WBCS Preliminary(Physics): Magnetism and Electricity

1. MAGNETISM:

   • The word magnet is derived from the name of an island in Greece called Magnesia where magnetic ore deposits were found, as early as 600 BC. Magnetite, an iron ore, is a natural magnet. It is called lodstone.
   • When a bar magnet is freely suspended, it points in the north-south direction. The tip which points to the geographic north is called the north pole and the tip which points to the geographic south is called the south pole of the magnet. There is a repulsive force when north poles ( or south poles ) of two magnets are brought close together. Conversely, there is an attractive force between the north pole of one magnet and the south pole of the other.
   • The properties of a magnet are
     1. it attracts small piece of iron towards it.
     2. it always cmes to rest in north-south direction when suspended freely.
     3. like poles repel, unlike poles attracts each other
     4. Magnetic poles always exist in pairs.
     5. the strength of a magnet is maximum at poles located near the poends
   • The phenomenon due to which an unmagnetized magnetic substance behaves like a magnet, due to the presence of some other magnet, is called magnetic induction. Magnetic induction takes place first then magnetic attraction.
   • Magnetic induction depends upon the nature of magnetic substance. Magnetic induction is inversely propotional to the distance between inducing magnet and the magnetic substance. More powerful the inducing magnet, the more strong will be the magnetism in magnetic substance.
   • The space around the magnet where its influence can be detected is called the magnetic field.
   • A curve in a magnetic field, along with a free north magnetic pole will move, is called magnetic line of force. The direction of magnetic lines of force is the direction in which free north pole will move in a magnetic field.
     • They are closed continuous curves.
     • They travel from north to south pole outside the magnet and from south to north pole inside the magnet.
     • They mutually repel each other
     • They never intersect with each other
   • The earth behaves as a magnet with the magnetic field pointing approximately from the geographic south to the north. At a particular place on earth, the magnetic north is not usually in the direction of the geographic north. The angle between the two directions called declination.
2. ELECTRICITY:
   • The phenomenon due to which a suitable combination of bodies on rubbing, get electrified is called electricity. If a charge on a body is not allowed to flow, it is called the static electricity.
   • Matters are made of atoms. An atom is basically composed of three different components -- electrons, protons, and neutrons. An electron can be removed easily from an atom. When two objects are rubbed together, some electrons from one object move to another object. For example, when a plastic bar is rubbed with fur, electrons will move from the fur to the plastic stick. Therefore, plastic bar will be negatively charged and the fur will be positively charged.
   • When two objects are rubbed together, some electrons from one object move to another object. For example, when a plastic bar is rubbed with fur, electrons will move from the fur to the plastic stick. Therefore, plastic bar will be negatively charged and the fur will be positively charged.
   • When you bring a negatively charged object close to another object, electrons in the second object will be repelled from the first object. Therefore, that end will have a negative charge. This process is called charging by induction.
   • When a negatively charged object touches a neutral body, electrons will spread on both objects and make both objects negatively charged. This process is called charging by conduction. The other case, positively charged object touching the neutral body, is just the same in principle.
   • Substances can be classified into three types -- insulators, conductors, and semiconductors
     1. Conductors are materials which electrical charges and heat energy can be transmitted very easily. Almost all metals such as gold, silver, copper, iron, and lead are good conductors.
     2. Insulators are materials which allow very little electrical charges and heat energy to flow. Plastics, glass, dry air and wood are examples of insulators.
     3. Semiconductors are materials which allow the electrical charges to flow better than insulators, but less than conductors. Examples are silicon and germanium.
   • There are two different types of electric charges namely the positive and negative charges. Like charges repel and unlike charges attract each other.
   • Electric current always flows from the point of high potential. The potential difference between two conductors is equal to the work done in conducting a unit positive charges from one conductor to the other conductor through a metalic wire.
   • The flow of charge is called the current and it is the rate at which electric charges pass though a conductor. The charged particle can be either positive or negative. In order for a charge to flow, it needs a push (a force) and it is supplied by voltage, or potential difference. The charge flows from high potential energy to low potential energy.
   • A closed loop of current, is called an electric circuit. The current [I] measures the amount of charge that passes a given point every second. The unit for current is Ampere [A]. 1 A means that 1 C of charge passes every second.
   • when current flows through a conductor it offers some obstruction to the flow of current The obstruction offered to flow of current by the conducting wire is called its resistance in passege of electricity.
   • The unit of resistance is ohm. The resistance varies in different materials. For example, gold, silver, and copper have low resistance, which means that current can flow easily through these materials. Glass, plastics, and wood have very high resistance, which means that current can not pass throught these materials easily.
   • Electromagnetism: The branch of physics which deals with the relationship between electricity and magnetism is called electomagnetism.
   • Whenever current is passed through a straight conductor it behaves like a magnet. The magnitude of magnetic effect increases with the increase in the strength of current.
   • Faraday's law of induction is one of the important concepts of electricity. It looks at the way changing magnetic fields can cause current to flow in wires. Basically, it is a formula/concept that describes how potential difference (voltage difference) is created and how much is created. It's a huge concept to understand that the changing of a magnetic field can create voltage.
   • He discovered that the changes in the magnetic field and the size of the field were related to the amount of current created. Scientists also use the term magnetic flux. Magnetic flux is a value that is the strength of the magnetic field multiplied by the surface area of the device.
   • Coulomb's Law is one of the basic ideas of electricity in physics. The law looks at the forces created between two charged objects. As distance increases, the forces and electric fields decrease. This simple idea was converted into a relatively simple formula. The force between the objects can be positive or negative depending on whether the objects are attracted to each other or repelled.
   • Coulomb's Law: When you have two charged particles, an electric force is created. If you have larger charges, the forces will be larger. If you use those two ideas, and add the fact that charges can attract and repel each other you will understand Coulomb's Law. It's a formula that measures the electrical forces between two objects. F=kq₁q₂/r². Where "F" is the resulting force between the two charges. The distance between the two charges is "r". The "r" actually stands for "radius of separation" but you just need to know it is a distance. The "q₂" and "q₂" are values for the amount of charge in each of the particles. Scientists use Coulombs as units to measure charge. The constant of the equation is "k."
   • There are two main types of current in our world. One is direct current (DC) which is a constant stream of charges in one direction. The other is alternating current (AC) that is a stream of charges that reverses direction. The current in DC circuits is moving in a constant direction. The amount of current can change, but it will always flow from one point to another. In alternating current, the charges move in one direction for a very short time, and then they reverse direction. This happens over and over again.

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