Physics Notes - PDF

**Physics** (2.9) The alteration in space caused by a mass is called its gravitational field. Likewise, the alteration in space caused by an electric charge is called electric field. Just as the space around a planet and every other mass is filled with a gravitational field, the space around every...

**Physics** (2.9) The alteration in space caused by a mass is called its gravitational field. Likewise, the alteration in space caused by an electric charge is called electric field. Just as the space around a planet and every other mass is filled with a gravitational field, the space around every electric charge is filled with a gravitational field. An electric field is a vector quantity and thus has both magnitude(strength) and direction. The magnitude of the field at any point is simply the force per unit charge. If a charge q experiences a force F at some point in space, the the electric field E at that point is A useful way to describe an electric field is with electric lines of force. Where the lines are further apart, the field is weaker. A field line points away from a positive charge and point towards a negative charge. For an isolated charge, the lines extend to infinity; for two opposite charges we represent the lines as emanating from the positive charge and terminating on a negative charge.  The phenomenon of electric current involves the ordered movement of electric charges. According to the theoretical model, electric current is the flow of negatively charged particles, positively charged particles, or both in an ordered manner. In metals, the electric charge carriers are the conduction electrons that break free from their atoms. (2.10) To obtain an electric current, charge needs to flow in a continuous path or circuit. Therefore, an electric current can be defined as the flow of electric charge.  (2.10.2) An electric current is the flow of negatively charged particles, or of positively charged particles, or of both. However, according to the accepted model, in metals, only free electrons move. All free electrons in a metallic conductor start moving at almost the same instant, in a way similar to a fluid moving in a pipe. They move at speed of the order of 1mm/s but they all do so at almost the same time anywhere in the wire. Scientists have agreed to adopt the convention that current always flows from the positive to the negative terminal of a generator. It does not make any difference which sense is chosen as conventional current. In metals negative (electrons) moves from -- to + that actually carry the current. (2.10.4) Energy tends to dilute itself over matter. Energy has the natural tendency to flow spontaneously from where it is concentrated to where it is dilute. Matter will accelerate in the direction of a net force, so a fluid, for example, will tend to flow from where it has greater potential energy to a point where its potential energy is less. The potential energy it loses will be converted (ultimately to heat) and spread over a larger quantity of matter. Although water does not flow spontaneously from a low place to a high place, it can be pumped up. In the process, the water pumped up will have more energy concentrated in it, but this only happens at the expense of another larger quantity of energy originally concentrated in another place becoming diluted over matter. When electrons are moved from one neutral conducting sphere A to another sphere B, energy should be supplied. 1. Energy must be supplied to pull the electrons away against the force that attracts them to the nuclei of their atoms in A 2. Energy must be supplied to push the electrons against the repulsive forces by the electrons of the atoms of B. 1. Each electron e in the wire will be pushed away from B ( by excess negative charge on B) and will be pulled towards A ( by the excess positive charge on A) 2. This results in all mobile electrons in the wire to flow from B towards A: a current of electrons flows in the wire. 3. Excess in B repel each other in the wire ( thus reducing the negative charge of B) and electrons will pour from the wire into A ( thus reducing the positive charge on A) (2.11) Two points A and B are said to have a potential difference between them if when charges move spontaneously from one to the other, electric potential energy is changed to some other form of energy. If the mass is high at point A, and it moves down to another point B lower than A, it will lose gravitational potential energy. The gravitational potential at A is higher than gravitational potential at B and a gravitational potential exists between A and B. The gravitational potential difference between A and B is defined as the gravitational potential energy per kilogram lost when mass moves from A to B. Similarly, if a coulomb of charge at point A moves to another point B, and in the process electric potential energy is lost, we say point A is at higher electric potential than point B, and an electric potential difference exists between A and B.  In a dipole (battery), the positive terminal has a higher potential and the negative has a lower potential. When the positive charge moves to a point of lower potential ( the -- terminal) it loses some of its potential energy as heat or as any form of energy. (2.11.1) Potential difference between two points is measured in terms of how much energy is given out by each coulomb of charge that passes across these points. For the same charge, the more energy released, the greater the potential difference. (2.12) Power is defined as work (energy) per unit time. Using the relation W = Q x V we can derive an expression for electric power. But the work done per unit time is the power P, and the charge flowing per unit time is the current I, therefore P = IV Electric power is measured in watts, just as mechanical power. For a constant power, energy is W = VIT (2.12.1) Work is expressed in joules, but when dealing with electric energy consumption, the kilowatt-hour (KWh) is more indicative and widely used unit. By definition, the kilowatt-hour is the energy consumed in one hour by an electric device having a power of 1 kW. kWh is often called a unit. 4 units of energy is 4kWh (2.12.2) Very small currents in the order of milli and micro amperes are common in electronic circuits. A coulomb is a small quantity of charge that may flow through an ordinary lamp every second. By contrast, a coulomb is not a reasonable amount of static electricity. If a sphere charged to 1 C were placed in the middle of the classroom, light objects would leap up and stick to it due to the effects of electric induction and everybody's hair would point towards the sphere. (2.12.3) (2.13) The flow of electric current is accompanied by the conversion of electric potential energy into various other forms. When two oppositely charged metallic objects are interconnected by a wire, an electric current will flow in the wire for a very short time and the electric energy initially stored in them will appear mainly in the form of internal energy of the wire. To maintain electric currents a generator are required. An electric generator is a device that converts a certain available form of energy to electric energy. Dry cell converts chemical potential energy stored in chemicals inside the cells to electric energy. A dynamo converts mechanical energy to electric energy and a solar cell converts light energy to electric energy. A generator is like a pump, which supplies energy to pump up electric charges from a low potential energy to a high potential. When charges flow outside the generator, electric energy is converted to another form. The energy supplied by a generator to transport 1C of positive charge from the negative to the positive terminal callled e.m.f ( electromotive force , not a force) of the generator. In other words, the electric charge gains electric potential energy in the generator and the energy gain per unit charge is the e.m.f. Since e.m.f is measured in joules per coulomb, its unit is the volt. The source of the energy depends on the kind of generator.  E.m.f is the total energy per unit charge transformed by the generator from other forms of energy to electrical energy. The potential difference between two points A and B is the total energy per unit charge transformed by the electrical elements between A and B from electric energy to other forms of energy.  (2.14.1) (2.14.2) Building circuits in electricity is very important to understand the study of electric currents, so special symbols have to be used to represent all the common devices that are going to be required to start building simple circuits.   (2.15) The ammeter is an instrument used to measure the electric current by its magnetic effect, Ammeter is represented by a circle with the letter A in it. It is important to note that the ammeter should always be included in series with the component through which the current is to be measured and the polarity must be respected. ( current flows outside the generator from + to - ) Moreover regardless of its own relative position in that branch( before or after other electric component) the ammeter always reads the current in that branch through the component) When reading an ammeter, it is important to minimize the parallax affect by making the needle cover its image in the mirror if available. Finally, one of the precautions to be observed, when dealing with ammeters, is to always start measuring with a meter that can handle large currents (or a multimeter switched to measure high currents). If the deflection is not enough to read, you can always substitute a more sensitive meter later or switch to a lower range of measurement. The reason for that is that the wires inside an ammeter are thin and in an enclosed space. If an excessive current passes through them they will convert electric potential energy to heat faster than they can transfer the heat to the surroundings, getting hotter and hotter until they burn or melt. Also a rapid deflection to the end of the scale can bend the pointer. (2.15.2) A voltmeter measure the potential difference between two points in a circuit and its terminals must be connected to these points. For example a voltmeter connected across a cell is both in series and parallel with the cell. The voltmeters, is the above figure, are connected in parallel with the devices whose potential difference they measure; that is their terminals are connected across the points between which the potential difference is to be measured. A voltmeter always measures the potential difference between two points but never the potential at a point. The latter is unobservable quantity; only potential differences can be measured. The situation here is similar to that with altitude, altitude can be defined You must learn to use electric terms correctly, a misuse of terms signifies lack of understanding. 1. Charge flows 2. The current in a conductor, or in a circuit 3. The current through a conductor 4. The current round a circuit 5. The p.d or voltage between two points 6. The p.d or voltage across a conductor 7. The p.d also means the potential drop between two points in a circuit. (2.16) Two laws, known as kichhoff's law, govern currents and voltages in electric circuits. Kirchhoff's first law is a relation between all currents flowing into and out of a point in the circuit. The total current entering a junction in a circuit equals the total current leaving it.  Kirchhoff's second law states that the sum of the p.d.'s in series equals the p.d. Across the whole branch. Note that the potential difference between two points doesn't depend on the path followed. (2.17) Generators can be grouped in parallel or in series. Recall that an electric cell is represented by a tall thin line (positive terminal) and a short thick line (negative terminal). The e.m.f of a grouping can always be calculated using kirchoff's second rule.  All conductors resist the flow of current in the sense that within them, some electric potential energy is changed to heat energy. We define a resistor to be a device that changes electric potential energy to heat energy. All conductors, whether good or poor, devices or simple wires, possess electric resistance.