Biophysics Lecture 2 PDF

Biophysics Lecture (2)  Coulomb's Law  Coulomb’s law relates the force between two single charges separated by a distance.  Current  Electricity is the flow of electrons in a conductor from one atom to the next atom in the same general direction.  This flow of electrons is r...

Biophysics Lecture (2)  Coulomb's Law  Coulomb’s law relates the force between two single charges separated by a distance.  Current  Electricity is the flow of electrons in a conductor from one atom to the next atom in the same general direction.  This flow of electrons is referred to as current and is designated by the symbol “I”.  Current is measured in amperes, which is often shortened to “amps”. The letter “A” is the symbol for amps.  Current that constantly flows in the same direction is called direct current (DC). Current that periodically changes direction is called alternating current (AC).  Voltage  The force that causes current to flow through a conductor is called a difference in potential, electromotive force (emf), or voltage.  Voltage is designated by the letter “E” or the letter “V.” The unit of measurement for voltage is volts which is also designated by the letter “V.”  All voltage sources share the characteristic of an excess of electrons at one terminal and a shortage at the other terminal.  This results in a difference of potential between the two terminals.  For a DC voltage source, the polarity of the terminals does not change, so the resulting current constantly flows in the same direction.  The terminals of an AC voltage source periodically change polarity, causing the current flow direction to change with each switch in polarity.  Resistance  A third factor that plays a role in an electrical circuit is resistance. Resistance is the property of a circuit, component, or material that opposes current flow.  All material resists the flow of electrical current to some extent.  Resistance is designated by the symbol “R.” The unit of measurement for resistance is the ohm, symbolized by the Greek letter omega Ω.  Ohm’s Law A simple electric circuit consists of a voltage source, some type of load, and conductors to allow electrons to flow between the voltage source and the load.  Ohm’s law defines the relationship between current, voltage, and resistance and shows that current varies directly with voltage and inversely with resistance.  Series Circuit Resistance  A series circuit is formed when any number of resistors are connected end-to-end so that there is only one path for current to flow.  Series Circuit Voltage and Current  The current in a series circuit can be determined using Ohm’s law.  First, total the resistance, and then divide the source voltage by the total resistance.  This current flows through each resistor in the circuit. The voltage measured across each resistor can be calculated using Ohm’s law.  The voltage across a resistor is often referred to as a voltage drop. The sum of the voltage drops across each resistor is equal to the source voltage.  Parallel Circuits  A parallel circuit is formed when two or more resistances are placed in a circuit side-by-side so that current can flow through more than one path.  There are two paths of current flow.  One path is from the negative terminal of the battery through R1 returning to the positive terminal.  The second path is from the negative terminal of the battery through R2 returning to the positive terminal of the battery.  Power in a DC Circuit  In an electrical circuit, voltage applied to a conductor causes electrons to flow.  Voltage is the force and electron flow is the motion. The rate at which work is done is called power and is represented by the symbol “P.”  Power is measured in watts, represented by the symbol “W.”