Circuits and Circuit Elements
Schematic Diagrams A diagram that depicts the construction of an electrical apparatus is called a schematic diagram These diagrams use symbols to represent the bulb, battery, wire, and other components of the system If you are familiar with the standard set of symbols you can read any electrical schematic diagram
Schematic Diagram Symbols Use your textbook to complete the table Component Wire or conductor Symbol Resistor or circuit load Bulb or lamp Plug Battery Switch Capacitor
Schematic Diagram Draw a diagram of each element in the photograph below and indicate all connections.
Electric Circuits In the previous diagram, the bulb, battery, and wire form an electrical circuit An electrical circuit is a path through which charges can be conducted Any element or group of elements in a circuit that dissipates energy is called a load In our example with the light, the battery is the source of energy and the bulb is the load. Note: the connecting wire and switch have negligible resistance so are not considered part of the load.
Types of Circuits Closed circuit: there is a closed-loop path for electrons to follow Open circuit: there is no charge flow and therefore no current
Short Circuits Without a load, a circuit contains little resistance to the movement of charges. This is called a short circuit This occurs when uninsulated wires connected to different terminals come into contact with each other Most wires can not handle this increased current, overheat and cause the remaining insulation to melt or even cause fire
EMF EMF is the energy per unit charge supplied by a source of electric current Any device that increases the potential energy of charges circulating in a circuit is a source of emf. Batteries and generators are examples of emf sources These sources are often referred to as a charge pump
Terminal Voltage The potential difference across the battery terminals is the terminal voltage Because of conservation of energy, the charge must gain as much energy as it loses in one trip around the circuit Electric potential energy gained in the battery must equal the energy dissipated by the load The potential increase across the battery must equal the potential decrease across the load
Resistors in Series What would the schematic diagram look like if we added another light bulb to our circuit? Because all charges in the circuit must follow the same conducting path, these bulbs are said to be connected in series. How does this affect the current in the circuit? V=IR What is the current for the circuit? Because there is only one path for a charge to follow, the amount of charge entering and exiting the first bulb must equal the amount of charge that enters and exits the second bulb in the same time interval.
Resistors in Series When many resistors are connected in series, the current in each resistor is the same The total current in a series circuit depends on how many resistors are present and on how much resistance each offers To find the total current first use the individual resistance values to find the total resistance of the circuit equivalent resistance Equivalent resistance can then be used to find the current ( V=IR eq )
Resistors In Series R eq =R 1 + R 2 + R 3 + R 4. Equivalent resistance equals the total of individual resistances in series The equivalent resistance of a series combination of resistors is always greater than any individual resistance The current in a series circuit: I= V/R eq
Practice: Resistors in Series A 9.0 V battery is connected to four light bulbs, as shown in Figure 20-12. Find the equivalent resistance for the circuit and the current in the circuit. R eq =18.0 Ω Ι = 0.50 A
Practice: Resistors In Series A 4.0 Ω resistor, and 8.0 Ω resistor, and a 12.0 Ω resistor are connected in series with a 24.0 V battery. Calculate the equivalent resistance. Calculate the current in the circuit. What is the current in each resistor? 24.0 Ω 1.00 A 1.00 A
Practice: Resistors in Series Several light bulbs are connected in series across a 115 V source of emf. What is the equivalent resistance if the current in the circuit is 1.70 A? 67.6 Ω If each light bulb has a resistance of 1.50 Ω, how many light bulbs are in the circuit? 45 bulbs
Series Circuits Why put load in series? Regulate the current in the device (decrease the current in each bulb) Several lesser resistances can be used to add up to a single greater resistance that is unavailable Some items it is important that if a component is broken that there be no current (some burglar alarms)
Resistors in Parallel A schematic drawing of a parallel circuit would look like Each left side of the bulb is connected to the positive terminal of the batter and the right side of each bulb is connected to the negative terminal. The potential difference across each bulb is the same because they are connected at common points
Resistors in Parallel If the common points are the battery s terminal like in our drawing the potential difference across each resistor would be equal to the voltage of the battery Unlike a resistor in series resistors in parallel do not always have the same current moving across them
Resistors in Parallel Current is conserved in any circuit system Due to this conservation, the sum of the currents in each bulb (any resistor in parallel) equals the current delivered by the battery I = I 1 + I 2 + I 3.. The equivalent resistance of two or more resistors connected in parallel can be calculated by 1/R eq = 1/R 1 + 1/R 2 + 1/R 3
Resistors in Series or in Parallel Circuit Type Schematic diagram Current Potential difference Equivalent resistance Series I=I 1 =I 2 =I 3. =same for each resistor V= V 1 + V 2 + V 3 =sum of potential differences R eq =R 1 +R 2 +R 3. =sum of individual resistances Parallel I = I 1 + I 2 + I 3 =sum of currents V= V 1 = V 2 = V 3.. =same for each resistor 1/R eq =1/R 1 +1/R 2 +1/R 3 =reciprocal sum of resistance
Complex Resistor Combinations Most circuits today use a combination of series and parallel wiring to utilize the advantages of each type A common example of a complex circuit is found in the electrical wiring of our home.
Complex Resistors To prevent excessive current, a fuse or circuit breaker must be placed in series with all of the outlets When tripped the circuit breaker will interrupt the current Because this is placed in series, the current in the fuse or circuit breaker is the same as the total current in the circuit To find this current you must determine the equivalent resistance To do this for a complex circuit you mist simplify the circuit into groups of parallel and series resistors and then find the equivalent resistance of series and parallel resistors.