Physics Notes

Topics: Atom, Electron, Chemical bond Pages: 6 (1687 words) Published: November 6, 2012
Current, Voltage and Power
* Electricity is the flow of electric charge. We can describe the flow of electric charge in several ways. These include the quantities Current, Voltage and Power. Current
* Current (I) is the rate of flow of Charge Carriers, such as electrons. Current is usually thought of as moving in the direction of positive charge, so from the positive power supply to the negative. However, since in metals it is electrons that carry electric charge, the actually flow is opposite to the way in which we think of it.

* Current it the amount of Charge, Q that passes a point in a set time, t. It is measured in Amps (A), and charge is measured in Coulombs (C). Since Amps are SI base units, Coulombs are defined as A×s, As. Voltage

* Voltage (V) or Potential Difference (p.d.) is a measure of the Energy transferred per Charge Carrier between two points.

* Voltage is the Energy, E per Charge, Q. Voltage is measured in Volts (V), which is defined as one Joule per Coulomb. Voltage can be defined in base units as Kgm2s-3A-1. Power
* Power (P) is the rate of Energy transfer. It is measured in watts (W), where one watt is defined as one Joule per Second. Hence watts can be expressed in base units as Kgm2s-3

* From this definition of Power, we can substitute the algebraic definitionsabove to produce a variety of other formulae, including 'Power = Current × Voltage'

* Ohm's Law states that 'Voltage = Current × Resistance'. We can use this to produce two more definitions of Power.  

Resistance and Resistivity
* Resistance it the opposition to the passage of current within a component. The Resistance of a component decides how much voltagewill be dropped across it for a particular current. * Resistance is measured in Ohms (Ω). According to Ohm's Law, voltage is the product of current and resistance. Therefore Ohms can be expressed in base units as Kgm2s-3A-2.

* Everything has Resistance, because everything has some opposition to the flow of Electric Charge. Components whose sole purpose is to provide a Resistance of a certain value are called Resistors.

* When Resistors are connected in Series, the total Resistance across them will be equal to the sum of each Resistor value. The total voltage will be equal to the sum of the voltages across each Resistor. This rule will also apply for other components.

* When Resistors are connected in Series, the reciprocal of the total resistance will be equal the sum of the reciprocals of each Resistor Resistance. The total voltage dropped will be the same as the voltages dropped across all the individual Resistors.

* Resistance is a Sample Constant, so is specific to individual components. However, there is a Material Constant that can be used to find the Resistance of any component of a specific material. This is Resistivity. Together with the length and cross-sectional area of a sample, it can calculate its resistance. * Resistivity is given the symbol ρ and is measured in Ohm Meters (Ωm, or Kgm3s-3A-2 in base units). * For example, copper has a Resistivity of 1.68 ×10-8 Ωm, and Germanium 4.6 ×10-1 Ωm. * The Resistance of a material of Resistivity ρ, length l and cross-sectional area A is calculated by the formula:

* Compounds are formed when two or more atoms join together, forming Bonds. There are different types of bonds that occur between atoms which give rise to different properties. Ionic Bonds
* Ionic Bonds form when electrons are transferred from one atom to another, forming charged Ions which are attracted to each other byElectrostatic Forces. Elements tend to loose or gain electrons, forming Ions, to get a 'full other shell'.

* Ionically bonded substances, such as Sodium Chloride, can from crystals known as Giant Ionic Lattices with bonds forming a network of connections between atoms.

* Giant Ionic Lattices have high melting and boiling points since...
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