Magnetism Handouts

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Lecture notes on magnetism for high school students

Text of Magnetism Handouts

  • Magnetism

    A phenomenon due to the presence of magnetic domains caused by the alignment of electron spins.

    This phenomenon of matter was first observed in Magnesia, Asia Minor (now Turkey) through large deposits of black stones known as lodestone (magnetite)

    Nature of Magnets

    Thales of Miletus first described that magnets is composed of two poles. This was described after suspending a piece of lodestone through a string. He noticed that the stone always point to the geographic north pole.

    Poles are regions in a magnet where the magnetic lines of force are strongest or concentrated.

    Magnetic lines of force always originate from the north pole of a magnet. These lines of force always face toward the south pole of a magnet.

    Like poles repel. Unlike poles attract. A monopole magnet does not exist.

    The Earth as a Magnet

    Our planet can also be considered as a large magnet.

    The magnetism is caused by the alignment of electron spins located at the inner core of the planet.

    The alignment is caused by the tremendous heat and pressure of the earths interior.

    The north pole of the earths magnetic field is south in nature. It is located 23.5 from the geographic north pole.

    Magnetic Field

    An area in space where magnetic flux (a collection of magnetic lines of force) exist.

    Magnetic Strength (B) This is a quantity that describes the

    distribution of the magnetic lines of force () across an area of space (A).

    This is expressed in tesla (T) or weber per square meter (Wb/m2). Tesla is the SI unit for magnetic strength named after Nikola Tesla.

    Mathematically, magnetic strength can be represented as:

    =

    =

    =

    = ()

    The mathematical statement implies that

    there is a direct relationship between magnetic strength and magnetic flux. On the other hand, magnetic strength varies inversely with area.

    The Link Between Electricity and Magnetism

    In 1821, Hans Christian Oersted, a Danish physicist discovered accidentally the deflection of a compass needle when placed near a current-carrying conductor.

    When he placed the wire around the periphery of the current-carrying wire, the compass needle follows a definite direction.

    When Oersted tried to change the direction of electric current passing the wire, the

  • direction to which the compass needle deflected also changed.

    Significance of Oersteds Observations

    A current-carrying conductor can be a direct source of magnetic field

    A temporary magnet that can be switched on and off is possible. We call this nowadays as electromagnet.

    How to determine the direction of the current-carrying conductors magnetic field?

    The Right-Hand Rule is a model used to determine the direction of the magnetic field produced by a current-carrying wire. It says that when you grasp a current-carrying wire with your right hand,

    o The thumb represents the direction of electric current

    o The other four fingers represent the direction of the magnetic field.

    What happens when a current-carrying wire when subjected to an external magnetic field?

    Since a current-carrying wire possesses a magnetic field of its own, it will interact with present magnetic field.

    If both like poles of each magnet interact, the wire will be squashed away by the external magnetic field.

    The strength of magnetic force is dependent on the following factors:

    o The length of the current-carrying wire exposed to an external magnetic field (L);

    o The amount of current passing through the wire (I); and

    o The strength of external magnetic field (B)

    This relationship can be expressed mathematically using the following equation:

    = F in newtons I in amperes L in meters B in tesla or N/Am

    This led to development of a device that converts electrical energy (the current passing through the wire surrounded by an external magnetic field) into mechanical energy (the movement of the wire).

    We call this as a motor. Examples of appliances with motors are

    electric fans, washing machines and farming equipment.

    How to determine the direction of the force applied on a wire subject to an external magnetic field?

    The left hand rule is used to determine the force applied on a current-carrying wire to an external magnetic field by using the left hand rule.

    This states that when a current-carrying

    conductor subjected to an external magnetic field is grasped by the left hand,

    o The thumb represents the direction of electric current,

    o The other four fingers represent the direction of the magnetic field, and

    o The palm represents that direction of the magnetic force.

    So we were able to prove that electricity can produce magnetic field. Is it now possible to do the reverse to convert magnetic field into electricity? Michael Faraday

    An English physicist who made a significant contribution in the field of electromagnetism.

    In 1835, he was able to determine how magnetic fields can create electric current.

    What was Faradays setup?

    A solenoid (loop of wire) is connected to a galvanometer (a device that detects the presence of electric current)

  • The galvanometers needle swings at a faster rate, more electric current is present.

    When a magnet is continuously moved inside the coil of wire, the needle of the galvanometer swings at a faster rate.

    Significance of Faradays Observation

    He was able to prove that a changing magnetic field is capable of producing electric current.

    The stronger the magnetic field in motion, greater electric current is generated.

    This phenomenon is known as electromagnetic induction.

    These findings led Faraday to invent a device that converts mechanical energy (movement of the magnet) into electrical energy (the continuous swinging the galvanometers needle).

    This device is known as the generator.

    Faradays first generator was known as the

    dynamo. A generator can be described as a motor

    working in reverse. Factors Affecting Electromagnetic Induction

    Strength of the magnet the stronger the magnet the more electric current is generated

    Number of loops in a wire more loops, more electric current is generated

    The rate (velocity) by which the source of magnetic field is moving the faster a

    magnetic field changes its direction, the more electric current is generated

    Transformer Not all appliances are designed to operate at a

    single voltage supply. Some may require lower or higher than the usual electric supply generated by a power plant.

    A device that can converts electric supply to higher or lower voltage is needed to serve this purpose.

    This device is known as the transformer.

    This is composed of an iron core with two

    coils primary and secondary. The primary coil is where electric supply

    enters the transformer. The secondary coil is where electricity at

    desired voltage exits the transformer. Each coil is composed of coils of wire. There are two kinds of transformer.

    o Step-up transformer Converts electricity from

    lower voltage to higher voltage (e.g. 110 V to 220 V

    Has more coils of wire at the secondary coil

    o Step-down transformer Converts electricity from

    higher voltage to lower voltage (e.g. 220 V to 110 V)

    Has less coils of wire at the secondary coil

    There is a mathematical relationship between number of coils and voltage in a transformer.

    =

    np is the number of loops in the primary coil ns is the number of loops in the secondary coil Ein is the voltage input Eout is the voltage output

    Power Generation, Transmission and Distribution

    Electricity is a very important commodity among consumers since our lives have heavily depended on this form of energy.

    Through the interactions of motors, generators and transformers, people were able to generate, transmit and distribute electricity to different households.

  • A power plant has a big generator. The source of magnetic field is connected to a turbine. This part should be continuously moved so that electromagnetic induction is possible.

    One way of moving the turbine is by letting steam pass through it. Because of the pressure exerted by steam, the turbine rotates.

    To turn water into steam, a fuel is needed. In our country, the principal fuel used to turn water into steam is coal because of its cheap price.

    The generated electricity is passed through high tension cables and sent through very high voltages.

    This is to avoid too much loss of energy as heat because of the resistance of wires. Higher voltage leads to higher current. Higher current means less resistance. Less resistance means minimized conversion of electrical energy into heat.

    The generated electricity is converted into different voltages through substations with large transformers.

    As the electricity passes through the household, it is then consumed to make some of our appliances and gadgets work.

    Problems Related to Power Generation

    Excessive burning of fossil fuels lead to too much emission of pollutants in the atmosphere (most of which are greenhouse gases which contribute to global warming).

    Our country imports fossil fuel to augment our need for electrical supply. Therefore, there must be measures done on how to minimize the cost of generating electricity by using alternative sources of energy li