Electric Circuits Have you checked out current events today?
Circuit Symbolism We can simplify this circuit by using symbols All circuits have an energy source and a load, with wires completing the loop
Current Direction Conventional Current flows from positive to negative The electron flow is actually the opposite!
AC/DC? DC: Direct current only flows in one direction AC or alternating current switches direction (60 times per second!)
Series vs. Parallel In a series circuit, there is only one pathway for current to travel Parallel circuits have more than one path
Measuring current The ammeter measures the current through 1 Ammeters are always connected in series with the circuit (or you ll blow it!) Current is a measure of flow rate Q/t I Q t
Ex 1: Find I What is the current if 5C of charge flows through the circuit in 10s? I Q t 5C 10s 0.5A
Ex 2: Find Q How much charge has flowed through the ammeter if it measures 200mA for 10min? How many electrons? Q It 0.20A600s 120C
Batteries Cells in series make up a battery with a voltage equal to the sum (usually 1.5V each) Cells in parallel make up a battery with no more voltage, but it can last longer (or even supply more current as we will find later with emf and internal resistance)
Ex 1: what are the voltages of these batteries if they are made of lithium (3.7V) cells? 63.7 22V 13. 7 3. 7
Measuring Voltage The Voltmeter measures the potential difference (Voltage) across 1 Voltmeters are always connected in parallel across the device
Circuit configuration This shows the voltmeter measuring the potential drop across resistor 1 The ammeter measures the current through 1 The long line on the cell indicates the anode: current flows from anode to cathode Electron flow is actually from cathode to anode!
Exercises p. 180 #1-3 Chapter 5 Corrections Prep lab p. 193
Which scenario offers less resistance?
Equivalent esistance esistors in series simply add up to give equivalent n i i1 esistance decreases when you add resistors in parallel n i1 1 i 1
esistors in series esistance in parallel... 3 2 1 1 n i i 1 1 1 n i i... 1 1 1 1 3 2 1
Ex 1: Find the equivalent resistance Each resistor has a resistance of
Ohm s Law The potential difference V required to run a circuit is proportional to current I and resistance V I
Ohm s Law V I Put another way, the more resistance, the more energy you need to push things along
Ex 1: Find the voltage of the battery if 2 amps are passing through the 3 ohm resistor V I 2A3 6V
Ex 2: Find the current passing through the 6 ohm resistor from the 1.5V cell V I I V 1. 5V 6 0.25A
Ex 3: Find the resistance if there are 0.5 Amps flowing from the 6V battery V I V I 6V 0.5A 12
Ohms Law activity Voltage (Volts) esistance (Ohms) Current (Amps) I (current x resistance) 1 Battery 10 27 2 Batteries 10 27
Ex 2: Find the current passing through the ammeter given resistors 1,2,3 are 6, 12, and 12 ohms respectively V I I V 1. 5V 12 0.125A
Power Since V=J/C and I=C/s, multiplying these we get P IV
Ex 3: What is the power delivered to the 6 ohm resistor? P IV
Ex 3: What is the power delivered to the 6 ohm resistor? V I 1 1 1 0.125A6 0.75V P IV 0.125A 0. 75V 0.094W
Exercises p. 184 #1-5 Prep lab p. 193
Internal esistance
EMF This is the maximum ( unbelievable ) potential difference the battery is capable of delivering As we place greater demands on the chemical reactions occurring inside the battery, we find terminal voltage V T drops This is equivalent to having an internal resistor, r in series with the EMF
What does this mean? The potential drop across the internal resistance depends on current V Ir T
Ex 1: find the terminal voltage V Ir T V T 6.0V 2.0A 0. 50 V 5. 0V T
Ex 2: find the 6.0V internal resistance I 0.0 A When the ammeter is (briefly!) connected, measure the terminal voltage and current V Ir T r V T I r 1.0V 6.0V 5A r 1. 0
Ex 2: find the 5.8V internal resistance I 0.0 A When the ammeter is (briefly!) connected, measure the terminal voltage and current V Ir T r V T I r 1.45V 6.0V 3.4A r 1. 34
Ex 2: find the internal resistance 6.2V When a current of 0.53A passes through the battery, there is a terminal voltage of 5.4V V Ir T r 1. 5 r V T I
Ex 3: find the maximum current 6.2V r 1.5 V Ir T 0 Ir Ir I 4. 1A I max r
Ex 2: find the internal resistance 5.4V When a current of 36mA passes through the battery, there is a terminal voltage of 0.5V V Ir T r 136 r V T I
Exercises p. 186 #1-3 p. 191 #1-3 Finish lab 6-2 p. 193-6
Kirchoff s Laws
Loop Law For a given loop, the sum of potential gains must equal the sum of potential drops, or V 0
What does this mean? There are no credit cards, or savings accounts in the circuit world. Every penny must be spent and you may not spend more than you have
Did I mention you can t have anything to show for it?
Ex 1: Find V2
Find V 2 given V 1 =3V and V 4 =5V V V V V 0 1 2 4 V 12V 3V 5V 2 V 4V 2
Kirchoff s Junction Law The sum of flow into a junction in the circuit must equal the current out I I in out
Ex 2: find I1
Ex 2: find I 1 V I 3 3 3 V 1.5A 40 3 I 2 V 2 2 60V 10 V 60V V 3 2 I 6. 0A 2 I I I 1 2 3 1.5A 6. 0A 7.5A
Ex 3: find given the 8 ohm resistor dissipates 45J in 5s
Ex 3: find given the 8 ohm resistor dissipates 45J in 5s E P 8 8 45J 9W t 5s 2 8 V P V P 8 8 8 8 8 V 20V 8.48V 11. 52V 5 I 5 V 5 5 11.52V 5 9W 8 2.3A 8.48V
Ex 3: find given the 8 ohm resistor dissipates 45J in 5s I 8 V 8 8 8. 48V 8 1.06A I x I I 5 8 2.3A 1. 06A1.24A x V I x x 8.48V 1.24A 6. 8
Ex 3: find given the 8 ohm resistor dissipates 45J in 5s T T T I V A V 2.3 20 69 8. 1 8 5 1 1 x T x T 1 1 8 1 5
Ex 3: find given the 8 ohm resistor dissipates 45J in 5s 85 6. x T 1 1 8 1 5 1 8 1 5 1 T x 1 1 8 1 5 8.69 x
Ex: 4 Find the terminal voltage and EMF
Find the terminal voltage and EMF Ex: 4 V 2 I 3 3 3 I V 2 2 V 0.14A50 7V 7V 33 I 4 4 4 I I I I 1 2 3 4 7V 68 0.21A 0.103A 0.14A0.212A 0. 103A 0.455A
Find the terminal voltage and EMF Ex: 4 V I 1 1 1 0.455A10 4.55V V V V 4.55V 7V T 1 2 11.5V V Ir T 11.55V 0.455A 0. 80 11.9V
Find the terminal voltage and EMF Ex: 4 T 10 0.8 33 1 1 1 1 50 68 T 26. 2 I 0 T 0.455A26. 2 11.9V
p. 191 #1-6 Exercises
How is your teacher feeling today? angry angry angry ok
Ex: 5 A) Find the EMF given terminal voltage is 5.8V B) When the switch is opened, how is V T affected and why?
A) Find the EMF given terminal voltage is 5.8V B) When the switch is opened, how is V T affected and why? T 8 11 1 1 1 16 14. 52 I T V T T 5.8V 14.52 0.40A Ir 5.8V 0.40A1. 0 V T 6.2V B) When the switch is opened, how is V T affected and why? Total resistance is increased This causes current drawn from the battery to decrease Potential drop across the internal resistance decreases Therefore terminal voltage increases (closer to emf)
Ex: 6 Find the terminal voltage
Find the terminal voltage V I 8 8 8 0.6A8 4.8V V V (looplaw) 5 7 (AKA lunch law) V I P 4.8V 5 1 7 1 8 1 2.25A P T T P V I 2.25A6 4 2. 14 27V T
-or- V I 8 8 8 0.6A8 4.8V V V (looplaw) 5 7 I 5 V 5 5 4.8V 5 0.96A I 7 V 7 7 4.8V 7 0.69A I I I I 0 5 7 8 0.6A 0.96A0.69A 2. 25A V I 6 6 6 2.25A613.5V
-or- V I 6 6 6 V I 4 4 4 2.25A613.5V 2.25A4 9V V T V V V 6 5,7,8 4 13.5V 4.8V 9V 27V
Ex: 7 Find the power dissipated in the 25 ohm resistor
Ex 7: find P 25 I 10 V 10 10 7. 1V 10 0.71A P 1 1 12 25 1 8. 11 V T I T T 0.71A18. 1 12.86V
Ex 7: find P 25 P 25 V 2 25 25 5.76V 25 2 1.32W
Ex 7: find P 25 I 25 V 25 25 12.86V 7.1V 25 0.23A P I V 25 25 25 P 0.23A5. 76V 25 P 3W 25 1.
You must now answer these questions Chapter eview Questions p. 197-200 1-24 (Bonus 25-26) Test Yourself p. 201-206 1-14