In nuclear and radiation physics, the curie (Ci) is a unit used to measure which physical quantity?

Introduction / Context:
In nuclear physics, medical imaging, and radiation safety, different units are used to describe radioactive decay and exposure. The curie, symbol Ci, is one of the older but still widely known units. This question tests your knowledge of which physical quantity is measured in curies, which is important for understanding discussions of radioactive sources and their strength.

Given Data / Assumptions:

• The symbol Ci refers to the curie.
• We are working within the context of radioactivity and nuclear phenomena.
• We must identify the physical quantity associated with this unit.

Concept / Approach:
Radioactivity is the process in which unstable atomic nuclei spontaneously emit particles or electromagnetic radiation. The activity of a radioactive source tells us how many disintegrations occur per unit time. Historically, the curie was defined so that 1 curie corresponds to 3.7 * 10^10 disintegrations per second, approximately equal to the activity of one gram of radium. So, the curie is a unit of activity or radioactivity, not of temperature, heat, or energy directly.

Step-by-Step Solution:
Step 1: Recall that temperature is measured in degrees Celsius, kelvin, or Fahrenheit, not in curies. Step 2: Recall that heat and energy are measured in joules or calories, while electrical resistance is measured in ohms. Step 3: Identify the physical quantity related to the rate of nuclear disintegrations per second. This is called activity or radioactivity of a source. Step 4: Note that the curie was defined as 3.7 * 10^10 disintegrations per second, making it clearly a unit of radioactivity.

Verification / Alternative check:
Modern SI practice replaces the curie with the becquerel (Bq), where 1 Bq is 1 disintegration per second. The relationship is 1 curie = 3.7 * 10^10 becquerel. Since the becquerel is defined as a unit of activity, the curie must also represent activity. This cross check strongly confirms that the curie measures radioactivity and not temperature, heat, or resistance.

Why Other Options Are Wrong:
Temperature: Units like kelvin and degree Celsius are used for this; curie is unrelated to thermal state. Heat energy: This uses joule, calorie, or similar units, not the curie. Electrical resistance: The standard unit is the ohm, not the curie.

Common Pitfalls:
Learners sometimes mix up units when first encountering nuclear physics. It is easy to confuse curie with other nuclear related terms like joule or gray. A helpful strategy is to remember that activity describes how many atoms decay per second and is measured by curie in older literature and becquerel in SI, while absorbed dose uses gray and Sievert for biological effect.

Final Answer:
The curie is a unit of the activity of a radioactive source, that is, of radioactivity.