Mastering Physics

If a study of physics or any other science is made in any detail, it is necessary to understand certain basic ideas and to know the meanings of the words used to express them. In this first chapter, we deal with some of them.

Most of you will have eaten walnuts at some time or other and possibly associate them with the Christmas season. But have you ever tried to crack one without a nutcracker just by squeezing it in your hand? Unless you find a particularly weak nut, it is very difficult indeed, if not impossible. However, if you take two nuts and squeeze them together as shown in Fig. 2.1, you will probably have little difficulty in cracking them. Can you explain this rather surprising effect? Why do you think drawing pins have sharp points, and knives with sharp blades cut better than knives with blunt blades? In order to answer these and similar questions, we must try to understand the idea and the meaning of pressure.

Does a mass of 3g added to a mass of 5g always produce a total mass of 8g? You may think this is a stupid question and say, ‘Of course it does’. You would be quite right. But does a force of 3N added to a force of 5N always produce a force of 8N? What would be the result if they pulled on a body in opposite directions? Two forces, one of 5N one way and the other of 3N the other way are equivalent to one single force of 2N in the first direction. The single force to which the other two added together are equivalent is termed the resultant force.

Velocity is the distance travelled in unit time in a particular direction, or in the specified direction (4.1) A common unit is metres per second, written m/s, in a stated direction.

Most of you at some time or other will have used a screwdriver to prize the lid off a paint tin. Does the length of the screwdriver matter? Would the lid of the paint tin come off just as easily if it were prised off using a coin?

A machine is any device which enables mechanical work to be done more easily. Very often this results in a small applied force being amplified into a large one, for example when a screwdriver is used to ‘lever’ a lid off a tin. Sometimes a small movement is amplified into a large one, as in a bicycle when a small pedal movement results in a much larger movement of the bicycle along a road.

Why is it that ships made of many tons of iron can float on water, while a piece of solid iron thrown into water will sink?

If you laze on a beach in the sunshine you are aware of the heat energy from the Sun falling on your body as you get hotter and hotter! This experience is an example of a basic physical fact; heat energy supplied to a body causes its temperature to rise.

The experiments illustrated in Fig. 10.1 demonstrate the expansion of (a) a rod, (b) water and (c) air, when each is heated. When the rod is heated, the straw rotates. Now a small rotation of the needle results in a much larger movement of the end of the straw. If the length of the rod increases by an amount equal to the circumference of the needle, the straw will make one complete rotation. Solids expand very little on heating compared with liquids or gases and hence some arrangement like that shown in Fig. 10.1a is needed to magnify the movement.

Many physical properties of matter can be discussed without regard to how matter is made up. But for a more complete discussion the nature of matter must be considered. The generally accepted theory is that:

Matter consists of myriads of tiny particles called molecules

The theory enables a wide variety of facts to be linked together, and we begin with the differences between solids, liquids and gases.

We are all familiar with the fact that heat energy can travel from one place to another. We can, for example, on sunny days feel the warmth from the Sun’s rays which have travelled millions of miles across space. In the winter, houses are kept warm by heat energy which travels from fires and radiators to other parts of the buildings.

If some ice cubes from a refrigerator are put in a beaker of water and the mixture stirred, the temperature will be 0°C. If the beaker is now put on a tripod and heated very gently with a bunsen burner it will be found that the temperature remains at 0°C until all the ice has melted. The temperature of the water then begins to rise and goes on rising until it reaches 100°C. Heat energy is still being supplied to the beaker from the bunsen burner but the temperature remains steady at 100°C (Fig. 13.1). What is happening to the heat energy being supplied by the bunsen burner when the temperature remains at 100°C?

A wave is a travelling disturbance which carries energy away from its source. Some waves, such as light waves and radio waves, can pass through an evacuated or empty space. In this way we receive energy from the Sun and other more distant stars.

A swimming pool viewed from a spring board vertically above its surface seems shallower than it really is; and to an observer looking across the pool from one side, it appears even shallower. The effect is caused by the refraction or bending of light.

The passage of light through a thin converging lens is illustrated in Fig. 17.1. The light is refracted at both surfaces of the lens, but the rays emerge just as if they had suffered a single refraction at a plane somewhere inside the glass. For the purposes of ray constructions we represent a thin lens by a single line (AB in Fig. 17.1), where the refraction is considered to take place.

The pinhole camera is a simple device for taking photographs. Light is admitted into a box through a small pinhole in one side, and the image is formed on a film on the opposite side of the box (Fig. 18.1). As in other cameras, the box has dull black internal surfaces to prevent reflected light forming secondary images on the film.

Light of many colours can be derived from white light, e.g. from sunlight. A band of colours or a rainbow appears when direct sunlight is refracted by raindrops in the sky; and a similar band of colours is produced when a ray of white light from a tungsten filament lamp is refracted on passing through a triangular glass prism (Fig. 19.1). The band of colours displayed on the white screen is called a spectrum. It is not a pure spectrum, however, as some of the colours in the middle of the band overlap one another.

All sources of sound have some part of them which is vibrating, for example, a violin string or the surface of a drum. Sound travels in the form of longitudinal waves, that is, molecules vibrate to and fro in the direction of travel of the sound (see Section 14.4). When sound is heard, energy is carried from the source of the waves to the ear of the listener; but the molecules of air in between do not move as a whole towards the listener (see Section 14.1). At any instant there are regions where the air is compressed (compressions), separated by regions where the air is rarefied (rarefactions or decompressions). Sound waves consist of a series of alternate compressions and rarefactions travelling away from a source at a certain speed determined by the nature of the medium in which they flow.

If you have played with magnets or magnetic toys, you will probably be familiar with two of their basic properties, namely the fact that they attract pieces of iron or steel (Fig. 21.1) and, secondly, if pivoted or sus-

pended, they always come to rest pointing in a definite direction. When pivoted the end which points towards the North of the Earth is called the North-seeking pole or simply the N-pole. The end which points South is called the South-seeking pole or the S-pole. (Two substances other than iron and steel which are attracted by magnets are cobalt and nickel.)

Conductors contain a large number of electrons which are free to move within them. When a conductor is connected to a battery, these free electrons, instead of moving in a random way, move in a particular direction.

Fig. 24.1 shows a diagram of a simple experiment to demonstrate that a magnetic field is produced by an electric current. It consists of a long piece of thick copper wire connected in series with a switch and a battery. The wire passes through a piece of cardboard on which is placed a number of plotting compasses.

Fig. 25.1 shows a power supply connected in series to a small piece of resistance wire XY, a variable resistor and an ammeter. The potential difference across the wire can be measured using the voltmeter.

Electromagnetic induction is the term used to describe the effect by which a voltage or e.m.f. can be induced in a conductor.

Radioactive substances have the ability to ionise the air surrounding them and it is this property that is used to distinguish them from materials which are not radioactive.