Electric Circuits. Physics 6 th Six Weeks

Transcription

1 Electric Circuits Physics 6 th Six Weeks

2 Electric Circuits (a review) A circuit is a path through which electricity can flow Electric Circuits always contain 3 things: a voltage source, a conductor (usually a wire), and 1 or more devices which use the electrical energy. If the pathway that the electrons must travel is broken it is an open circuit. If the pathway that the electrons must travel is complete then it is a closed circuit. There are two forms of electric circuit: series circuits and parallel circuits.

3 Basic Circuit Diagram Symbols Note: an Ammeter is a tool that is used to measure current or amps. a Voltmeter is a tool that is used to measure voltage.

4 Series Circuits Series Circuit a circuit in which the current (or flow of electrons) has only one loop to flow through. Example: Christmas lights and a flashlight.

5 Series Circuits and Current I battery = I 1 = I 2 = I 3 =... Current can be compared to how many soldiers march past a given point in a period of time. The current in a series circuit everywhere is the same. Therefore, the rate at which charges flow (aka the current) remains the same everywhere If you know the voltage and resistance at that point in a series circuit, you can use Ohm s Law to calculate it at that point.

6 Series Circuits and Resistance To calculate the total resistance of a series circuit, simply Add the amount of resistance of each device together. For example if you have a series circuit with a light bulb that Has a resistance of 10 Ω and another light bulb that has a resistance Of 20 Ω, then the total resistance in that circuit is 30 Ω Since resistance is larger in a series circuit, light bulbs in A series circuit would be dimmer than those in a parallel circuit.

7 Series Circuits and Resistance In a series circuit, the equivalent resistance of the circuit (R eq ) is equal to the sum of the resistors. R eq = R 1 + R 2 + R

8 Series Circuits and voltage drop ΔV battery = ΔV 1 + ΔV 2 + ΔV As charges move through a circuit, they may lose energy from one resistor to another it is as if an army of marching soldiers get tired and march less energetically along a road from place A to place B. This loss in electric potential is referred to as a voltage drop. It occurs as the electrical energy of the charge is transformed to other forms of energy (thermal, light, mechanical, etc.) within the resistors or loads. Voltage drop in a series circuit is equal to Current x Resistance at each point. Current in series circuit is voltage divided by total resistance In a parallel circuit voltage is the same everywhere only dropping when it gets back to the battery or power source

9 Series Circuits and voltage drop If an electric circuit powered by a 1.5-volt cell is equipped with more than one resistor, then the cumulative loss of electric potential is 1.5 volts. There is a voltage drop for each resistor, but the sum of these voltage drops is 1.5 volts - the same as the voltage rating of the power supply. Consider the two circuits shown below in Diagrams A and B. Suppose that you were to asked to determine the two unknown values of the electric potential difference across the light bulbs in each circuit. The sum of the voltage drop is equal to the voltage difference of the power source. ΔV battery = ΔV 1 + ΔV 2 + ΔV

10 Series Circuits and voltage drop The voltage difference from one end of the battery (produced by the chemical reaction within) is 12 V which causes the charges to flow through the circuit. The voltage leaves the battery (at Point A) with 12 V of energy and reaches the first resistor where it experiences a voltage drop of 3 V The voltage now down to 9 V and reaches the second resistor where it experiences a voltage drop of 7 V The voltage now down to 2V and reaches the third and final resistor where it experiences the final voltage drop of 2V before the electrons return back to the battery with a voltage of 0 V. The cycle will continue as the newly weak electrons are energized and sent out from the battery through the circuit until the chemical reaction has run its course.

11 Voltage Drop: Series v Parallel

12 Total Resistance of a Series Circuit (as seen on the STAAR Formula Chart)

13 Example 4: Total Equivalent Resistance for resistors in a series circuit What is the total resistance if R 1 = 2 Ω, R 2 = 3 Ω, and R 3 = 4 Ω R Total = 2 Ω + 3 Ω + 4 Ω R Total = 9 Ω

14 Series Circuits Review Pit Stop One pathway Series Circuits Voltage drop occurs total voltage equals the sum of the voltage values at each resistor Current is the same everywhere Every device must function to have a complete circuit Total Resistance is the sum of the individual resistances Ex: If a battery was hooked up to a 6V battery in a series circuit with 3 resistors (each with 2 Ω of resistance), the voltage would drop by 2V through each until it was 0 V back at the battery. The current is the same through all

15 Parallel Circuits Parallel Circuits circuits that contain 2 or more branches for current (or the flow of electrons) to move through. The advantage of a parallel circuit is that when one branch of the circuit is open, the current continues to flow through the other branches (thus individual parts can be turned off and on). Examples: home wiring, car wiring.

16 Total Resistance of a Parallel Circuit (as seen on the STAAR Formula Chart)

17 Parallel Circuits and Resistance To find the Total Resistance of a Parallel Circuit, calculate the reciprocal of the sum of the reciprocals of the resistance of each pathway. therefore In the calculator: R = ( ) -1 = 9.23 Ω ~ 9 Ω

18 Parallel Circuits and Resistance Remember: The total equivalent resistance in a parallel circuit Is equal to the reciprocal of the sum of the reciprocals of the resistors.

19 Example 5: Total Resistance of a Parallel Circuit What is the total resistance if R 1 = 20 Ω and R 2 = 30 Ω 1 R = R = ( ) 1 R 12 Ω

20 Parallel Circuits and Current The current outside the branches is equal to the sum of the current in the individual branches.. Thus the branch of Resistor 1, with a current of 2 A and the branch of Resistor 2 with a current of 4 A will yield the current of 6 A outside of both branches

21 Parallel Circuits and voltage drop V battery = V 1 = V 2 = V 3 =... In a parallel circuit voltage is the same everywhere only dropping when it gets back to the battery or power source A charge does not pass through every resistor rather it passes through a single resistor Thus, the entire voltage drop across that resistor must match the battery voltage. It matters not whether the charge passes through resistor 1, resistor 2, or resistor 3, the voltage drop across the resistor that it chooses to pass through must equal the voltage of the battery. If three resistors are placed in parallel branches and powered by a 12-volt battery, then the voltage drop across each one of the three resistors is 12 volts.

22 Parallel Circuits A Review Pit Stop Multiple pathways Parallel Circuits No voltage drop occurs voltage is the same everywhere at each component Current differs at each resistor each branch is like its own series circuit Total current = the sum of the current of each branch Individual parts may fail and the circuit remains functional Total Resistance is equal to the inverse of the sum of the reciprocals of the individual resistances Ex: the same battery & 3 resistors are in a parallel circuit, the currents through each resistor combine to total the overall current. There would only be one, overall amount of voltage drop 6V back at the battery.

23 Short Circuits A short circuit is a circuit path with zero or very low resistance. A short circuit can be created by connecting a wire directly between two ends of a battery. Short circuits are often created accidentally by connecting a wire between two other wires of different voltages, creating a parallel path of very low resistance. Short circuits are dangerous because they can cause dangerous amounts of current. Large amounts of current could cause a wire to melt instantly and cause fires or burns.

24 Series vs. Parallel in summation Series Circuits One pathway Voltage drop occurs total voltage equals the sum of the voltage values at each resistor Parallel Circuits Multiple pathways No voltage drop occurs voltage is the same everywhere at each component Current is the same everywhere Every device must function to have a complete circuit Total Resistance is the sum of the individual resistances Ex: If a battery was hooked up to a 6V battery in a series circuit with 3 resistors (each with 2 Ω of resistance), the voltage would drop by 2V through each until it was 0 V back at the battery. The current is the same through all Current differs at each resistor each branch is like its own series circuit Total current = the sum of the current of each branch Individual parts may fail and the circuit remains functional Total Resistance is equal to the inverse of the sum of the reciprocals of the individual resistances Ex: the same battery & 3 resistors are in a parallel circuit, the currents through each resistor combine to total the overall current. There would only be one, overall amount of voltage drop 6V back at the battery.

25 Types of Current Direct Current Current that flows in only one direction through a wire. Known as DC. Direct Current is produced by batteries. Alternating Current The electric current that reverses its direction regularly. Known as AC. Alternating Current is the electric current produced by electric generators and is used in home wiring.