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Circuits Current, Voltage and Resistance TOP Current Batteries Basic Circuit Elements Resistors 1 Circuits Current, Voltage and Resistance Current is a measure of the movement of charge TOP 2 Circuits Current, Voltage and Resistance TOP Current is defined as the amount of charge that passes through a surface in a given amount of time. Q I t The units of current are amps Q The direction of the current is opposite to the flow of electrons. It may be thought of as the flow of positive charge. 3 Current always flows from higher potential to lower potential. Circuits Current, Voltage and Resistance TOP A battery uses chemical reactions to cause a potential difference between the terminals + - The maximum value of the potential difference of a battery is called its voltage 4 Circuits Current, Voltage and Resistance This is how an alkaline cell (AA battery) works. TOP Zn + 2 OH- —> ZnO + H2O + 2 e2 MnO2 + H2O + 2 e- —>Mn2O3 + 2 OH- Cathode (MnO2) Mn2O3 MnO2 MnO2 Anode (Zn) ZnO OH HOH O 2 - Zn Zn + 2MnO2 —> ZnO + Mn2O3 + 1.5 V 5 Circuits Current, Voltage and Resistance TOP Current is defined as the amount of charge that passes through a surface in a given amount of time Q I t The units of current are amps (A) The direction of current is opposite to the flow of electrons The current may be though of as the flow of positive charge Current always flows from higher potential to lower potential In other words, current flows from the positive terminal of the battery through the circuit to the negative terminal V In a circuit diagram we draw it like this 6 + I - Circuits Current, Voltage and Resistance TOP The following circuit elements and their symbolic representation are the ones most commonly found in circuits. Name Battery Symbol - + V Resistor Capacitor Inductor V R Ω F C H L Switch S 7 Units Circuits Current, Voltage and Resistance TOP 8 Circuits Current, Voltage and Resistance Resistors slow down the electrons TOP I 9 Circuits Current, Voltage and Resistance Resistors slow down the electrons TOP + - - 10 + Circuits Current, Voltage and Resistance TOP In most instances, the voltage across a piece of material is proportional to the current that flows through it The constant of proportionality is called resistance, R, and has units of Ohms (Ω) This is known as Ohm’s law and is usually written as V IR In a circuit diagram we draw resistance like this R 11 Circuits Current, Voltage and Resistance TOP As area increases, more electrons pass through it. Thus, the current is greater and the resistance is smaller As length increases, the electrons are blocked for a longer time, slowing them down even more. Thus, the current is smaller and the resistance is greater For many materials, the resistance is proportional to the length of the resistor and inversely proportional to the cross-sectional area L R A The constant of proportionality is called resistivity, ρ, and is different for different materials 12 Table 28.2 in the text gives resistivity for some materials Circuits Current, Voltage and Resistance Power is defined as the change in energy per unit time TOP In the case of electric potential energy, we can write U qV P IV t t The power may be written in different forms using Ohm’s law P IV PI R 2 13 V2 P R