C. Matter and Energy · Fluids

What Affects How Fluids Flow?

C2.8 C2.1 A1.2 A1.5About 45 minutes

Learning goals

I can describe the factors that affect the flow of fluids through pipes and tubes.
I can explain everyday flow problems, like a kinked hose or a clogged drain.
I can analyse data from a flow investigation and draw a conclusion.

Words to know

flow
The movement of a fluid from one place to another.

diameter
The distance across a circle through its centre. For a pipe, how wide it is inside.

pressure difference
A difference in pressure between two places. Fluids flow from higher pressure to lower pressure.

obstruction
Something that blocks or partly blocks a flow, like a clog in a drain.

friction
A force that resists motion where two things rub. Fluids rub against the walls of a pipe.

The garden hose mystery

You're filling a wading pool in July. Suddenly the water slows to a trickle. You follow the hose and find a kink in it. You straighten it, and the water gushes out again.

Nothing changed at the tap. So what controls how fast a fluid flows? In the last lesson you learned that viscosity matters. Today you'll find out that the path the fluid takes and the push behind it matter just as much.

A green garden hose with a sharp kink in it, letting only a trickle of water into a blue wading pool. — The kink makes the path narrow, so only a trickle of water gets through even though nothing changed at the tap. AI-generated image

Fluids flow from high pressure to low pressure

A fluid moves only when something pushes it. That push comes from a pressure difference: fluids flow from where the pressure is higher to where it is lower.

A pump (like your heart, or the pump in a well) makes a pressure difference.
Gravity makes a pressure difference too. Many towns store water in a tall water tower. The water's weight pushes it through the pipes and out of your taps. The higher the water, the bigger the push.
When you drink with a straw, you lower the air pressure in your mouth. The higher air pressure on the drink pushes it up the straw.

A bigger pressure difference makes a faster flow.

Gravity makes the push. The weight of the water high in the tower makes a high pressure in the pipes, so water flows out of the tap, where the pressure is lower. (Not to scale.)

Six factors that affect flow

Pressure difference (the push). Bigger push → faster flow.
Viscosity of the fluid. Higher viscosity → slower flow. Honey flows through a straw much more slowly than water.
Temperature. For liquids, warmer usually means lower viscosity → faster flow.
Diameter of the pipe. A wider pipe lets much more fluid through each second than a narrow one. Fire hoses are wide so they can deliver lots of water fast.
Length of the pipe. The fluid rubs against the pipe walls (friction). A longer pipe means more friction → slower flow.
Obstructions, bends and rough walls. Kinks, sharp bends, clogs and rough inner surfaces all slow the flow. Hair in a shower drain is an obstruction.

With the same push and the same length, the wide pipe carries much more water each second than the narrow pipe. (Not to scale.)

Worked example

Let's analyse flow data

Maya pokes one hole in the bottom of each of two identical 500 mL plastic bottles. Hole A is about 3 mm across. Hole B is about 6 mm across. She fills each bottle with 500 mL of water and times how long it takes to drain. Bottle A takes 80 s. Bottle B takes 22 s. What can she conclude?

Step 1. What changed? Only the hole diameter (the independent variable). Same bottles, same volume of water, same temperature (the controlled variables).
Step 2. Calculate the flow rates. A: 500 mL ÷ 80 s ≈ 6.3 mL/s. B: 500 mL ÷ 22 s ≈ 22.7 mL/s.
Step 3. Compare. 22.7 ÷ 6.3 ≈ 3.6. Doubling the diameter made the water flow more than three times as fast.
Step 4. Explain. A wider hole has a much bigger opening area, so more water can get through at once.
Step 5. Check reliability. One trial each isn't much. She should repeat each test at least twice and average the times.

A wider opening gives a much faster flow. Doubling the hole's diameter made the flow rate about 3.6 times as large (6.3 mL/s → 22.7 mL/s).

Situation	Factor	Effect on flow
Kinked garden hose	obstruction / narrowing	slower
Hair clog in a drain	obstruction	slower
Cold motor oil on a January morning	temperature and viscosity	slower
Water tower built on a hill	bigger pressure difference (height)	faster
Two straws taped end to end	longer tube (more friction)	slower
Fire hose instead of a garden hose	wider diameter	faster

Everyday examples of factors that affect flow.

Flow in pipelines

Some oil from Alberta's oil sands, called bitumen, is so viscous that it can't flow through a pipeline on its own. Companies mix it with a thinner, lighter fluid to lower its viscosity so it can be pumped. This diluted bitumen is sometimes called dilbit. You'll meet it again in the lesson Fluid Spills: Impacts and Cleanup.

Just for fun

I was going to tell you a joke about a kinked garden hose...

...but it just didn't flow.

Hands-on activity

Straw lab: what slows a flow?

You need: water, a glass, a regular drinking straw, a thinner straw or coffee stirrer (or a regular straw pinched almost closed with a clothes peg), tape, a second regular straw, and a timer. Use clean straws and your own glass.

Pour the same amount of water (for example 100 mL) into the glass for each trial.
Trial 1: drink the water through one regular straw. A helper times how long it takes.
Trial 2: drink through the thin straw or stirrer.
Trial 3: tape two regular straws end to end to make a longer straw, and drink through it.
Optional Trial 4: try 100 mL of a thicker drink, such as a smoothie or chocolate milk, through one regular straw.
Try to suck with the same effort each time. Record the times, calculate the flow rates (mL/s) and rank the trials.
Communicate: write a conclusion that names each factor you tested (diameter, length, viscosity).

What to look for: the thin straw is slowest (narrow diameter), the long straw is slower than the regular straw (more length, more friction), and the thick drink is slower than water (higher viscosity).

Optional with a grown-up: a grown-up pokes three small holes up the side of an empty plastic bottle with a pushpin. Fill it with water over the sink. The water from the lowest hole comes out with the most force, because water deeper down is under more pressure.

Safety: use only food-safe drinks and your own clean straws. A grown-up does any hole-poking.

Your turn

Fluids flow from where the pressure is ___ to where it is ___.

Name two things that can create the pressure difference that makes a fluid flow.

How does the diameter of a pipe affect flow?

Why does water flow more slowly through a very long hose than a short one?

Your bathroom sink drains slowly. Which factor is probably the cause?

Why is a town's water tower built high up?

More practice: warm-up

Which flows faster through a straw: water or honey?

True or false: bends and kinks make fluids flow faster.

Which carries more water each second: a wide fire hose or a thin garden hose (same push)?

What do we call something that blocks a flow?

Does warm syrup or cold syrup flow faster through a tube?

More practice: on your own

A plumber replaces an old, narrow, rusty pipe with a new, wider, smooth one. Name two reasons the water will flow better.

500 mL drains from a bottle in 40 s. After the bottle cap's hole is made wider, 500 mL drains in 25 s. Find both flow rates.

Why does it get harder to drink a thick milkshake through a long, thin straw than a short, wide one?

In January, an outdoor pump moving vegetable oil at a farm slows down. Explain why.

Name the independent and dependent variables in the straw lab when you compare the thin straw and the regular straw.

Challenge

A cottage gets water from a tank on a hill. The owners move the tank higher up the hill and also replace a long, winding hose with a shorter, straighter one. Predict the effect on flow and explain both changes.

Why do companies mix very viscous bitumen with a lighter fluid before pumping it through a pipeline? Name one cost or risk of doing this.

Data from a test: 3 mm hole → 6 mL/s; 6 mm hole → 23 mL/s; 9 mm hole → 51 mL/s. Describe the pattern. Is the relationship between diameter and flow rate a straight-line (proportional) one? Explain.

Check your understanding

Quiz

Fluids flow from...

Which change would make water flow faster through a pipe?

A kinked garden hose slows the water mainly because...

Why is water flow stronger from a tank placed high on a hill?

Two straws are taped end to end. Compared with one straw, the flow is...

Which factor does not affect how fast a liquid flows through a pipe?

For parents and teachers

Big idea (C2.8): the flow of a fluid depends on the pressure difference (the push), the fluid's viscosity and temperature, and the pipe's diameter, length, bends, roughness and obstructions. The straw lab practises controlling variables (A1.2) and communicating results (A1.5).

How to help: point out flow problems and fixes at home: a slow drain, a kinked hose, a low-flow showerhead, a clogged vacuum hose (air is a fluid too!). Ask your child to name the factor.

Common difficulties:

Thinking the pipe's width only matters a little. In fact, a small change in diameter makes a big change in flow, as the challenge data shows.
Forgetting that gases flow too. Air in a vacuum cleaner hose, a furnace duct or a drinking straw follows the same rules.
Mixing up viscosity and flow rate. Viscosity is a property of the fluid. Flow rate depends on the fluid and the pipe and the push.

Safety: use clean, food-safe drinks and straws. You poke any holes in bottles.