Physical phenomena and how we investigate them

Two pieces of ice were placed in glasses. One melted sooner. You might immediately think, “The water was warmer.” But what if the other piece was larger? Let’s design a test that can distinguish these explanations.

A piece of ice loses its shape and becomes liquid water. What has changed? Its state has changed, but the substance is still water. An observable change in motion, state or another physical property is called a physical phenomenon. A falling ball, a warming spoon, a vibrating string’s sound and a glowing lamp are all examples. Before looking for a cause, describe the change: what was the situation at the start, and what was different at the end?

Imagine breaking a drinking glass. It is no longer one intact object, but the material, glass, is still there. This separates the object from its material: a physical object is a distinct item, while a substance is what it is made of. A drinking glass and a bottle are different objects made from the same material. Spoons with the same shape can be made from different materials. So “What object is it?” and “What is it made of?” ask different questions.

How can a magnet attract a paperclip without glass, water or another visible link between them? We describe the interaction using a physical field. The magnet’s field exists around it and acts on the paperclip; the field is not a substance. Substances and physical fields are both part of physical reality, which exists whether or not we observe it. We will study fields in detail later. For now, remember that physical reality includes more than visible objects.

Simply watching ice melt is an observation. To explain different melting times, we can propose a hypothesis: warmer water makes ice melt faster when the other conditions are the same. A hypothesis is a tentative explanation that leads to a prediction we can test. If we change everything at once, the result cannot tell us which change mattered. We need an experiment in which the two possible explanations predict different results.

Imagine putting a small piece of ice in warm water and a large piece in cold water. If the first melts sooner, that does not isolate the effect of temperature: both differences might contribute. Instead, use identical glasses, equal volumes of water, and ice with the same mass and starting temperature. Change only the water’s initial temperature and keep the other conditions as similar as possible. Controlling the conditions in this way turns an observation into an experiment.

When should we start the stopwatch: when the ice enters the water, or a few seconds later? Different rules make the readings difficult to compare. Let’s measure time t from putting the ice in the water until it has completely melted; the unit is the second, с. Time is the result we measure, not a condition we keep equal. Repeat the comparison several times: a late start or an unnoticed difference between the pieces can change an individual reading.

If warmer water consistently shortens the melting time in repeated trials, the results support our explanation. They do not prove it for every piece of ice and every container: we have tested particular conditions. To reason further, we use a model that keeps the important features and leaves out details for now. A diagram of two identical glasses, for example, lets us focus on temperature and time without drawing every scratch in the glass.

Two identical dark plates are heated by a lamp. How could you test the effect of their distance from the lamp?

We want to test distance from the lamp, so keep the plates’ material, size, colour, orientation and initial temperature the same. Keep the lamp’s operating conditions unchanged too.

Place one plate closer and the other farther away, without letting either block the light reaching the other. Measure the temperature rise over the same time interval. Comparing final temperatures without allowing for starting temperatures would not be a fair test.

Repeat the experiment and swap the plates’ positions. This checks whether the difference follows the distance rather than a particular plate.

Change the distance and compare the temperature rise over the same time interval, keeping the other conditions the same.

Before looking for a cause, distinguish the object, its material and the change taking place.

An experiment gives stronger evidence when you change the selected condition, control the others and repeat the measurements.

The model simplifies the situation for the sake of a testable explanation; the conclusion refers to those conditions that are actually tested.

Next, we will learn to read a scale and judge measurement uncertainty so that our comparisons are meaningful.