Understanding PADI IDC Physics Exam – Tips

The PADI Instructor Development Course (IDC) is a challenging yet rewarding journey for aspiring dive instructors. Among the various components of the IDC, the physics exam often stands out as one of the most daunting. Understanding the principles of physics as they apply to diving is crucial for ensuring safety and competence as an instructor, but many candidates find this exam particularly challenging.

This blog is designed to complement our first physics blog, “Strategies to Help You Through the PADI IDC Physics Exam.” Here, we’ll delve deeper into some practical tips and insights to help you not only understand the material but also approach the exam with confidence. Whether you’re just beginning your IDC journey or looking to refine your knowledge, these tips are aimed at getting you through the physics exam with flying colors.

Here, you’ll find simplified explanations of some key physics concepts that you’ll encounter during the physics exam in your PADI Instructor Examinations. We’ve aimed to break down these topics to make them easier to understand, especially since the physics equations in scuba diving can sometimes be complex. Because PADI only requires a basic understanding of these principles, a simplified approach should be sufficient to help you pass the exam.

1. Water Weight and Pressure

Water has weight, and weight creates pressure. In the context of scuba diving, this pressure can exert force both upwards and downwards. A simple experiment I often do during the IDC is to have a candidate lay down while I place a 1.5-liter bottle of water on them, followed by one or two 6-liter bottles. This allows them to physically feel the weight of the water pressing down on them—this is pressure in action.

When we dive, a similar concept applies: the deeper you go, the more water there is above you, which increases the pressure. Now, here’s where science comes into play: because water is incompressible, the pressure exerted on your body is balanced by the water pressure inside your body. Since our bodies are mostly made up of water, we don’t actually feel this external pressure. Instead, the water in your body becomes one with the surrounding water.

In the scuba diving world, what does compress is air, which is why you’ll encounter questions about the compression of flexible containers like wetsuits and BCDs.

Additionally, remember that salt water is heavier than fresh water, so it creates more pressure. Always keep in mind that 1 liter of fresh water weighs 1.0 kg, while 1 liter of salt water weighs 1.03 kg—this extra weight is due to the salt content.

2. Displacement

Displacement can be understood through a simple analogy: imagine yourself in water. You can float on the water’s surface if you spread out wide, like doing a starfish impersonation. But you can’t walk on water with just the soles of your feet touching the surface.

In this scenario, the water and your body weight remain constant, but what changes is how much of your body is in contact with the water (displacement). When you spread out, you displace more water, which increases your chances of floating. In physics, we often represent this with forces: the force pushing up (buoyancy) and the force pushing down (weight). Whichever force is greater determines whether something is positively buoyant (floats), negatively buoyant (sinks), or neutrally buoyant (hovers in mid-water).

Before tackling any displacement question, identify the forces at play—what’s pushing up and what’s pushing down. You need to know the object’s weight (in kilograms) and the weight of the water it displaces (displacement in liters multiplied by the weight of the water). This will help you determine if the object will float, sink, or remain neutrally buoyant.

3. Salt Water vs. Fresh Water – Kilograms vs. Liters

Two of the most common mistakes candidates make in physics exams involve not converting between kilograms and liters correctly or confusing salt water with fresh water. These errors typically result from not reading the question carefully or rushing through it.

Take your time—slow down and read each question thoroughly. Make sure your diagram clearly indicates whether you’re dealing with salt water or fresh water and whether your answer should be in kilograms or liters.

4. Conduction, Radiation, and Convection

Heat transfer in scuba diving occurs through three mechanisms: conduction, convection, and radiation. Most PADI physics exams include questions on this topic, with conduction and convection being the most relevant to diving.

Conduction: This occurs when two objects come into direct contact. For example, when a diver in cold water loses heat as their body comes into contact with the water.

Convection: This is the transfer of heat in fluids, such as water. When water near your skin heats up, it becomes less dense and rises, being replaced by cooler water, thus carrying heat away from your body.

Radiation: While less relevant to diving, radiation is the transfer of heat from one object to another without direct contact, like the warmth you feel from the sun.

5. Refraction and Visual Reversal

Refraction: Refraction occurs when light passes from water into air (such as in your scuba mask), making objects appear closer and larger than they actually are.

Visual Reversal: Visual reversal makes objects appear farther away, and is often caused by turbidity—a combination of water clarity, contrast, and light.

6. Molecules and Temperature

The behaviour of atoms and molecules changes with temperature: they move faster when hot and slower when cold. This principle affects scuba diving in various ways, especially in relation to the pressure inside a diving cylinder. For example, a cylinder left in the sun will heat up, causing the pressure inside to increase. Conversely, when the cylinder cools, the pressure decreases.

This is why you might notice a significant drop in pressure shortly after starting a dive—it’s not just due to air consumption but also the cooling of the cylinder. Understanding this relationship between temperature and pressure is key to answering related questions on the exam.

7. Ambient Pressure, Total Pressure, and Gauge Pressure

Total Pressure: This includes both water pressure and atmospheric pressure and is the most commonly used measure in scuba diving.

Absolute Pressure: Essentially the same as total pressure, and for exam purposes, you can treat them as identical.

Gauge Pressure: This uses 1 ATA/bar as a zero point, meaning it doesn’t include atmospheric pressure. It’s primarily used for measuring the usable pressure in diving cylinders.

8. Partial Pressure

As you learned in your Open Water course, the deeper you dive, the denser the air you breathe becomes. On the surface, air consists of 79% nitrogen and 21% oxygen. As you dive, these gases become denser, but their percentages remain the same. Instead of expressing these changes as percentages, we use partial pressure (pp). For example, at 10 meters, where the air is twice as dense, the partial pressure of nitrogen would be 1.58 pp, and oxygen 0.21 pp.

Remember, while the air becomes denser as you dive, the percentage composition stays the same. What changes is the partial pressure, not the proportion of gases.

9. Key Calculations

1 liter of salt water weighs 1.03 kg
1 liter of fresh water weighs 1.0 kg
Pressure increases by 1 ATM every 10 meters of salt water
Pressure increases by 1 ATM every 10.3 meters of fresh water
Sound travels 4 times faster underwater than in air
Water conducts heat 20 times faster than air
Also, remember these key laws:

Archimedes’ Principle relates to buoyancy.
Dalton’s Law refers to partial pressure.
Boyle’s Law relates to pressure and volume.