CHAPTER ELECTRICAL QUANTITIES IN CIRCUITS 13 KEY QUESTION

CHAPTER ELECTRICAL QUANTITIES IN CIRCUITS 13 KEY QUESTION: How are electrical circuits used? Key Concepts • Circuits are a part of electrical technology. They can be connected in series and in parallel. • Current is the rate of electron flow. • Potential difference is a measure of the difference in electric potential energy per unit of charge between two points. • Resistance is a measure of the opposition to the flow of electrons. • Connecting loads in series or parallel affects the current, potential difference, and total resistance. • Current, voltage, and resistance measurements can be used to test circuits.

Circuit Diagrams 13. 1 • An electric circuit can be represented using a diagram. • Each part of the circuit is represented with a symbol. • By reading a circuit diagram, it is possible to determine how the circuit works and how to repair it when it does not function properly. circuit diagram

Series Circuits 13. 1 • In a series circuit, electrons flow through only one path. • Loads are connected in a chain, one after another in one continuous loop. • When one load does not work, the whole chain of loads stops working because electrons cannot flow. series circuit

Parallel Circuits • In a parallel circuit, electrons can flow through more than one path. • Loads are connected in different branches that are connected to the energy source. • When a load does not work in one branch, the loads in the other branches continue to function because electrons can flow through another path. 13. 1 parallel circuit

13. 2 CONNECTING MULTIPLE LOADS If you dried your hair with a hairdryer or used a computer, you were using circuits that contained more than one load. In this activity, you will design two circuits to light two lamps. Your circuits will consist of a power supply, connecting wires, and lamps. Purpose To construct and operate series and parallel circuits. Always disconnect the energy source before connecting a circuit. Do not adjust the power supply unless instructed to do so by your teacher.

Electric Current 13. 3 Electric Current Vocabulary • Electric current is the rate of electron flow past a given point in a circuit. • Current is represented by the symbol I. • The current through a circuit flows from the negative terminal of an energy source through the loads to the positive terminal of the energy source. The current continues moving in this way as long as the energy source is not disconnected from the circuit. electrical current

How to Measure Electric Current • Current is measured in amperes (A) using a device called an ammeter. • An ammeter must always be connected in series in a circuit. 13. 3 ammeter

Safety with Electric Currents • • • 13. 3 Large currents can damage electrical devices, start a fire, and even cause death Vocabulary if the current passes through your body. ammeter Fuses and circuit breakers regulate the amount of current in a circuit. – If the current is greater than the rating of the fuse, the fuse will blow. In the case of a circuit breaker, the breaker trips. – Fuses and circuit breakers prevent large currents from damaging the rest of a circuit and possibly starting a fire. When measuring current, always set the ammeter to the highest current setting to avoid damaging the meter.

COMPARING THE CONDUCTIVIY OF CONDUCTORS 13. 4 In this activity, you will design a circuit to measure the current fl owing through a conductor to compare the electrical conductivity of different materials. Purpose To design a circuit to measure conductivity and compare the conductivity of different materials. Always disconnect the energy source before connecting or disconnecting any meters or parts to a circuit.

13. 5 Potential Difference (Voltage) • Potential difference, or voltage, is the difference in electric potential energy per unit of charge measured at two points. • Voltage is represented by the symbol V. • The potential difference between the two terminals of an electric cell creates the flow of electrons through a circuit. potential difference (voltage) – Electrons leave the negative terminal of the electric cell with high electric potential energy. – That energy gets converted by the load into another form of energy. – When the electrons reach the positive terminal of the electric cell, they have low electric potential energy. – The chemical reactions within the electric cell re-energize the electrons, and the electrons once more flow from the negative terminal through the circuit back to the positive terminal.

How to Measure Potential Difference • Voltage is measured in volts (V) using a device called a voltmeter. • A voltmeter must always be connected in parallel with a load or energy source. 13. 5 voltmeter

MEASURING VOLTAGE AND CURRENT IN CIRCUITS 13. 6 In this activity, you will test a DC circuit. You will measure the current and the potential difference using an ammeter and a voltmeter, or a multimeter. Meters allow you to determine which part of a circuit is causing the problem. Purpose To measure electric currents and potential differences in DC circuits using meters. Always disconnect the energy source before connecting or disconnecting any meters or parts to a circuit.

Resistance • Electrical resistance is the characteristic of a material that prevents electrons from flowing through it. • Resistance is represented by the symbol R. • There are five major factors that affect the resistance of a material: – Type of material—Electrons move more easily through some materials compared to others. – Cross-sectional area—The thicker a wire, the lower will be its resistance. – Length—The longer a wire, the greater will be its resistance. – Temperature—The warmer the wire, its resistance will likely be greater. 13. 7 electrical resistance

DETERMINING THE RELATIONSHIP BETWEEN CURRENT AND POTENTIAL DIFFERENCE 13. 8 In this activity, you will use a resistor to determine the relationship between the current and the potential difference as the values of I and V are changed. Purpose To determine the relationship between the current and the potential difference.

Relating Current, Voltage and Resistance • With conductors and some types of loads, the current, voltage, and resistance are related. • If the voltage across one of these loads increases, the current through the load also increases. • The graph of voltage vs current looks like a straight line. The slope of the graph is the resistance of the load. • This straight-line relationship is written as 13. 9 Ohm’s law

Loads in Series: Current and Resistance 13. 10 • Vocabulary When more loads are added in series to a circuit, the electrons experience more “bumps” along their path. Ohm’s law • So the total resistance of the circuit increases and the current decreases.

Loads in Series: Voltage 13. 10 • The energy source of a circuit provides electrons with electric potential energy. Vocabulary That energy gets converted into kinetic energy as the electrons flow through the Ohm’s law circuit. • The energy source provides electrons with the same amount of electric potential energy no matter how many loads are connected in the circuit. • When two identical loads are connected in series, the electric potential energy is divided in half between the two loads. • So the voltage drop across each load (Vload) is half the voltage drop across the terminals of the energy source (Vsource). • When more identical loads are connected in series, the voltage drop across each load is given by Vload= .

Loads in Parallel: Current 13. 10 • Vocabulary When two identical loads are connected in parallel to a circuit, the electrons leaving the energy source split into two paths. Ohm’s law • So the current through each load (Iload) is half the current coming from the energy source (Isource). • When more identical loads are connected in parallel, the current through each load is given by Iload =

Loads in Parallel 13. 10 Resistance • In a parallel circuit, electrons flowing through one branch experience “bumps” Vocabulary only through the load(s) in that branch. Ohm’s law • So the total resistance of a parallel circuit is less than if the loads were connected in series. Voltage • In a parallel circuit with two branches, some electrons flow through one branch, while other electrons flow through the other branch. • In each branch, all of the electric potential energy provided to the electrons by the energy source gets converted into kinetic energy. • So the voltage drop across each parallel branch is the same as the voltage drop across the terminals of the energy source.

CHAPTER 13 LOOKING BACK KEY CONCEPTS SUMMARY • LOOKING BACK Circuits are a part of electrical technology. They can be connected in series and in parallel. – Series circuits allow electrons to flow along one path only.   – Parallel circuits allow electrons to flow along more than one path.   – When a load fails in a series circuit,  all the loads will stop working. – When a load fails in a parallel circuit,  the rest of the loads will  continue to work.   

CHAPTER LOOKING BACK KEY CONCEPTS SUMMARY • 13 LOOKING BACK Current is the rate of electron flow. – Electric current (/) is the amount of electrons passing a point in a  circuit in a given time period.   – Electric current is measured in amperes (A) using an ammeter  connected in series.

CHAPTER LOOKING BACK KEY CONCEPTS SUMMARY • 13 LOOKING BACK Potential difference is a measure of the difference in electric potential energy per unit of charge between two points. – The potential difference between two points in a circuit determines  whether or not charges will flow.   – The potential difference across a load or an energy source is  measured using a voltmeter connected in parallel.

CHAPTER LOOKING BACK KEY CONCEPTS SUMMARY • 13 LOOKING BACK Resistance is a measure of the opposition to the flow of electrons. – Resistance (R) is the amount of opposition to the flow of current.   Resistors are electrical devices that oppose the flow of electric current. – The resistance of a conductor is affected by the cross-sectional area,   the type of material used,  the temperature,  and the length of the  conductor.   – For conductors,  the graph of voltage versus current yields a straight  line.  The slope of the graph represents the resistance of the load, .

CHAPTER 13 LOOKING BACK KEY CONCEPTS SUMMARY • LOOKING BACK Connecting loads in series or parallel affects the current, potential difference, and total resistance. – Connecting loads in series increases the total resistance and  decreases the electric current coming from the energy source. – The voltage drop across the energy source is split between the loads  connected in series,  while the current is constant throughout the circuit. – Connecting loads in parallel decreases the total resistance of the  circuit. – The current is split among loads connected in parallel,  while the  voltage drop across each parallel branch remains the same.  

CHAPTER LOOKING BACK KEY CONCEPTS SUMMARY • 13 LOOKING BACK Current, voltage, and resistance measurements can be used to test circuits. – Hypotheses can be written to answer a Testable Question to compare  total resistance,  current,  and potential difference across resistors.   – Hypotheses and predictions are tested using voltmeters,  ammeters,   ohmmeters,  or multimeters to measure current,  potential difference,   and resistance.

CHAPTER QUIZ 13 MULTIPLE CHOICE For each question, select the best answer from the four alternatives. 1. Which variable is not part of Ohm’s relationship? (a) voltage (b) temperature (c) current (d) resistance 2. Which statement is incorrect? (a) Electrons follow only one path in a series circuit, but can follow more than one path in a parallel circuit. (b) The total resistance of a series circuit is greater than that for a parallel circuit. (c) Ohm’s law applies to both series and parallel circuits. (d) The current in a series circuit does not change when you add loads to the circuit.

CHAPTER 13 QUIZ FILL IN THE BLANKS Copy each of the following statements into a notebook. Fill in the blanks with a term from the box below that correctly completes the sentence. series circuit, parallel circuit, voltmeter, series, electric current, rest of the circuit, resistance, voltage, parallel, ohmmeter, whole circuit, ammeter, potential difference 3. ________ is measured using an ________ connected in ________ with a load. ________ or ________ is measured using a ________ connected in ________ with a load. ________ is measured using an ________ connected in ________ with a load and the circuit is switched off. 4. In a ________, when a load does not work, the ________ does not work. In a ________, when a load in a branch does not work, the ________ still works.

CHAPTER QUIZ 13 SHORT ANSWER 5. Look at the two circuits below. (a) Which circuit has loads connected in parallel? (b) Which circuit has loads connected in series? (c) Which circuit has the greater total resistance?

CHAPTER QUIZ ANSWERS 13 MULTIPLE CHOICE 1. Which variable is not part of Ohm’s relationship? (a) voltage (b) temperature (c) current (d) resistance 2. Which statement is incorrect? (a) Electrons follow only one path in a series circuit, but can follow more than one path in a parallel circuit. (b) The total resistance of a series circuit is greater than that for a parallel circuit. (c) Ohm’s law applies to both series and parallel circuits. (d) The current in a series circuit does not change when you add loads to the circuit.

CHAPTER QUIZ ANSWERS 13 FILL IN THE BLANKS 3. Electric current is measured using an ammeter connected in series with a load. Potential difference or voltage is measured using a voltmeter connected in parallel with a load. Resistance is measured using an ohmmeter connected in parallel with a load and the circuit is switched off. 4. In a series circuit, when a load does not work, the whole circuit does not work. In a parallel circuit, when a load in a branch does not work, the rest of the circuit still works.

CHAPTER QUIZ ANSWERS 13 SHORT ANSWER 5. Look at the two circuits below. (a) Which circuit has loads connected in parallel? ANSWER: circuit B (b) Which circuit has loads connected in series? ANSWER: circuit A (c) Which circuit has the greater total resistance? ANSWER: circuit A