Science · Pressure and heat
Why does the same force produce more pressure over a smaller area? Can energy enter water while its temperature stays almost constant? These questions become clearer when force and pressure, heat and temperature, and warming and phase change are kept separate.
Read the quantities in an equation before substituting numbers. After calculating, check the unit and the conditions. Specific heat capacity and boiling use some secondary-school ideas; the examples explain those ideas rather than treating their names as answers.
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Pressure P is perpendicular force F divided by the area A over which it acts: P = F/A. The pascal, Pa, corresponds to N/m². In the example, 20 N acts over 0.01 m², so P = 20/0.01 = 2000 Pa. Dividing by that small area is different from multiplying by it.
For an original comparison, spread the same 20 N over 0.02 m². The pressure is 1000 Pa. Doubling area while keeping force constant halves pressure. State both quantities when explaining how strongly a surface is pressed.
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Temperature describes a state; heat is energy transferred as a result of a temperature difference. With no phase change, approximately constant specific heat capacity c and negligible mechanical work such as expansion, heat mainly changes temperature and Q = mcΔT applies. Here m is mass and ΔT is temperature change. Specific heat capacity describes the energy needed per kilogram for a one-kelvin rise and depends on the material and its phase.
In an original calculation, m = 0.5 kg, c = 400 J/(kg·K) and ΔT = 10 K give Q = 0.5 × 400 × 10 = 2000 J. Rearranging gives ΔT = Q/(mc): for the same heat and mass, a larger c means a smaller rise. A temperature difference of 1 K equals a difference of 1°C; their zero temperatures are different.
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While pure water boils at constant pressure and liquid and vapor coexist, added heat mainly changes liquid into vapor. Its temperature stays approximately constant. The warming equation Q = mcΔT cannot by itself calculate the energy for this phase change.
A steady thermometer reading does not mean that energy stopped moving. Ask whether the question describes warming within one phase or a change of phase before choosing an equation.
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A useful boiling criterion is that the liquid’s vapor pressure reaches the external pressure. Vapor pressure is the pressure of the liquid’s own vapor in equilibrium with the liquid. In a gas mixture it is that vapor’s partial pressure: the part of the total pressure contributed by that vapor. Vapor pressure increases with temperature. Lowering external pressure lets pure water reach that pressure at a lower temperature, so its boiling point usually falls.
The familiar boiling temperature of about 100°C for pure water includes an external pressure of about one atmosphere. The name “water” alone does not fix its boiling point under every pressure. In the example, external pressure is the changed condition; merely mentioning how much heat is supplied does not resolve the comparison.
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Example 1 · Physics: pressure and heat
Answer: 2000 Pa
P = F/A = 20/0.01 = 2000 Pa.
Definitions, laws, and equations in this chapter. The Japanese questions and explanations were newly authored rather than copied from the source or existing exercises.
Source checked: 2026-10-03Example 2 · Physics: pressure and heat
Answer: Its temperature rises less
Q = mcΔT, so a larger c gives a smaller temperature rise for the same Q and m.
Definitions, laws, and equations in this chapter. The Japanese questions and explanations were newly authored rather than copied from the source or existing exercises.
Source checked: 2026-10-03Example 3 · Physics: pressure and heat
Answer: It decreases
Boiling occurs when the water's vapor pressure reaches the external pressure.
Definitions, laws, and equations in this chapter. The Japanese questions and explanations were newly authored rather than copied from the source or existing exercises.
Source checked: 2026-10-03Source checked: 2026-10-05
Source checked: 2026-10-05
Source checked: 2026-10-05
Source checked: 2026-10-05
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