NOTES: Electric Current, Ohm s Law, Batteries, and Series Circuits

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1 1 NOTES: Electric Current, Ohm s Law, Batteries, and Series Circuits ELECTRIC CHARGE is generated by the transfer of electrons: Electric charge is a fundamental quantity like mass, distance, or time. Charge is observable and measurable by the force it exerts on other charges. There are two types of charges: positive (+) and negative (-). Like charges repel one another: positive repels positive, negative repels negative. Opposite charges attract one another: positive attracts negative, negative attracts positive. Electric charge is measured in Coulombs (C), where 1 C consists of natural units of electric charge, such as individual electrons or protons, and is defined as the quantity of electricity transported in one second by a current of one ampere. Since all matter contains protons and electrons, there is charge present (often in great quantities) in every object. Typically, however, the number of protons in an object essentially equals the number of electrons adding the amounts of charge gives a total of zero the object is said to have no net charge and to be neutral. An object is charged or has a net charge when there are unequal numbers of protons and electrons. This occurs almost always as a result of electrons (e - ) being transferred to or from an object. Charge is conserved! Charge is neither created nor destroyed electrons are merely transferred. The net amount of electric charge produced in any process is zero. Means of becoming charged: o Conduction is the transfer of charge from one object to another, usually as a result of contact. o Induction involves the rearrangement of charge within an object due to the presence of an external charge (or electric field). There is no contact. Insulators vs. Conductors: Both insulators and conductors can possess charge. However, Charge travels freely and easily through CONDUCTORS, thereby generating current: while INSULATORS resist the flow of charge, impeding the passage of current. o Explanation: It is now known that electrons are carriers of charge. In conductors (metals) the electrons are not tightly bound to nuclei and easily roam from one atom to another. In insulators (e.g. rubber, plastic, wood, etc.) electrons are held more tightly in orbits around nuclei and do not easily move from one atom to another. o Examples of conductors: most metals (such as copper wire) and electrolyte solutions (an aqueous solution containing charged particles) o Examples of insulators: rubber, plastics, wood, pure water (containing no dissolved electrolytes) ELECTRIC CURRENT is the flow of electric charge: An electric current is generated by any movement of any electric charge carrier o May be subatomic charged particles: electrons having negative charge (-) or protons having positive charge (+); or o Ions: atoms that have lost or gained one or more electrons Positive ions have lost one or more electrons. Negative ions have gained one or more electrons. The rate of the flow of charge is defined as the electric current. Electric current is measured in Amperes (A), where 1 Ampere, 1 A represents the flow of one coulomb of charge per second, or electrons per second. In other words, a current of 1 A is flowing in a circuit if a charge of 1 coulomb passes any point in the circuit every second. We can write this formula as: Current (I) = Charge (Q) / Time (t)

2 Examples of the different levels of electric current include: o Lightning is thousands of amps. o Commercial power lines make available about 100 amps to a typical home o A standard light bulb requires about 1 amp, a one-room air conditioner about 15 amps o Electric currents in an electric clock will be milliamps, where 1 A = 1000 milliamp (ma). o Electric current in electronic circuits are a few micro amps, 1 A = 1,000,000 micro amps (μa). The path through which the electric current flows is called the circuit. In order for a sustained electric current to flow, a circuit must be complete i.e. a loop without breaks. Electric circuit: A path through which electrons can flow through a conductor. o A path not having any gaps in it that allows electron flow is called a closed circuit. o A path with one or more gaps is called an open circuit and electrons will not flow through it o In order for electrons to flow in a closed circuit, they need some sort of device that maintains a potential difference such as a battery (see VOLTAGE, below). BIG IDEA: The electric current of a circuit is the number of electrons that pass by a given point in a circuit in a given period of time, measured in amperes (A), using an ammeter. Types of current in circuits you use everyday. o AC (alternating current): Current that reverses direction multiple times each second. Wall outlets supply AC at 60Hz (Hertz). The current reverses direction 60 times a second. Most devices around the home that plug into outlets use AC. o DC (direct current): Current that flows in only one direction through a circuit. Batteries provide DC. Many portable electical devices can use batteries. Some electronic devices, like computers, need DC. S o do cars, planes, and small boats. Electric current is driven by VOLTAGE: Current flows through a circuit as a result of a difference in electric potential energy in the circuit. The electrical potential difference between two points in the circuit is defined as the voltage. The voltage between two ends of a path is the total energy required to move a small electric charge along that path, divided by the magnitude of the charge being moved. Voltage = potential energy charge Electric current flows through a circuit, because of an applied voltage across the circuit. In other words the potential difference is the difference in potential energy between any two places in the circuit, which is measured in volts (V) using a volt meter, which value compares how much energy electrons have in a circuit between any two points. BIG IDEA: Voltage produces current (if there is a complete circuit). The source of the potential difference may be a power supply, battery, 9 V thermocouple or any device capable of creating a voltage. Battery: a device to maintain a potential difference in a circuit. o As electrons in the circuit move through the conductor or a device in the circuit they lose energy. The battery supplies new energy but eventually it will exhaust itself and will need to be either recharged or replaced. o Types of batteries: Dry cell: typically consists of a zinc container ( - ) filled with an electrolytic paste with a carbon rod in the center ( + ). Current is produced when the zinc reacts with chemicals in the paste. Flashlights and portable CD players use this type of battery. Wet cell: two different metal plates immersed in a liquid electrolyte solution. In a car battery, the electrolyte is sulfuric acid and the metal plates are lead and lead dioxide. 2

3 The ELECTRIC POWER of a circuit is equal to the voltage times the electric current: The moving charges of an electric current are a source of energy that can be used to perform work. For example, electrical energy may be transformed in to mechanical energy (as in a motor), to light (as in a lamp), to thermal energy (as in a heater), or to other forms. Power is defined as the rate at which energy is used to perform work: thus power is the energy transformed divided by elapsed time. In the case of a circuit, electric power is equal to the current multiplied by the voltage: Power = current X voltage, or P = IV When current (I) is measured in amperes and voltage (V) is in volts, then power is in watts (W), thus Watts = amperes X volts This value (in Watts) indicates that rate at which the electrical device transforms energy Examples of different levels of electric power include: o A typical household incandescent light bulb has a power rating of 25 to 100 watts; fluorescent lamps typically consume 5 to 30 watts to produce a similar amount of light. o A typical coal power station produces around megawatts, where a megawatt is equal to one million (1 X 10 6 ) watts. o A typical unit in a nuclear power plant has an electrical power output of megawatts. 3 Electric current is affected by RESISTANCE: Amount of charge flow through a circuit also depends upon the electrical resistance of the circuit. Resistance is the tendency for a material to oppose the flow of electrons and therefore to resist the flow of an electric current. Amount of resistance within a circuit is influenced by a variety of factors: o Length and thickness of a conductor: Narrow wires have higher resistance than wider wires Long wires have higher resistance than shorter wires For example, the long, thin wires in a toaster have a high resistance to electron flow causing the electrons to give up their energy in the form of light and heat which toasts your bread. o Material of the circuit: Conductor vs. Insulator? Copper, an excellent conductor, has low resistance Rubber, an insulator, has high resistance o Temperature: Higher temperature = higher kinetic energy = greater movement of atoms within a conductor = greater resistance The resistance of some materials reaches zero at very low temperatures. These materials are called superconductors Resistance is measured in units of Ohms (Ω) using a device called an ohmmeter. Drawing a 1000 Ω resistor

4 4 OHM S LAW explains the relationship between current, voltage, and resistance in a circuit: Ohm s Law shows the relationship between the three quantities of electric circuits: voltage (V, measured in volts), current (I, measured in Amperes) and resistance (R, measured in Ohms). Ohm's law: Current = voltage resistance Or in units form: Amperes = volts or in abbreviations I = V Ohms R Thus, Ohm s Law tells us that 1V across a circuit with a resistance of 1 Ω produces a current of 1 A. Note that for a circuit with constant resistance, current and voltage are directly proportional to one another: the higher the voltage, the higher the current. For example, you get twice the current for twice the voltage Similarly, the higher the resistance, the lower the current. Thus, if you double the resistance for a circuit, the current is reduced by half. Circuits and BATTERIES: As discussed above, the path through which the electric current flows is called the circuit. In order for a sustained electric current to flow, a circuit must be complete i.e. a loop without breaks. Electric circuit: A path through which electrons can flow through a conductor. o A path not having any gaps in it that allows electron flow is called a closed circuit. o A path with one or more gaps is called an open circuit and electrons will not flow through it o In order for electrons to flow in a closed circuit, they need some sort of device that maintains a potential difference (see VOLTAGE, above). o In many cases, this voltage is provided by a battery. As discussed above, a battery maintains voltage (the potential difference) in a circuit, and thereby drives current through the circuit. Two commonly used battery types are the dry cell battery and the wet cell battery (also known as a voltaic cell or a galvanic cell). See below for more details on each. Batteries are designed to possess a negatively charged (-) anode and a positively charged (+) cathode Batteries produce current when attached to a circuit because electrons (e - ) flow from the anode, through the circuit, and into the cathode. o ANODE: The negatively charged terminus Represented with a symbol. Negatively charged electrons and ions (anions) move away from the anode, Thus, the anode is the source of electrons in the circuit (the electron donor). Positively charged ions (cations) move towards the anode o CATHODE: The positively charged (+) terminus Represented with a + symbol. Negatively charged electrons and ions (anions) move towards the cathode, Thus the cathode is the electron (e - ) acceptor (receives the electrons) in a circuit Positively charged ions (cations) move away from the cathode. BIG IDEA: When a battery is attached to a circuit electrons (e - ) flow from the anode to the cathode. e - e -

5 5 DRY CELL BATTERIES: The batteries with which you a probably most familiar: flashlights and portable CD players use this type of battery Typically, a zinc anode outer case filled with an electrolyte paste and a cathode carbon rod in the center Current is generated by a spontaneous redox reaction, in which: the anode zinc metal is oxidized to release free electrons (e - ), and as the electrons (e - ) travel into the carbon rod cathode, the electrolyte paste is reduced As this redox reaction occurs, electrons (e - ) flow from the anode to the cathode generating measurable current. + - WET CELL BATTERIES (also known as VOLTAIC CELLS or GALVANIC CELLS): Two different metal plates immersed in liquid electrolyte solutions that are linked by a salt bridge E.g., in a car battery the electrolyte is sulfuric acid and the metal plates are lead and lead dioxide. Current is generated by a spontaneous redox reaction, in which the anode metal is oxidized from a solid to make an ion in aqueous solution plus free electrons (e - ) the cathode metal is reduced from an ion in aqueous solution to make a solid. As this redox reaction occurs, electrons (e - ) flow from the anode to the cathode generating measurable current. The circuit is completed by a SALT BRIDGE linking the two electrolyte solutions: the salt bridge contains a strong electrolyte, such as NaCl or KNO 3 salt, that allows for the flow of ions to neutralize the charge imbalance caused by the redox reaction, allowing it to continue negative ions from the salt bridge flow to neutralize accumulation of positive charge at the anode positive ions from the salt bridge flow to neutralize accumulation of negative charge at the cathode

6 CIRCUITS: As discussed above, the path through which the electric current flows is called the circuit. In order for a sustained electric current to flow, a circuit must be complete, i.e. a loop without breaks Electric circuit: A path through which electrons can flow through a conductor. o A PATH not having any gaps in it that ALLOWS ELECTRON FLOW is called a CLOSED circuit. o A path with gaps is called an OPEN circuit and ELECTRONS WILL NOT FLOW through it In their simplest forms, CIRCUITS contain: o A VOLTAGE source (such as a battery) o A PATH made of a conductor (such as a metal wire) through which the electrons flow as they travel from anode to cathode of the voltage source o A SWITCH that regulates whether the path is CLOSED and current flows (and therefore the CIRCUIT IS ON ) or OPEN and current does NOT flow (and therefore the CIRCUIT is OFF ) o Some sort of LOAD (such as a light bulb) representing the element of the circuit that is powered by the current and/or creates RESISTANCE in the circuit (NOTE: that any LOAD will inherently create RESISTANCE). Circuits consisting of just one battery and one load resistance are very simple to analyze, but they are not often found in practical applications. Usually, we find circuits where more than two components are connected together. There are two ways in which to connect more than two circuit components: SERIES and PARALLEL. First, an example of a SERIES CIRCUIT: o Here, we have three resistors (labeled R 1, R 2, and R 3 ), connected in a long chain from one terminal of the battery to the other. o The subscript labeling -- those little numbers to the lower-right of the letter "R" -- serves to identify one resistor from another. o The defining characteristic of a series circuit is that there is only one path for electrons to flow. o In this circuit the electrons flow in a counter-clockwise direction, from point 4 to point 3 to point 2 to point 1 and back around to 4. o BIG IDEA: In a "SERIES" connection the components are connected end-to-end in a line to form a single path for electrons to flow: 6

7 Now, let's look at the other type of circuit, a PARALLEL configuration: o Again, we have three resistors, but this time they form more than one continuous path for electrons to flow. o There's one path from 8 to 7 to 2 to 1 and back to 8 again. o There's another from 8 to 7 to 6 to 3 to 2 to 1 and back to 8 again. o And then there's a third path from 8 to 7 to 6 to 5 to 4 to 3 to 2 to 1 and back to 8 again. o Each individual path (through R 1, R 2, and R 3 ) is called a branch. o BIG IDEA: In a "PARALLEL" connection all components are connected across each other's leads. In a purely parallel circuit, there are never more than two sets of electrically common points, no matter how many components are connected. There are many paths for electrons to flow, but only one voltage across all components: REVIEW: o In a series circuit, all components are connected end-to-end, forming a single path for electrons to flow. o In a parallel circuit, all components are connected across each other, forming exactly two sets of electrically common points. o Because of these differences: Series components have the same current flowing through each. Parallel components have the same potential difference (voltage) across them 7