and Higher tier candidates are assessed on overlapping but not identical content, check which specific outcomes within each sub-topic apply to your tier before relying on any single source of revision material, since Higher tier candidates are expected to handle more demanding calculations and a wider range of contexts than Foundation tier candidates.

Official syllabus

OCR GCSE (9-1) Physics A (Gateway Science) (J249) specification, accredited PDF, version 5.0, for first teaching September 2016 — ocr.org.uk.

Density

Density is mass per unit volume:

density = mass / volume       rho = m / V
kg/m^3  or  g/cm^3            1 g/cm^3 = 1000 kg/m^3

For a regular solid, measure dimensions and calculate volume. For an irregular solid, use a displacement can and measure the water displaced. For a liquid, find the mass of an empty measuring cylinder, add the liquid, reweigh and subtract.

Solids and liquids have similar densities because their particles are close together; gases are roughly a thousand times less dense because the particles are far apart.

The particle model and changes of state

Changes of state are physical: the particles themselves are unchanged, only their arrangement and energy differ, so the change is reversible and mass is conserved.

Heating a substance either raises its temperature (increasing the kinetic energy of the particles) or changes its state (increasing potential energy as bonds between particles are overcome) — never both at once. This is why a heating curve has flat sections at the melting and boiling points despite energy still being supplied.

Development of the atomic model

Ideas about the structure of the atom changed as new evidence emerged. Dalton described atoms as solid spheres. Thomson’s discovery of the electron led to the “plum pudding” model — a ball of positive charge with electrons embedded in it. Rutherford’s alpha-scattering experiment (carried out with Geiger and Marsden) fired alpha particles at thin gold foil: most passed straight through, but a small fraction were deflected through large angles, showing that an atom’s mass and positive charge are concentrated in a tiny, dense nucleus with the rest of the atom being empty space. Bohr then proposed that electrons occupy fixed orbits (shells) at set distances from the nucleus, which explained why atoms did not collapse. Further experiments established that the nucleus itself contains protons and neutrons. An atom has a radius of the order of 10⁻¹⁰ m, with the nucleus around 10,000 times smaller still.

Specific heat capacity and latent heat

Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 degree Celsius:

E = m c delta-theta

Specific latent heat is the energy needed to change the state of 1 kg without a temperature change:

E = m L

Latent heat of fusion applies to melting and freezing; latent heat of vaporisation to boiling and condensing, and is the larger of the two because all the forces between particles must be overcome rather than merely loosened.

Gas pressure

Gas particles collide with container walls, and each collision exerts a small force. Pressure is the total force per unit area from these collisions.

Raising temperature at constant volume increases particle speed, so collisions are more frequent and more forceful, and pressure rises. Reducing volume at constant temperature increases the frequency of collisions, so pressure rises:

p1 V1 = p2 V2      at constant temperature

Pressure in liquids and the atmosphere

Pressure in a liquid increases with depth and with the liquid’s density, because a deeper or denser column of liquid above a point has more weight pressing down on it:

p = h rho g       (pressure = depth x density x gravitational field strength)

The pressure difference between two depths can be found by applying this equation to each depth and subtracting, or by using the depth difference directly in the same formula.

The atmosphere can be modelled as a layer of air surrounding the Earth whose density decreases with altitude. Atmospheric pressure at a point equals the weight of the column of air above it, so atmospheric pressure decreases with height as there is progressively less air, and less dense air, above.

Upthrust is the net upward force on an object in a fluid, caused by the pressure at the bottom of the object being greater than the pressure at the top (since the bottom is deeper). An object floats when the upthrust acting on it equals its weight; it sinks if its weight exceeds the maximum upthrust the fluid can provide.

Worked example

How much energy is needed to turn 0.50 kg of ice at 0 degrees Celsius into water at 20 degrees Celsius? (L of fusion = 334 000 J/kg, c of water = 4200 J/kg C)

Melting:  E = m L = 0.50 x 334 000        = 167 000 J
Heating:  E = m c dt = 0.50 x 4200 x 20   =  42 000 J

Total = 209 000 J

Two stages, two equations. Melting takes four times as much energy as the heating that follows — the point of the question.

Common mistakes

Using E = m c delta-theta during a change of state, where temperature is not changing. Forgetting to convert g/cm^3 to kg/m^3, or cm^3 to m^3. Saying particles “expand” when heated — the particles do not change size, their spacing and speed do. Explaining gas pressure by saying particles “push harder” without mentioning collision frequency. Treating a change of state as chemical.

Quick revision checklist

• Calculate density and describe how to measure it for regular solids, irregular solids and liquids.
• Explain changes of state using the particle model and state why mass is conserved.
• Interpret a heating curve, explaining the flat sections.
• Apply E = m c delta-theta and E = m L, including two-stage problems.
• Explain gas pressure in terms of collisions and apply p1 V1 = p2 V2.
• Describe how the model of the atom developed from Dalton to Bohr, and state the order-of-magnitude size of an atom.
• Apply p = h rho g to find pressure and pressure differences in a liquid, and explain why atmospheric pressure decreases with height.
• Explain floating and sinking in terms of upthrust and weight.