Physics · Thermal Properties Of Matter · NEET
Either works and gives the same number. delta T is a temperature difference, and one Celsius degree equals one Kelvin in size. So a change of 20 degrees C is the same as a change of 20 K. You only must convert to Kelvin when a formula uses the absolute temperature T itself (like the gas law or Stefan's law), not when it uses a difference delta T.
Specific heat capacity c is per unit mass: heat to raise 1 kg by 1 K, unit J per kg per K. Heat capacity (also called thermal capacity) is for the whole body: C = m c, unit J per K. So Q = m c delta T = C delta T. Specific heat depends only on the material; heat capacity depends on the material and how much of it you have.
c is the specific heat capacity of the substance. It is the amount of heat needed to raise the temperature of unit mass (1 kg) by one unit (1 K), so its SI unit is J per kg per K. For water c is about 4186 J per kg per K, which is unusually high, so water heats up slowly and cools slowly.
No. During a change of state the temperature stays constant, so delta T = 0 and this formula gives zero even though heat is flowing. For melting and boiling you use latent heat, Q = m L. Use Q = m c delta T only while the substance stays in one state and its temperature is actually changing.
Because water has a very high specific heat capacity (about 4186 J per kg per K) while metals like copper (about 386) or aluminium (about 900) are much lower. From Q = m c delta T, a bigger c means more heat for the same mass and same temperature rise. This is why water is used as a coolant in radiators and why the sea warms and cools slowly.
The quantities of heat required to raise the temperature of two solid copper spheres of radii r1 and r2 (r1 = 1.5 r2) through 1 K are in the ratio Q1 : Q2 equal to:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Q = m c delta T, where Q is heat in joules, m is mass in kg, c is specific heat capacity in J per kg per K, and delta T is the temperature change. It applies only when there is no change of state.
Joule per kilogram per kelvin (J per kg per K). Water has about 4186 J per kg per K.
No. During melting or boiling the temperature does not change, so delta T is zero. Use the latent heat formula Q = m L for those phase changes instead.
Heat capacity of a body is C = m c, with unit J per K. So Q = m c delta T can also be written as Q = C delta T. Specific heat is per kilogram; heat capacity is for the whole object.
Because the two scales differ only by a shift of 273.15, and a difference cancels that shift. One degree Celsius equals one Kelvin in size, so a change of 30 degrees C equals a change of 30 K.
