Heating and cooling
Heat a measured mass with a known power and watch the temperature. The plateaus are where the energy is going into melting or boiling instead.
Gases & thermodynamics — Pressure, volume and temperature, and the particles whose motion they average.
Experiments already set up on it
The specific heat capacity of water
Heat a known mass with a known power and plot the energy against the rise.
- c comes from the gradient of the energy supplied against the temperature: E = mcΔθ
- heat leaking into the room makes the measured c come out too large
- a gradient is better than a single pair of readings, because it ignores the starting value
The latent heat of melting ice
Heat ice from below zero: the temperature stops dead while it melts, then starts again.
- a change of state happens at constant temperature: the energy breaks the solid apart instead
- L_f = Pt/m, with t the length of the plateau
- the two sloping parts of the curve have different gradients, because ice and water have different capacities
How a hot drink cools
Heater off, and nothing to do but lose heat to the room.
- the rate of cooling is proportional to how far above the room it is, so it slows as it cools
- the curve is an exponential, and ln(θ − θ_room) against t is a straight line of gradient −k/mc
- it never quite reaches room temperature, which is what an asymptote means
Does the mass matter?
The same heater and the same time, on more and more water.
- the same energy raises twice the mass by half as much: Δθ ∝ 1/m
- plotting Δθ against 1/m straightens it, and the gradient is Pt/c
- controlling the mass matters, because it changes the answer and not just the scatter
What you can measure
Each of these can be recorded over time, aggregated over a run, or read at the moment something happens — through an instrument that carries its own uncertainty.
- Timet (s)
- Temperatureθ (K)
- Temperature riseΔθ (K)
- Temperature above the roomT_x (K)
- Fraction meltedf_m (1)
- Fraction boiled awayf_b (1)
- Energy suppliedE (J)
- Energy lost to the roomQ_l (J)
- Energy gained by the sampleQ (J)
- Heater powerP (W)
- Net power into the sampleP_n (W)
- Power lost to the roomP_l (W)
What you can change
Any of these can be the thing you vary, and every combination of the ones you choose is run as its own trial.
- Massm (kg)
- Starting temperatureθ_0 (K)
- Heater powerP (W)
- Room temperatureθ_r (K)
- Heat lost to the roomk (W/K)
The equations behind it
What heating and cooling is built on. Every one of these is in the lab's formula reference too, where it copies straight into a write-up.
Thermal energy transfers
- Heating something up — c is the specific heat capacity.
- Changing state — L is the specific latent heat; the temperature does not change.
- Density
- Kelvin from Celsius
Also in Gases & thermodynamics
Ideal gases
Squeeze, heat and cool a box of atoms. The pressure is measured from the hammering on the walls and the temperature from how fast they are going.
Heat engines
Take a gas round a closed cycle and measure the area of the loop. How much of the heat you pay for comes back as work — and why can none of them do better than Carnot?
Thermal radiation
A hot body radiating to a sensor: the fourth power of the temperature, the inverse square of the distance, and the colour it glows.