Circuits Current, Voltage and Resistance 1 TOP Current Batteries Basic Circuit Elements Resistors

Circuits Current, Voltage and Resistance 2 TOP Current is a measure of the movement of charge

Circuits Current, Voltage and Resistance 3 TOP Current is defined as the amount of charge that passes through a surface in a given amount of time. The units of current are amps The direction of the current is opposite to the flow of electrons. It may be thought of as the flow of positive charge. Current always flows from higher potential to lower potential.

Circuits Current, Voltage and Resistance 4 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

Circuits Current, Voltage and Resistance 5 TOP Zn + 2 OH - —> ZnO + H 2 O + 2 e - 2 MnO 2 + H 2 O + 2 e - —>Mn 2 O OH - Zn + 2MnO 2 —> ZnO + Mn 2 O V Cathode (MnO 2 ) Anode (Zn) H2OH2O MnO 2 Zn OH - ZnO Mn 2 O 3 This is how an alkaline cell (AA battery) works.

Circuits Current, Voltage and Resistance 6 TOP Current is defined as the amount of charge that passes through a surface in a given amount of time 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 In a circuit diagram we draw it like this The units of current are amps (A) - + V I + -

Circuits Current, Voltage and Resistance 7 TOP The following circuit elements and their symbolic representation are the ones most commonly found in circuits. Battery V -+ V NameSymbolUnits Resistor Capacitor Inductor Switch R Ω C F L H S

Circuits Current, Voltage and Resistance 8 TOP

Circuits Current, Voltage and Resistance 9 TOP Resistors slow down the electrons I

Circuits Current, Voltage and Resistance 10 TOP Resistors slow down the electrons

Circuits Current, Voltage and Resistance 11 TOP In most instances, the voltage across a piece of material is proportional to the current that flows through it This is known as Ohm’s law and is usually written as The constant of proportionality is called resistance, R, and has units of Ohms (Ω) In a circuit diagram we draw resistance like this R

Circuits Current, Voltage and Resistance 12 TOP For many materials, the resistance is proportional to the length of the resistor and inversely proportional to the cross-sectional area Table 28.2 in the text gives resistivity for some materials The constant of proportionality is called resistivity, ρ, and is different for different materials 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 As area increases, more electrons pass through it. Thus, the current is greater and the resistance is smaller

Circuits Current, Voltage and Resistance 13 TOP Power is defined as the change in energy per unit time In the case of electric potential energy, we can write The power may be written in different forms using Ohm’s law