Dickson Phy

ELECTRONICS

ELECTRONICS.

Conductors, insulators and semi-conductors

i) An insulatoris a material or object which resists flow of heat (thermal insulator) or electrical charges (electrical insulators). Examples are paraffin, wood, rubber, plastics etc.
ii) Conductorsare materials that contain free electrons which carry an electrical charge from one point to another.

Examples are metals and non-metals like carbon, graphite etc.

iii) Semi-conductors are materials or objects which allow the flow of electrical heat or energy through them under certain conditions i.e. temperature. Examples are germanium, silicon, cadmium sulphide, gallium arsenide etc.

RADIOACTIVITY

RADIOACTIVITY.

Introduction

Radioactivity was discovered by Henri Becquerel in 1869. In 1898, Marie and Pierre Curie succeeded in chemically isolating two radioactive elements, Polonium (z=84) and Radium (z= 88).

Radioactivity or radioactive decay is the spontaneous disintegration of unstable nuclides to form stable ones with the emission of radiation.

Unstable nuclides continue to disintegrate until a stable atom is formed.

Alpha (α) and beta (ϐ) particles are emitted and the gamma rays (ϒ) accompany the ejection of both alpha and beta particles.

PHOTOELECTRIC EFFECT

PHOTOELECTRIC EFFECT.

Photoelectric effect was discovered by Heinrich Hertz in 1887. Photoelectric effect is a phenomenon in which electrons are emitted from the surface of a substance when certain electromagnetic radiation falls on it.

Metal surfaces require ultra-violet radiation while caesium oxide needs a visible light i.e. optical spectrum (sunlight).

X-RAYS

X-RAYS.

X-rays were discovered by a German scientist named Roentgen in 1985. They can pass through most substances including soft tissues of the body but not through bones and most metals. They were named X-rays meaning ‘unknown rays’.

CATHODE RAYS

CATHODE RAYS.

What is cathode rays?. These are streams of electrons emitted at the cathode of an evacuated tube containing an anode and a cathode.

Production of cathode rays

They are produced by a set up called a discharge tube where a high voltage source usually referred to as extra high tension (EHT) supply connected across a tube containing air at low pressure thereby producing a luminous electron discharge between the two brass rods placed at opposite ends of the tube. These electron discharges are called cathode rays which were discovered by J.J Thomson in the 18th century.

MAINS ELECTRICITY

MAINS ELECTRICITY.

Sources of mains electricity

Mains electricity comes from a power station and its current is the alternating current which can either be stepped up or down by a transformer.

A.c is produced when a conductor is rotated in a magnetic field or when a magnetic field is rotated near a conductor.

This method is known as electromagnetic induction. The source of energy for rotating the turbine is the actual source of electrical energy.

Most of the electricity in East Africa is generated from water.

ELECTROMAGNETIC INDUCTION

ELECTROMAGNETIC INDUCTION.

Electromagnetism is the effect resulting from the interaction between an electric current and a magnetic field. This effect brings about induced electromagnetic force (e.m.f) and the resulting current is called induced current.

Experiments on electromagnetic induction

When the wire is moved up the galvanometer deflects in one direction then the opposite direction when moved downwards.

When moved horizontally or held in a fixed position there is no deflection in the galvanometer.

This shows that e.m.f is induced due to the relative motion of the wire or the magnet.

ELECTROMAGNETIC SPECTRUM

ELECTROMAGNETIC SPECTRUM.

What is Electromagnetic?. Electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength.

It includes visible light, ultra-violet rays, microwaves, X-rays, radio waves and gamma rays.

Electromagnetic waves are produced when electrically charged particles oscillate or change energy in some way. The waves travel perpendicularly to both electric and magnetic fields.

FLOATING AND SINKING

FLOATING AND SINKING.

Any object in a liquid whether floating or submerged experiences an upward force from the liquid; the force is known as upthrust force.

Upthrust force is also known as buoyant force and is denoted by letter ‘u’.

Archimedes’ principle

Floating and Sinking

Archimedes, a Greek scientist carried out first experiments to measure upthrust on an object in liquid in the third century. Archimedes principle states that ‘When a body is wholly or partially immersed in a fluid (liquid/ gas), it experiences an upthrust equal to the weight of the displaced fluid”.

UNIFORM CIRCULAR MOTION

UNIFORM CIRCULAR MOTION.

Introduction

Circular motion is the motion of bodies travelling in circular paths. Uniform circular motion occurs when the speed of a body moving in a circular path is constant. This can be defined as motion of an object at a constant speed along a curved path of constant radius. When acceleration (variation of velocities) is directed towards the centre of the path of motion it is known as centripetal acceleration and the force producing this centripetal acceleration which is also directed towards the centre of the path is called centripetal force.

THIN LENCES

THIN LENCES.

What is a Len?. A lens is conventionally defined as a piece of glass which is used to focus or change the direction of a beam of light passing through it.

They are mainly made of glass or plastic. Lens are used in making spectacles, cameras, cinema projectors, microscopes and telescopes.

Types of thin lenses.

A lens which is thicker at its centre than at its edges converges light and is called convex or converging lens.

A lens which is thicker at its edges than at its centre diverges light and is known as concave or diverging lens.

THE GAS LAWS

Post Views: 0 THE GAS LAWS Pressure law: This law states that “the pressure of a fixed mass of a… Read More »THE GAS LAWS

QUANTITY OF HEAT

QUANTITY OF HEAT.

What is heat?. Heat is a form of energy that flows from one body to another due to temperature differences between them.

Heat capacity

Heat capacity is defined as the quantity of heat required to raise the temperature of a given mass of a substance by one degree Celsius or one Kelvin. It is denoted by ‘C’.

Heat capacity, C = heat absorbed, Q / temperature change θ.

The units of heat capacity are J / 0C or J / K.

HEATING EFFECT OF AN ELECTRIC CURRENT

HEATING EFFECT OF AN ELECTRIC CURRENT.

When current flows, electrical energy is transformed into other forms of energy i.e. light, mechanical and chemical changes.

Factors affecting electrical heating

Energy dissipated by current or work done as current flows depends on,

a) Current
b) Resistance
c) Time

ELECTROSTATICS

ELECTROSTATICS.

Electric fields

An electric field is the space around a charged body where another charged body would be acted on by a force. These fields are represented by lines of force.

This line of force also called an electric flux line points in the direction of the force.

WAVES

WAVES.

Properties of waves

Waves exhibit various properties which can be conveniently demonstrated using the ripple tank.

It consists of a transparent tray filled with water and a white screen as the bottom. On top we have a source of light.

A small electric motor (vibrator) is connected to cause the disturbance which produces waves.

The wave fronts represent wave patterns as they move along.

CURRENT ELECTRICITY

CURRENT ELECTRICITY.

Electric potential difference and electric current

Electric current

Electric potential difference (p. d) is defined as the work done per unit charge in moving charge from one point to another. It is measured in volts.

Electric current is the rate of flow of charge. P. d is measured using a voltmeter while current is measured using an ammeter. The SI units for charge is amperes (A).

ENERGY, WORK, POWER AND MACHINES

ENERGY, WORK, POWER AND MACHINES.

Energy

This is the ability to do work.

Forms of energy.

Chemical energy: – this is found in foods, oils charcoal firewood etc.
Mechanical energy:- there are two types;
Potential energy – a body possesses potential energy due to its relative position or state
Kinetic energy – energy possessed by a body due to its motion i.e. wind, water

iii. Wave energy – wave energy may be produced by vibrating objects or particles i.e. light, sound or tidal waves.

NEWTON LAWS OF MOTION

NEWTON’S LAWS OF MOTION.

Newton’s first law (law of inertia)

This law states that “A body continues in its state of rest or uniform motion unless an unbalanced force acts on it”.

The mass of a body is a measure of its inertia. Inertia is the property that keeps an object in its state of motion and resists any efforts to change it.

REFRACTION OF LIGHT

REFRACTION OF LIGHT.

Introduction

Refraction is the change of direction of light rays as they pass at an angle from one medium to another of different optical densities.

Exp. To investigate the path of light through rectangular glass block.

Apparatus: – soft-board, white sheet of paper, drawing pins (optical), rectangular glass block.