Notes1
TOPIC 1 – WAVES
TOPIC 2 – ELECTROMAGNETISM
TOPIC 3 – RADIOACTIVITY
TOPIC 4 – THERMIONIC EMISSION
TOPIC 5 – ELECTRONIC
TOPIC 6 – ELEMENTARY ASTRONOMY
TOPIC 7 – GEOPHYSICS
IMPORTANCE OF PHYSICS IN OUR DAILY LIFE – PART 4
19. Enhancing critical thinking
Doing experiments encourages learners to think critically and
analyse data before coming up with conclusions. This helps them solve complex
problems and make better decisions.
20. Developing observational skill
Experiments in Physics require students to observe and analyse
data to come up with conclusions. This helps them develop important
observational skills which will be useful throughout their lives.
21. Physics makes the modern world possible
Have you ever driven a car? Watched a show on a flat screen TV?
Called your best friend on your mobile phone? Without physics, you wouldn’t
have been able to do any of these things!
Even non-technical objects have been indirectly supported by
physics. For example, the mass production of clothes would be impossible
without physics. Completing a physics degree will allow you to learn more about
the history of some of your favourite objects.
22. Understand the Universe. Physics centres around understanding the
world. You will learn about matter and energy, study planets and galaxies and
discover the workings. and limitations of nature’s laws.
23. Stimulating creativity
Doing experiments in a lab can stimulate creativity as students
are encouraged to think outside the box when trying to solve problems. This
encourages students to be innovative and come up with unique solutions.
24. Increasing confidence. Experiments can help boost student confidence as they see
the results of their efforts first-hand. This will also motivate them to keep
exploring and developing their skills.
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RADIOACTIVITY
The Nucleus of an Atom
The Structure of the Nucleus of an Atom
Describe the structure of the nucleus of an atom
The word atom is derived from the Greek word ‘atomos’ which means indivisible. The Greeks concluded that matter could be broken down into particles too small to be seen. These particles were called atoms.
Atoms are composed of three type of particles: protons, neutrons, and electrons. Protons and neutrons are responsible for most of the atomic mass. The mass of an electron is very small (9.108 X 10-28 grams).
Both the protons and neutrons reside in the nucleus. Protons have a positive (+) charge, neutrons have no charge i.e they are neutral. Electrons reside in orbitals around the nucleus. They have a negative charge (-).
It is the number of protons that determines the atomic number, e.g., H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92) but neutron number may vary, so mass number (protons + neutrons) may vary.
The same element may contain varying numbers of neutrons; these forms of an element are called isotopes. The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different.
The Atomic Number, Mass Number and Isotopes of an Element and their Symbols
Explain the atomic number, mass number and isotopes of an element and their symbols
The atomic number of a chemical element (also known as its proton number) is the number of protons found in the nucleus of an atom of that element.Therefore it is identical to the charge number of the nucleus. It is conventionally represented by the symbol Z.
The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.
The atomic number, Z, should not be confused with the mass number, A.
Mass number is the number of nucleons, i. e the total number of protons and neutrons in the nucleus of an atom. —The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N (these quantities are always whole numbers).
Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the “relative isotopic mass”), is roughly (to within 1%) equal to the whole number A.
Isotopes
Isotopes are atoms with the same atomic number Z but different neutron numbers N, and hence different atomic masses.
A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element’s standard atomic weight.
Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century.The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different.
Some isotopes are radioactive-meaning they “radiate” energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into stable form. Another example is oxygen, with atomic number of 8 can have 8, 9, or 10 neutrons.
Forces Holding the Nucleus
Mention forces holding the nucleus
Stable and unstable atoms
There are forces within the atom that account for the behavior of the protons, neutrons, and electrons. Without these forces, an atom could not stay together.
Recall that protons have a positive charge, electrons a negative charge, and neutrons are neutral. According to the laws of physics, like charges repel each other and unlike charges attract each other. A force called the strong force opposes and overcomes the force of repulsion between the protons and holds the nucleus together.
The net energy associated with the balance of the strong force and the force of repulsion is called the binding energy. The electrons are kept in orbit around the nucleus because there is an electromagnetic field of attraction between the positive charge of the protons and the negative charge of the electrons.
In some atoms, the binding energy is great enough to hold the nucleus together. The nucleus of this kind of atom is said to be stable. In some atoms the binding energy is not strong enough to hold the nucleus together, and the nuclei of these atoms are said to be unstable. Unstable atoms will lose neutrons and protons as they attempt to become stable.
- Binding energy is the net energy that is the result of the balance with the strong force and the repulsive force, and this is the amount of energy that holds the nucleus together.
- A stable atom is an atom that has enough binding energy to hold the nucleus together permanently.
- An unstable atom does not have enough binding energy to hold the nucleus together permanently and is called a radioactive atom.
Natural Radioactivity
The Concept of Radioactivity
Explain the concept of radioactivity
Radioactive decay, also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy by emitting ionising radiation.
A material that spontaneously emits such radiation (alpha particles, beta particles, gamma rays and conversion electrons) is considered radioactive.
Radioactive decay is a stochastic (i.e. random) process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay.
The chance that a given atom will decay never changes, that is, it does not matter how long the atom has existed. For a large collection of atoms however, the decay rate for that collection can be calculated from their measured decay constants or half-lives. The half-lives of radioactive atoms have no known limits for shortness or length of duration.
Properties of the Radiations Emitted by Radio-active Substances
Describe properties of the radiations emitted by radio-active substances
There are many types of radioactive decay. A decay, or loss of energy, results when an atom with one type of nucleus, called the parent radionuclide (or parent radioisotope), transforms into an atom with a nucleus in a different state, or with a nucleus containing a different number of protons and neutrons. The product is called the daughter nuclide. In some decays, the parent and the daughter nuclides are different chemical elements, and thus the decay process results in the creation of an atom of a different element. This is known as a nuclear transmutation.
The Nuclear Changes due to the Emission of Alpha α, Betaβ and Gammaγ Radiations
Explain the nuclear changes due to the emission of Alpha α, Betaβ and Gammaγ radiations
Properties of Alpha Rays
- Alpha rays or alpha particles are the positively charged
- Alpha particles have the least penetration power. They cannot penetrate the skin but this does not mean that they are not dangerous.
- Since they have a great ionisation power, so if they get into the body they can cause serious damage. They have the ability of ionising numerous atoms a short distance. It is due to this reason that the radioactive substance that releases alpha particles needs to be handled with rubber gloves. It should not be inhaled, eaten or allowed near open cuts.
Properties of Beta Rays.
- Beta particles are highly energetic electronswhich are released from inside of a nucleus.
- They are negatively charged and have a negligible mass.
- Beta particles have a greater penetration power than the alpha particles and can easily travel through the skin.
- Though beta particles have less ionisation power than the alpha particles but still they are dangerous and so their contact with the body must be avoided.
Properties of Gamma Rays
- They have greatest power of penetration.
- They are the least ionizing but most penetrating and it is extremely difficult to stop them from entering the body.
- These rays carry huge amount of energy and can even travel through thin lead and thick concrete.
The Detection of α,β andγ Radiations
Explain the detection of α,β andγ radiations
Geiger Counter, with Geiger-Mueller (GM) Tube or Probe
A GM tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when radiation interacts with the wall or gas in the tube. These pulses are converted to a reading on the instrument meter.
If the instrument has a speaker, the pulses also give an audible click. Common readout units are roentgens per hour (R/ hr), milliroentgens per hour (mR/hr), rem per hour (rem/hr), millirem per hour (mrem/hr), and counts per minute (cpm).
GM probes (e.g., “pancake” type) are most often used with handheld radiation survey instruments for contamination measurements. However, energy-compensated GM tubes may be employed for exposure measurements.
Further, often the meters used with a GM probe will also accommodate other radiation-detection probes. For example, a zinc sulfide (ZnS) scintillator probe, which is sensitive to just alpha radiation, is often used for field measurements where alpha-emitting radioactive materials need to be measured.
Spark counter
This consists of a fine metal gauze mounted about a millimetre away from a thin wire.A voltage is applied between the two so that sparking takes place between them – this usually requires some 4000 – 5000 V. The voltage is then reduced until sparking just stops.
If an alpha-source is brought up close to the gauze it will ionise the air, and sparks will occur between the gauze and wire. With beta and gamma sources insufficient ions are usually produced for sparking to take place.The spark counter can be used to measure the range of alpha-particles.
Cloud chamber
The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionising radiation.
Rare picture shows in a single shot the 4 particles that we can detect in a cloud chamber: proton, electron, muon (probably) and alpha. In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol.
When a charged particle (for example, an alpha or beta particle) interacts with the mixture, the fluid is ionized. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation).
The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle’s track is broad and shows more evidence of deflection by collisions, while an electron’s is thinner and straight).
When any uniform magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law with two particles of opposite charge.
Other devices used to detect radiation include:
- Photographic film
- Bubble chamber
- Gold-leaf electroscope
Half-Life as Applied to a Radioactive Substance
Describe half-life as applied to a radioactive substance
Half life can be defined as the time taken for the number of nuclei in a radioactive material to halve. It can also be defined as the time taken for the count rate of a sample of radioactive material to fall to half of its starting level. It is simply the time taken for the radioactive material to decay by half.
The count rate is measured by using an instrument called a Geiger-Muller tube over a period of time. A Geiger-Muller tube detects radiations by absorbing the radiation and converting it into an electrical pulse which triggers a counter and is displayed as a count rate.
The release of radiation by unstable nuclei is called radioactive decay. This process occurs naturally and cannot be influenced by chemical or physical processes.
The release of radiation is also a random event and overtime the activity of the radioactive material decreases. It is not possible to predict when an individual nucleus in a radioactive material will decay.
But it is possible to measure the time taken for half of the nuclei in a radioactive material to decay. This is called the half life of radioactive material or radioisotope.
The Half-Life of a Radioactive Element
Determine the half-life of a radioactive element
An exponential decay process can be described by any of the following three equivalent formulas:
where
- N0is the initial quantity of the substance that will decay (this quantity may be measured in grams, moles, number of atoms, etc).
- N(t) is the quantity that still remains and has not yet decayed after a time
- t1⁄2is the half-life of the decaying quantity.
- τis a positive number called the mean lifetime of the decaying quantity.
- λis a positive number called the decay constant of the decaying quantity.
Where ln (2) is the natural logarithm of 2 (approximately 0.693).
By plugging in and manipulating these relationships, we get all of the following equivalent descriptions of exponential decay, in terms of the half-life:
The Application of a Natural Radioactive Substances
Identify the applications of a natural radioactive Substances
Medical Uses
Hospitals, doctors, and dentists use a variety of nuclear materials and procedures to diagnose, monitor, and treat a wide assortment of metabolic processes and medical conditions in humans. In fact, diagnostic x-rays or radiation therapy have been administered to about 7 out of every 10 Americans. As a result, medical procedures using radiation have saved thousands of lives through the detection and treatment of conditions ranging from hyperthyroidism to bone cancer.
The most common of these medical procedures involves the use of x-rays — a type of radiation that can pass through our skin. When x-rayed, our bones and other structures cast shadows because they are denser than our skin, and those shadows can be detected on photographic film. The effect is similar to placing a pencil behind a piece of paper and holding the pencil and paper in front of a light. The shadow of the pencil is revealed because most light has enough energy to pass through the paper, but the denser pencil stops all the light. The difference is that x-rays are invisible, so we need photographic film to “see” them for us. This allows doctors and dentists to spot broken bones and dental problems.
X-rays and other forms of radiation also have a variety of therapeutic uses. When used in this way, they are most often intended to kill cancerous tissue, reduce the size of a tumor, or reduce pain. For example, radioactive iodine (specifically iodine-131) is frequently used to treat thyroid cancer, a disease that strikes about 11,000 Americans every year.
X-ray machines have also been connected to computers in machines called computerized axial tomography (CAT) or computed tomography (CT) scanners. These instruments provide doctors with color images that show the shapes and details of internal organs. This helps physicians locate and identify tumors, size anomalies, or other physiological or functional organ problems.
In addition, hospitals and radiology centers perform approximately 10 million nuclear medicine procedures in the United States each year. In such procedures, doctors administer slightly radioactive substances to patients, which are attracted to certain internal organs such as the pancreas, kidney, thyroid, liver, or brain, to diagnose clinical conditions.
Academic and Scientific Applications
Universities, colleges, high schools, and other academic and scientific institutions use nuclear materials in course work, laboratory demonstrations, experimental research, and a variety of health physics applications. For example, just as doctors can label substances inside people’s bodies, scientists can label substances that pass through plants, animals, or our world. This allows researchers to study such things as the paths that different types of air and water pollution take through the environment. Similarly, radiation has helped us learn more about the types of soil that different plants need to grow, the sizes of newly discovered oil fields, and the tracks of ocean currents.
In addition, researchers use low-energy radioactive sources in gas chromatography to identify the components of petroleum products, smog and cigarette smoke, and even complex proteins and enzymes used in medical research.
Archaeologists also use radioactive substances to determine the ages of fossils and other objects through a process called carbon dating. For example, in the upper levels of our atmosphere, cosmic rays strike nitrogen atoms and form a naturally radioactive isotope called carbon-14. Carbon is found in all living things, and a small percentage of this is carbon-14. When a plant or animal dies, it no longer takes in new carbon and the carbon-14 that it accumulated throughout its life begins the process of radioactive decay. As a result, after a few years, an old object has a lower percent of radioactivity than a newer object. By measuring this difference, archaeologists are able to determine the object’s approximate age.
Industrial Uses
We could talk all day about the many and varied uses of radiation in industry and not complete the list, but a few examples illustrate the point. In irradiation, for instance, foods, medical equipment, and other substances are exposed to certain types of radiation (such as x-rays) to kill germs without harming the substance that is being disinfected and without making it radioactive. When treated in this manner, foods take much longer to spoil, and medical equipment (such as bandages, hypodermic syringes, and surgical instruments) are sterilized without being exposed to toxic chemicals or extreme heat. As a result, where we now use chlorine a chemical that is toxic and difficult-to-handle we may someday use radiation to disinfect our drinking water and kill the germs in our sewage. In fact, ultraviolet light (a form of radiation) is already used to disinfect drinking water in some homes.
Similarly, radiation is used to help remove toxic pollutants, such as exhaust gases from coal-fired power stations and industry. For example, electron beam radiation can remove dangerous sulphur dioxides and nitrogen oxides from our environment. Closer to home, many of the fabrics used to make our clothing have been irradiated (treated with radiation) before being exposed to a soil-releasing or wrinkle-resistant chemical. This treatment makes the chemicals bind to the fabric, to keep our clothing fresh and wrinkle-free all day, yet our clothing does not become radioactive. Similarly, nonstick cookware is treated with gamma rays to keep food from sticking to the metal surface.
The agricultural industry makes use of radiation to improve food production and packaging. Plant seeds, for example, have been exposed to radiation to bring about new and better types of plants. Besides making plants stronger, radiation can be used to control insect populations, thereby decreasing the use of dangerous pesticides. Radioactive material is also used in gauges that measure the thickness of eggshells to screen out thin, breakable eggs before they are packaged in egg cartons. In addition, many of our foods are packaged in polyethylene shrink-wrap that has been irradiated so that it can be heated above its usual melting point and wrapped around the foods to provide an airtight protective covering.
All around us, we see reflective signs that have been treated with radioactive tritium and phosphorescent paint. Ionizing smoke detectors, using a tiny bit of americium-241, keep watch while we sleep. Gauges containing radioisotopes measure the amount of air whipped into our ice cream, while others prevent spillover as our soda bottles are carefully filled at the factory.
Engineers also use gauges containing radioactive substances to measure the thickness of paper products, fluid levels in oil and chemical tanks, and the moisture and density of soils and material at construction sites. They also use an x-ray process, called radiography, to find otherwise imperceptible defects in metallic castings and welds. Radiography is also used to check the flow of oil in sealed engines and the rate and way that various materials wear out. Well-logging devices use a radioactive source and detection equipment to identify and record formations deep within a bore hole (or well) for oil, gas, mineral, groundwater, or geological exploration. Radioactive materials also power our dreams of outer space, as they fuel our spacecraft and supply electricity to satellites that are sent on missions to the outermost regions of our solar system.
Nuclear Power Plants
Electricity produced by nuclear fission — splitting the atom — is one of the greatest uses of radiation. As our country becomes a nation of electricity users, we need a reliable, abundant, clean, and affordable source of electricity. We depend on it to give us light, to help us groom and feed ourselves, to keep our homes and businesses running, and to power the many machines we use. As a result, we use about one-third of our energy resources to produce electricity.
Electricity can be produced in many ways — using generators powered by the sun, wind, water, coal, oil, gas, or nuclear fission. In America, nuclear power plants are the second largest source of electricity (after coal-fired plants) — producing approximately 21 percent of our Nation’s electricity.
The purpose of a nuclear power plant is to boil water to produce steam to power a generator to produce electricity. While nuclear power plants have many similarities to other types of plants that generate electricity, there are some significant differences. With the exception of solar, wind, and hydroelectric plants, power plants (including those that use nuclear fission) boil water to produce steam that spins the propeller-like blades of a turbine that turns the shaft of a generator. Inside the generator, coils of wire and magnetic fields interact to create electricity. In these plants, the energy needed to boil water into steam is produced either by burning coal, oil, or gas (fossil fuels) in a furnace, or by splitting atoms of uranium in a nuclear power plant. Nothing is burned or exploded in a nuclear power plant. Rather, the uranium fuel generates heat through a process called fission.
Nuclear power plants are fueled by uranium, which emits radioactive substances. Most of these substances are trapped in uranium fuel pellets or in sealed metal fuel rods. However, small amounts of these radioactive substances (mostly gases) become mixed with the water that is used to cool the reactor. Other impurities in the water are also made radioactive as they pass through the reactor. The water that passes through a reactor is processed and filtered to remove these radioactive impurities before being returned to the environment. Nonetheless, minute quantities of radioactive gases and liquids are ultimately released to the environment under controlled and monitored conditions
The U.S. Nuclear Regulatory Commission (NRC) has established limits for the release of radioactivity from nuclear power plants. Although the effects of very low levels of radiation are difficult to detect, the NRC’s limits are based on the assumption that the public’s exposure to man-made sources of radiation should be only a small fraction of the exposure that people receive from natural background sources.
Experience has shown that, during normal operations, nuclear power plants typically release only a small fraction of the radiation allowed by the NRC’s established limits. In fact, a person who spends a full year at the boundary of a nuclear power plant site would receive an additional radiation exposure of less than 1 percent of the radiation that everyone receives from natural background sources. This additional exposure, totaling about 1 millirem (a unit used in measuring radiation absorption and its effects), has not been shown to cause any harm to human beings.
In agriculture
Radioisotopes are used to induce mutations in plants in order to develop superior varieties that are harder and more resistant to diseases.
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ELECTROMAGNETISM
Magnetic Fields due to a Current-carrying Conductor
How Electric Current Produce a Magnetic Field
Explain how electric current produces a magnetic field
Electromagnetism is the effect produced by the interaction of an electric current with a magnetic field. The interaction can result in a force on the conductor carrying the current.
If, on the other hand, a force is applied to a conductor (with no current) in a magnetic field the resulting movement can result in a current being noticed in the conductor.
When the switch is closed an electric current flows through the conductor. The electric current generates magnetic field around the conductor. This will cause a deflection on the compass needle. The magnetic field around a current-carrying conductor can be shown by means of magnetic field lines.
The Pattern of the Magnetic Field Lines around a Straight Conductor
Identify the pattern of the magnetic field lines around a straight conductor
The magnetic field pattern is usually given in a plain view. In the plain view, the conductor is represented by a circle. A dot in circle shows that the current is coming out of the plane. A cross the circle shows that the current is moving into the plane.
The strength of the magnetic field on the magnitude of the electric current. The higher the current, the stronger the magnetic field, and therefore the greater the deflection. The strength of the magnetic field decreases as you move further from the conductor. There will be less deflection as the compass is drawn from the current-carrying conductor.
The Direction of Magnetic Field around a Current-Carrying Conductor
Determine the direction of magnetic field around a current-carrying conductor
The direction of the field is determined by applying two rules, these are:
- Right-hand Grip Rule
- Maxwell’s cork screw rule
Right-hand Grip Rule
The Right-hand Grip Rule can be applied to a straight conductor or a solenoid-carrying an electric current. For a straight conductor, the Right-hand Grip Rule can be stated as:
“Imagine the wire carrying the current is gripped by the right hand with the thumb pointing in the direction of the conventional current (from positive to negative), the fingers will curl around the wire pointing in the direction of the magnetic field.”
For a solenoid, the Right-hand Grip Rule states that:
“When you wrap your right hand around a solenoid with your fingers pointing in the direction of convectional current, your thumb point in the direction of the magnetic North pole.”
A solenoid is a long coil containing a large number of close turns of illustrated copper wire.
Maxwell’s –Right –hand screw rule states that:
“If a right-hand screw advances in the direction of the current, then the direction of rotation of the screw represents the direction of the magnetic field due to the current.”
The Presence and Direction of a Force on a Current carrying Conductor in a Magnetic Field
Determine the presence and direction of a force on a current carrying Conductor in a magnetic field
The direction of the force on a current-carrying conductor in a magnetic field can be determined using Fleming’s Left –Hand Rule.
Fleming’s Left –Hand Rule states that:
“If you hold the index finger, the middle finger and the thumb of your left hand mutually perpendicular to each other so that the index finger points in the direction of the magnetic field and the middle finger points in the direction of current in the conductor, then the thumb will point in the direction of the force acting on the conductor.”
The Direction of Force due to two Current carrying Conductors when the Current Flowing in the Same or Opposite Direction
Determine the direction of force due to two current ’82air-carrying conductors when the current flowing in the same or opposite direction
If two current-carrying conductors are placed side by side close to one another, the currents in the conductors will interact with the magnetic fields produced by the two conductors. A force may result depending on the direction of the two currents.
When the currents are flowing in opposite directions, the conductors repel one another. When the currents are flowing in the same direction, the conductors attract one another, the conductors attract each other.
When the currents flow in the same direction, the magnetic field between the conductors cancel out, thus reducing the net field. However, on the outside, the magnetic fields add up, thus increasing the net field. Therefore, the magnetic field is weaker between the conductors that on the outside. The resultant force pushes the conductor towards each other.
When the currents are in the opposite directions, the fields between the conductors add up, while they cancel out on the outside. The field between them is stronger than on the outside. The resultant force is toward the outside of each conductor, hence repulsion.
ElectroMagnetic Induction
The Concept of Electromagnetic Induction
Explain the concept of electroMagnetic induction
Electromagnetic Induction is the production of e. m. f whenever there is a change in the magnetic flux linking a conductor”. The e. m. f produced is called induced e. m. f and the resulting current induced current.
The Laws of ElectroMagnetic Induction
State the laws of electroMagnetic induction
Lenz’s Law
This explains the direction of the induced e. m. f and it states that: “The direction of the induced e. m. f is such that the resulting induced current flows in such a direction that it opposes the change that causes it.”
Faraday’s Law.
It relates the magnitude of induced e. m. f and the rate of change of the magnetic flux linking the conductor. The magnitude of the induced e. m. f depends on:
- the strength of the magnetic field.
- the rate of change of the magnetic flux(speed of motion)
- the area of the conductor that is in the magnetic field.
Faraday’s law states that: “The e. m. f induced in the conductor in a magnetic field is proportional to the rate of change of magnetic flux linking the conductor.”
The Concepts of Self and Mutual Induction
Explain the concepts of self and mutual induction
When the current flowing through a conductor varies it creates a varying magnetic field that cuts across the conductor itself.
This results to self-induced e. m .f in the conductor that is opposite in direction to the original e. m. f. This voltage, usually referred to as back e. m .f, tends to limit or reverse the original current.
If the original current is increasing, then the induced current subtracts from it and then measured current is smaller than it would be if no self-induced magnetic field was produced in the conductor. If original current is decreasing, then the original current adds to it and the measured current is greater than it would be if self-induced magnetic field was produced in the conductor. This process is called self-induction since the changing current creates a back e. m. f in itself.
Consider a coil of wire wrapped around a cardboard tube. When the coil is connected to a battery a current flows in the coil producing a magnetic field as shown below.
If the current in the coil begins to increase, the magnetic flux increases. This induces an e. m. f that opposes the battery resulting in a back current that impedes the increase in current. If the current in the coil decreases, the magnetic flux decreases. This induces an e. m .f that adds to the battery resulting in an induced current that impedes the decrease in current.
If we place two coils near each other, a varying current in one coil will induce a current in the other. This is called mutual induction. The coil with a changing current is referred to as the primary coil while that in which a current is induced is the secondary coil.
The e.m.f induced in the secondary coil is proportional to the rate of change of the current in the primary coil.
The Mode of Action of Induction Coil
Describe the mode of action of induction coil
Is an electrical device consisting of two coils, the primary coil and the secondary coil, wound one over the other on an iron core.
It is used to produce high-voltage alternating current from low-voltage direct current. The primary coil is made up of tens or hundreds of turns of coarse wire while the secondary coil consists of thousands of turns of fine wire. The secondary coil is wound on top of the primary coil.
Mode of action
An induction coil produces high voltage in its secondary coil by electromagnetic induction. The direct current in the primary is switched on and off by a make-and-break mechanism. This produces change in current and magnetic field which is necessary for electromagnetic induction to occur in the secondary coil.
When the current in the primary coil is switched on, the induced magnetism in the iron core attracts the soft-iron armature.The moving iron armature opens a gap between the two contacts which breaks the primary coil circuit. This switches off the current. As the induced magnetism fades away, the armature springs back, closes the contacts and completes the circuit again. This allows the current to flow in the primary coil again. This cycle of events is repeated automatically.
The induced a.m. is very large, usually in the order of hundreds of kilovolts (kV).Such a high voltage is achieved because of two things:
- The secondary coil has a large number of turns compared to the primary coil.
- The rapid change in the primary current when it is switched on and off causes a rapid in the magnetic field through the secondary coil.
Applications of the induction coil
- It is used in the ignition system of internal combustion engines.
- It is used to trigger the flash tubes used in cameras and strobe lights.
- It is also used in wireless telegraphy.
The Mode of Action of a.c and d.c Generator
Describe the mode of action of a.c and d.c generator
A generators is the device which produces electricity on the basis of electromagnetic induction by the continuous motion of either a coil or a magnet.
A.C Generator or alternator
An arc generator utilizes Faraday’s law of induction, spinning a coil at a constant rate in a magnetic field to induce an oscillating a.m.
The arc generator consists of an armature made up of several turns of insulated wire wound on a soft-iron core. The armature revolves freely on an axis between the poles of a powerful magnet, which provides a strong magnetic field. Two slip rings are connected to the ends of the armature and two carbon brushes rest on the slip rings.
When the coil is vertical, no cutting of the magnetic lines of force takes place although the number of lines linking the coil is maximum. The rate of change magnetic flux is zero and as a result, no a.m. is induced in the coil.
When the armature is parallel to the magnetic field, the rate of change of magnetic flux is maximum and the motion of the coil is perpendicular to the magnetic field, hence an a.m. is induced along the sides of the coil.
After a 180° turn, starting from the vertical position, the sides of the loop interchange and the current in the loop is reversed. This means that the a.m. is positive for one half of the cycle and negative for the half. The maximum induced a.m. is at 90° rotation from the vertical position and the minimum is at 270° rotation. If there is an external circuit, the current through it would also have a maximum value at 90° and minimum at 270°.
This kind of current is called an alternating current and the corresponding a.m. is the alternating e.m.f.The number of cycles produced per second is called the frequency of the arc. The arc obtained is led to an external circuit through the slip rings and the carbon brushes.
D.C generator
It is made by replacing the slip rings in the arc generator with a commentator. Each half of the commentator ring is called a commentator segment and is insulated from the other half. Each end of the rotating loop of the wires connected to a commentator segment. Two carbon brushes connected to the outside circuit rest against the rotating commentator.
In the deck generator, the commentator rotates with the loop of wire, just as the slip rings do with the rotor of an arc generator.
When the loop is rotated in the magnetic field, the induced e.m.f is still in alternating form. However after rotation of 180° instead of the current reversing, the connections to the external circuit are reversed so that the current direction in the external circuit remains the same.
The output of a d.c generator is shown below.
Note: The lower half of the cycle is not cut off but is reversed.
Simple Step-up and Step-down Transformer
Construct a simple step-up and step-down transformer
A transformer is the device that uses mutual induction between two coils to convert an a.c across one coil to a larger or smaller a.c across the other coil.
A transformer is made up of two coils, each with a different number of loops linked by an iron core so that the magnetic flux from one passes through the other. When the flux generated by one coil changes the flux passing through the other will change, inducing a voltage in the second coil.
The coil that provides the flux that is the coil connected to the a.c power source is known as the primary coil while the coil in which the voltage is induced is known as the secondary coil.
When the number of turns in the primary coil (N) is lower than the number in the secondary coil (N),the secondary voltage will be lower than the secondary voltage. This is called the step-down transformer. The opposite of this is called the step-up transformer.
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