PHYS1055 How Things Work

Tutorial 5

18 April 2019

Intended learning outcomes:

1. Describe and discuss the physical principles

that are behind the household appliances

and the scientific issues in daily life

2. Demonstrate the knowledge to related

topics qualitatively

3. Express views in logical and effective ways

In this tutorial

1. Optics in life

2. Mechanics in life

3. Applications of electromagnetics

4. The power of sound

Physics extends well into everyday life, describing the motion, forces and energy of ordinary experience. In actions such as walking, driving a car or using a phone, physics is at work.

Look at the black spot on the left for at least 30 seconds, then quickly move your eyes to the black spot on the white area on the right. What do you see?

1. Optics in Life

Complementary colors

At this point, you will see color circles - blue and green. If you close your eyes quickly after looking at the picture on the left, this phenomenon will also occur. That is to say, if you look at a color for a long time, when you look away, you will see Complementary colors with the color you just saw.

Question: Why doctors wear green or blue operating clothes in the operating room?

1.1

It is not easy to cause visual tired by using light blue or green surgical clothes. Another reason is that when you stare at red or pink organs for a long time, your brain tends to get tired, and you can't see red clearly. Turning your eyes to green from time to time can keep your brain sensitive to red and ensure the precise position of the wound.

When taking pictures of TV, the camera flash and indoor lighting should be turned off so that the pictures can be taken more clearly. This is because:

1.2

Flashes, lights, and TV monitors are all light sources that emit light. The TV screen is a lens. When the light of flash and illumination illuminates the TV screen, light reflection occurs, which interferes with the transmission light emitted by the TV itself.

What color do astronauts see in outer space? Why?

1.3

Black, there is no 'atmosphere‘ (medium) in the space, hence there are no particles to scatter the lights coming from the stars, and so the visual blackness. The daytime sky is blue because light from the nearby Sun hits molecules in the Earth's atmosphere and scatters off in all directions. The blue color of the sky is a result of this scattering process.

2. Mechanics in life

Question: Under the same conditions, which is easier to break, an empty bottle or a bottle full of water?

2.1

When an empty bottle lands on the ground, it produces an elastic force on the bottle. The bottle deforms from outside to inside and is easy to break. A glass bottle filled with water and tightened the cap is not easy to compress, thus reducing the deformation of the bottle and making the bottle not easy to break.

Question: Why do we put stones or straw bags under the wheels when the car is stuck in the mud?

2.2

The main purpose is to increase friction. When the wheel is turning, the soil will be thrown out by the wheel. Something under the wheel can increase the friction to prevent the wheel from sinking deeper and deeper.

Question: Why the bottom of badminton is heavier than the upper?

2.3

This is to reduce the center of gravity of badminton, thus protecting the upper feathers.

G=mg

Life Communication Military

Compass Memory

Magnetic tape

Mobile TV

Bluetooth Electromagnetic gun

medical treatment, agriculture, material processing……

3. Applications of electromagnetics

electromagnetic oven

Question: The principle of electromagnetic oven

3.1

The surface of the electromagnetic oven is a heat-resistant ceramic plate. The alternating current generates magnetic field through the coil under the ceramic plate. When the magnetic line in the magnetic field passes through the bottom of the iron pot and stainless steel pot, it generates eddy current, which makes the bottom of the pot heat rapidly and achieves the purpose of heating food.

4.1

4. The power of sound

5 main fire extinguisher types: Water, Foam, Dry Powder, CO2 and Wet Chemical.

Question: The principle of fire extinguisher

5 main fire extinguisher types: Water, Foam, Dry Powder, CO2 and Wet Chemical.

The principle of fire extinguisher is to isolate combustible materials from air to achieve the effect of fire extinguishing.

Question: The working principle of this sound fire extinguisher

4.2

There are two forces in action here: 1) The sound waves can increase the velocity of the air, reduce

the oxygen concentration and thin the boundary layer where combustion occurs.

isolate combustible from air 2) The waves disrupt the surface of the fuel, which increases the width of the flame, disrupting combustion as the same amount of heat is spread over a wider area. decrease the heat density