Current, Voltage/Electromotive Force, and Resistance | MCAT Physics Prep

Need help preparing for the MCAT physics section? MedSchoolCoach expert, Ken Tao, will teach you what you need to know about current, voltage/electromotive force, and resistance within circuits. Watch this video to learn how to do well on the physics section of the MCAT exam! Current (I) can be defined as the net flow of electrical charge. Current is most often transmitted by wires made of metal, and metal atoms have their electrons described by a model known as the electron sea model. In the electron sea model, electrons move freely between nearby neighboring metal atoms, and are extremely mobile. Under normal conditions, there is no net movement of electrons in a metal lattice, and thus no current. However, if there was a source of positive charge on one side of the metal lattice, negatively charged electrons would naturally move in that direction. The flow of electrons in a particular direction would be creating a current. We said that current is the net flow of electrical charge. However, more specifically, current is defined as the direction of flow of positive charges. If the electrons in a metal wire are moving to the right, current is defined as moving to the left. Current is calculated using the equation below, where ΔQ represents a change in charge and ΔT represents a change in time. Therefore, current is a change in the charge at some point over a period of time, with units of coulombs per second (C/s), or amperes (A). Voltage/Electromotive Force Previously, we noted that charges spontaneously move in the direction of electric potential (Φ) which will produce a decrease in electric potential energy (ΔPE is less than 0). Specifically, electrons, having a negative charge, will move spontaneously towards higher electric potential. Therefore, current exists when one side of a metal wire is at a higher potential than the other side, initiating the spontaneous movement of electrons. The differences in electric potential (ΔΦ) are supplied by a battery that has some voltage (V). For example, a galvanic cell is a battery that could spontaneously supply a voltage to a circuit, initiating the flow of electrons through the circuit. On a circuit diagram, a battery is indicated by two parallel lines - one longer, one shorter. The longer line is always the positive side at a higher potential, and the smaller line is always the negative side at a lower potential. Electrons are therefore induced to move from the negative side toward the positive side. Resistance Resistance (R) is a measure of current repulsion, with units of ohms (Ω). Circuit elements that supply resistance are called resistors. On a circuit diagram, a resistor is drawn with a zig-zagged line. The equation for resistance is shown below, where L is the length of the resistor, A is the cross sectional area of the resistor, and ρ stands for resistivity. Resistivity is an intrinsic property of a material. A conductive metal will likely have low resistivity, whereas a resistant non-metal will likely have high resistivity. The longer a resistor, the longer the path the electrons have to travel, increasing resistance. The bigger the cross sectional area of a resistor, the more spacious the path electrons will have to travel through the wire, decreasing resistance. Ohm’s Law Ohm's law says that if you have current passing through a resistor, the current is going to be directly proportional to the voltage. This relationship is described by the equation below. Another interpretation of this equation is that if current passes through a resistor, the voltage in the circuit will drop across that resistor. Terminal Voltage Terminal voltage (Vterminal) is the literal output of voltage we can measure as coming from a battery and being supplied to a circuit. It’s not the same as the voltage of a battery, because batteries themselves have an internal resistance that causes a slight voltage drop that takes away from the voltage of the battery. As a side note, the term “electromotive force” will crop up on the MCAT, and it refers to the voltage of the battery (Vbattery). MEDSCHOOLCOACH To watch more MCAT video tutorials like this and have access to study scheduling, progress tracking, flashcard and question bank, download MCAT Prep by MedSchoolCoach IOS Link: https://play.google.com/store/apps/details?id=com.htd.medschoolcoach&hl=en_US Apple Link: https://apps.apple.com/us/app/mcat-prep-by-medschoolcoach/id1503000883 #medschoolcoach #MCATprep #MCATstudytools

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