Temperature difference (ΔT) is the gap in temperature between two objects or regions, found by subtracting one temperature from the other. In AP Physics 2, ΔT is the driving force behind heat transfer: energy flows from the hotter region to the colder one until ΔT reaches zero at thermal equilibrium.
Temperature difference, usually written ΔT, is exactly what it sounds like. You take two temperatures and subtract. But in AP Physics 2, ΔT isn't just arithmetic. It's the reason heat flows at all. Energy spontaneously moves from the higher-temperature object to the lower-temperature one, and it keeps moving as long as a temperature difference exists. No ΔT, no net heat transfer.
Think of ΔT like the voltage of thermodynamics. Just as a potential difference pushes charge through a circuit, a temperature difference pushes thermal energy through a material. That analogy is built right into the conduction rate equation, where the rate of energy transfer through a slab is proportional to ΔT (along with the material's thermal conductivity, the cross-sectional area, and the inverse of the thickness). Double the temperature difference across a wall and you double the rate heat leaks through it. One handy note on units: because Kelvin and Celsius degrees are the same size, a ΔT of 20°C is also a ΔT of 20 K, so you don't need to convert when only the difference matters.
Temperature difference lives in Topic 2.6, Heat and Energy Transfer, in Unit 2 (Thermodynamics). It's the variable that determines both the direction of heat flow (hot to cold, always) and the rate of heat flow by conduction. It also explains thermal equilibrium, which is just the special case where ΔT = 0 and net energy transfer stops. On the exam, almost every conduction problem and every equilibrium argument runs through ΔT. If you can identify which object is hotter, predict which way energy flows, and reason about how the flow rate changes as the gap shrinks, you've got the core of Topic 2.6.
Keep studying AP Physics 2 Unit 2
Thermal Equilibrium (Unit 2)
Thermal equilibrium is what happens when the temperature difference runs out. Two objects in contact exchange energy until ΔT = 0, and at that point net heat flow stops. Equilibrium isn't a separate idea from ΔT; it's the endpoint of ΔT shrinking to zero.
Heat Flux (Unit 2)
Heat flux is the rate of energy transfer per area, and it's directly proportional to ΔT in conduction. This is the quantitative payoff of temperature difference: ΔT doesn't just say heat flows, it tells you how fast. A bigger gap means a steeper flow.
Kinetic Energy (Units 1-2)
Temperature measures the average kinetic energy of a substance's particles, so a temperature difference is really a difference in average particle KE. When fast molecules collide with slow ones, energy passes from fast to slow. That microscopic picture is why heat flows from hot to cold.
Thermocouple (Unit 2)
A thermocouple turns a temperature difference into a measurable voltage, which is a nice real-world echo of the ΔT-as-voltage analogy. It's the lab tool you'd use to actually measure ΔT in an experimental design question.
No released FRQ uses the phrase "temperature difference" as a standalone term, but ΔT is baked into how Topic 2.6 gets tested. Multiple-choice questions ask you to predict the direction of heat flow between objects at different temperatures, rank conduction rates when ΔT (or area, thickness, or conductivity) changes, or identify when a system has reached thermal equilibrium. On FRQs, ΔT shows up in qualitative reasoning: you might explain why heat transfer slows as two objects approach the same temperature, or justify, using the conduction relationship, how changing ΔT changes the energy transfer rate. The skill being graded is proportional reasoning. If ΔT triples while everything else stays fixed, the conduction rate triples, and you need to say so explicitly with a reference to the relationship, not just the answer.
Temperature difference and heat are not the same thing. ΔT is a condition (a gap between two temperatures), while heat Q is the energy that gets transferred because that gap exists. ΔT is the cause; Q is the effect. Two objects can have a huge ΔT but transfer little heat if they're barely in contact, and a tiny ΔT can move lots of energy over a long time. On the exam, say "energy is transferred as heat because of a temperature difference," never "the object contains heat."
Temperature difference (ΔT) is found by subtracting one temperature from another, and it determines the direction of heat flow: energy always moves from the hotter region to the colder one.
The rate of heat conduction is directly proportional to ΔT, so doubling the temperature difference doubles how fast energy transfers.
Thermal equilibrium is just the case where ΔT = 0, meaning there is no longer any net heat transfer between the objects.
Because Celsius and Kelvin degrees are the same size, a temperature difference has the same numerical value in either scale, so no conversion is needed when only ΔT matters.
ΔT is the cause and heat (Q) is the effect: the temperature difference drives the transfer, and the transferred energy is the heat.
Microscopically, a temperature difference is a difference in average particle kinetic energy, and collisions pass energy from faster particles to slower ones.
Temperature difference (ΔT) is the gap in temperature between two objects or regions, calculated by subtraction. In Topic 2.6, it's the driving force for heat transfer, determining both the direction energy flows and how fast it flows by conduction.
No. Temperature difference is a condition (how far apart two temperatures are), while heat is the energy transferred because of that condition. ΔT is the push; heat (Q) is the energy that actually moves.
Yes, net heat flow stops. When ΔT = 0 the objects are in thermal equilibrium, so there's no longer a net transfer of energy between them, even though particles on both sides are still moving and colliding.
No. A Celsius degree and a Kelvin are the same size, so a ΔT of 15°C is also a ΔT of 15 K. You only need Kelvin when an equation uses absolute temperature itself, like the ideal gas law.
The conduction rate is directly proportional to ΔT. If the temperature difference across a material doubles while area, thickness, and conductivity stay the same, energy transfers twice as fast. That's also why heat flow slows down as two objects approach equilibrium.