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Electric Circuits

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Electric Circuits

Physics Unit 9

Current

- Rate of flow of charge
- Amount of charge per unit time that crosses one point

- Symbol: (I)
- Unit: ampere (A)

- Small computer speakers often have power supplies that give 12 VDC at 200 mA. How much charge flows through the circuit in 1 hour and how much energy is used to deliver this charge?
- = 720 C
- E = 8640 J

Conventional Current

- Electrons are the charge that flows through wires
- Historically thought positive charges move
- Conventional current imaginary flow of positive charges
- Flows from positive terminal and into negative terminal
- Real current flows the opposite way

Drift Velocity

- Electrical signals travel near speed of light, but electrons travel much slower
- Each new electron pushes one ahead of it, so current is actually like wave
- q = charge of each electron
- n = free charge density
- A = cross-sectional area
- = drift velocity

- Think of water pumps
- Bigger pumps more water flowing
- Skinny pipes (more resistance) less water flow

- Electrical Circuits
- Bigger battery voltage more current
- Big electrical resistance less current

Ohm’s Law

- V = emf
- I = current
- R = resistance
- Unit: V/A = ohm ()

Resistors

- Device that offers resistance to flow of charges
- Copper wire has very little resistance
- Symbols used for
- Resistor
- Wire

- Our speakers use 200 mA of current at maximum volume. The voltage is 12V. The current is used to produce a magnet which is used to move the speaker cone. Find the resistance of the electromagnet.
- R = 60

- Hopefully these circuit problems won’t have you running around in circles
- 20P1-4, 7a, 8-10, 18-19, 22-23
- Read 20.3
- 20CQ9-11
- Answers
- 1) 0.278 mA
- 2) 333 mA

- 3) 0.250 A
- 4) 4.00 mA
- 7a) 1.67 k
- 8) 5.00 V
- 9) 0.120 C, electrons
- 10) s, electrons/s
- 18) 0.833 A
- 19) 6.75 k
- 22) 0.300 V, 1.50 V
- 23) 0.200 mA

Another way to find resistance

- The resistance varies directly with length and inversely with width (or cross-sectional area) a wire
- Kind of like trying to get a lot of water through a pipe

- Short, thick wire small resistance
- Long, skinny wire large resistance

- = resistivity
- Unit: m

- Table 20.1 lists resistivities of some materials
- Metals small resistivity (1x10-8 m)
- Insulators large resisitivity (1x1015 m)
- Semi-conductors medium resistivity

Why are long wires thick?

- Wire thicknesses are measured in gauges. 20-gauge wire is thinner than 16-gauge wire. If 20-gauge wire has and 16-gauge wire has , find the resistance per meter of each if they are copper.
- 20-guage
- 16-guage

Resistivity and Temperature

- = resistivity at temperature T
- = resistivity at temperature T0
- = temperature coefficient of resistivity
- Unit: 1/°C (or 1/K)

- Metals
- Resistivity increases with temperature
- is positive

- Semiconductors
- Resistivity decreases with temperature
- is negative

Resistance and Temperature

- R = resistance at temperature T
- R0 = resistance at temperature T0
- = temperature coefficient of resistivity
- Unit: 1/°C (or 1/K)

- A heating element is a wire with cross-sectional area of and is 1.3 m long. The material has resistivity of at 200°C and a temperature coefficient of 1/°C. Find the resistance of the element at 350°C.
- R = 1430

Superconductors

- Materials whose resistivity = 0
- Metals become superconductors at very low temperatures
- Some materials using copper oxide work at much higher temperatures
- No current loss
- Used in
- Transmission of electricity
- MRI
- Maglev
- Powerful, small electric motors
- Faster computer chips

- Resistance is futile
- 20P24-26, 28-31, 33, 37a
- Read 20.4, 20.5
- 20CQ14-17
- Answers
- 24) 0.104
- 25) 0.322

- 26) m
- 28) A
- 29) 276 C
- 30) 25 C
- 31) /C
- 33)
- 37a) 40.0 C

Power

- Unit: Watt (W)
- Other equations for electrical power

- Let’s say an electric heater has a resistance of 1430 and operates at 120V. What is the power rating of the heater? How much electrical energy does it use in 24 hours?
- P = 10.1 W
- E = 873 kJ

Kilowatt hours

- Electrical companies charge you for the amount of electrical energy you use
- Measured in kilowatt hours (kWh)
- If electricity costs $0.15 per kWh how much does it cost to operate the previous heater (P = 10.1 W) for one month?
- $1.09

Alternating Current

- Charge flow reverses direction periodically
- Due to way that power plants generate power
- Simple circuit

Periodicity

- Voltage, Current, and Power fluctuate with time
- So we usually talk about the averages

Average Power

- DC
- AC
- Often P is used to represent average power in all AC circuits.

Root Mean Square (rms)

- and are called root mean square current and voltage
- Found by dividing the max by

Convention in USA

- V0 = 170 V
- Vrms = 120 V
- Most electronics specify 120 V, so they really mean Vrms
- We will always (unless noted) use average power, and root mean square current and voltage
- Thus all previously learned equations work!

- A 60 W light bulb operates on a peak voltage of 156 V. Find the Vrms, Irms, and resistance of the light bulb.
- Vrms = 110 V
- Irms = 0.55 A
- R = 202

- Why are you not supposed to use extension cords for devices that use a lot of power like electric heaters?
- P = IV
- P is large so I is large

- The wire has some resistance
- The large current and little resistance can cause heating
- If wire gets too hot, the plastic insulation melts

- Don’t write down just answers. Alternatively show your work, too.
- 20P40-44, 50-51, 72-74, 79-80
- Read 20.6, 20.7
- 20CQ19-21, 24-26
- Answers
- 40) W
- 41) 6.00 kW

- 42) W
- 43) 1.00 W
- 44) 1.50 W, 7.50 W
- 50) $438/y
- 51) 0.18 cents
- 72) ,
- 73) 480 V
- 74) 21.2 A
- 79) 2.40 kW
- 80) 5.89 A

- Thermal Hazards
- Electric energy converted to thermal energy faster than can be dissipated
- Happens in short circuits
- Electricity jumps between two parts of circuits bypassing the main load

- Low R so high P
- Can start fires
- Circuit breakers or fuses try to stop

- High resistance (thin)
- Or are coiled

- Shock Hazards
- Factors
- Amount of Current
- Path of current
- Duration of shock
- Frequency of current

- Factors
- Human body mainly water, so decent conductor

- Muscles are controlled by electrical impulses in nerves
- A shock can cause muscles to contract
- Cause fist to close around wire (muscles to close, stronger than to open)

- Can cause heart to stop
- Body most sensitive to 50-60 Hz

- Don’t let these problems shock you.
- 20P85-86, 87 (part b change 4000 k to 4500 ), 88-92
- Read 21.1
- 21CQ2-5, 8, 10, 12
- Answers
- 85) 230 kW, 960 A

- 86) 12 V
- 87) 0.400 mA (no effect), 26.7 mA (muscular contraction)
- 88) 400 V
- 89)
- 90) 6.00 mV
- 91) 1.00 , 14.4 kW
- 92) C, 10.0 kV, 1.00 k, C

Series Wiring

- More than one device on circuit
- Same current through each device
- Break in device means no current
- Form one “loop”
- The resisters divide the voltage between them

- V divide among resistors
- V = V1 + V2 + V3
- V = IR
- V = IR1 + IR2 + IR3
- V = I(R1 + R2 +R3)
- V = IRS
- RS = R1 + R2 + R3 + …

- A 807 resistor and a 569 resistor are connected in series. What is the equivalent resistance?
- 1.376 k

- Bathroom vanity lights are often wired in series. V = 120 V and you install 3 bulbs with R = 8 and 1 bulb with R = 12. What is the current, voltage of each bulb, and the total power used?
- I = 3.33 A
- V = 26.7 V, 40 V
- Ptotal = 400 W

Parallel Wiring

- Same voltage across several devices
- Typical house wiring
- Break in device has no effect on current
- Resistors divide current

Derivation

- Each branch draws current as if the other wasn’t there
- Each branch draws less current than the power supply gives
- R = V / I
- Overall circuit: Large I Small R
- Smaller resistance than either branch

Parallel Resistors

- A 569 resistor and a 807 resistor are connected in parallel. What is the equivalent resistance?
- 334
- If they were connected to a 1.5 V battery, how much total current would the battery supply?
- 4.5 mA
- How much current through each resistor?
- 2.6 mA and 1.9 mA

Circuits Wired Partially in Series and Partially in Parallel

- Simplify any series portions of each branch
- Simplify the parallel circuitry of the branches
- If necessary simplify any remaining series

10

8

7

27 V

3

5

- Find the equivalent resistance and the total current of the following circuit.

2

1

20

1.5 V

5

4

15

3

2.5

- Find the equivalent resistance and the voltage in the far right branch.

- These series of problems parallel the lesson.
- 21P1-4, 6, 10, 12-13
- Read 21.2
- 21CQ14, 16, 17
- Answers
- 1) 2.75 k, 27.5

- 2) 6.60 k, 93.9
- 3) 786
- 4) 15 A, 11.7 A, 0.63 A, yes
- 6) 0.400 A, 3.84 W, 15.4 W; 2.5 A
- 10) , W,
- 12)
- 13)

- Emf
- Electromotive force
- Not really a force
- Really voltage produced that could drive a current

Internal Resistance

- Batteries and generators have resistance
- In batteries due to chemicals
- In generators due to wires and other components
- Internal resistance is connected in series with the equivalent resistance of the circuit

- Internal resistance causes terminal voltage to drop below emf
- Internal resistance is not necessarily negligible
- terminal voltage
- emf
- current of circuit
- internal resistance

- A string of 20 Christmas light are connected in series with a 3.0 V battery. Each light has a resistance of 10 . The terminal voltage is measured as 2.0 V. What is the internal resistance of the battery?
- 100

- A battery has an internal resistance of 0.02 and an emf of 1.5 V. If the battery is connected with five 15 light bulbs connected in parallel, what is the terminal voltage of the battery?
- 1.49 V

- If batteries are connected in series, their emfs add, but so do the internal resistances
- If batteries are connected in parallel, their emfsstay the same, but the currents add and the combined internal resistance is less

- Hard work takes lots of emf!
- 21P14-19, 21, 23, 27
- Read 21.3
- 21CQ19, 20, 22
- Answers:
- 14) 2.00 V
- 15) 6, 9.24 V, r’s add so smaller V

- 16) 2.9994 V
- 17) 1.34 V, 3.08 W, 2.68 W
- 18) 0.375
- 19) 18.0 V, 18.0 V
- 21) 0.658 A, 0.997 W, 0.997 W
- 23) 200 A, 10.0 V, 2.00 kW, 80.0 A, 4.0 V, 320 W
- 27) 51 A

Kirchhoff’s Rules

- Junction Rule
- Total current into a junction must equal the total current out of a junction

- Loop Rule
- For a closed-circuit loop, the total of all the potential rises total of all potential drops = 0
- (or the total voltage of a loop is zero)

Reasoning Strategy

- Draw the current in each branch of the circuit (flows out of positive terminal of battery). Choose any direction. If you are wrong you will get a negative current.
- Mark each element with a plus and minus signs at opposite ends to show potential drop. (Current flows from + to – through a resistor)
- If the current leaves the element at +, voltage rise
- If the current leaves the element at -, voltage drop

- Apply junction rule and loop rule to get as many independent equations as there are variables.
- Solve the system of equations.

12V

5

3

I

8V

4

2

- Find the current through the circuit

30

I1

45 V

40

1

I3

20

I2

1

80 V

- Find I1, I2, and I3.

- Currently, you need to work on these problems
- 21P31-32, 35-36, 38
- Read 21.4
- 21CQ25-27
- Answers
- 31)

- 32)
- 35)
- 36)
- 38) A, A, A

- Analog (non-digital) meters
- Main component galvanometer

- Ammeters
- Measures current
- Inserted into circuit so current passes through it
- Connected in series

- Coil usually measures only little current
- Has shunt resistors connected in parallel to galvanometer so excess current can bypass
- A knob lets you select which shunt resistor is used

- Problems with Ammeters
- The resistance of the coil and shunt resistors add to the resistance of the circuit
- This reduces the current in the circuit
- Ideal ammeter has no resistance
- Real-life good ammeters have small resistance so as only cause a negligible change in current

- Voltmeters
- Connected in parallel to circuit since parallel has same voltage
- The coil works just like in the ammeter
- Given the current and the resistance of the coil V = IR
- To give more range, a large resistor is connected in series with the coil

- Problems with Voltmeters
- The voltmeter takes some the voltage out of the circuit
- Ideal voltmeter would have infinitely large resistance as to draw tiny current
- Good voltmeter has large enough resistance as to make the current draw (and voltage drop) negligible

- See if you measure up to these meter problems
- 21P42-47
- Read 21.6
- 21CQ30, 32, 34-35, 39

- Answers
- 42) 30.0 A
- 43) 4.00 mA
- 44) 1.98 k
- 45)
- 46)
- 47)

Charging a Capacitor

- Circuit with a capacitor, battery, and resistor
- Initially capacitor is uncharged
- When battery connected current flows to charge capacitor
- As charges build up, there is increased resistance because of the repulsion of the charges on the parallel plates
- When capacitor is fully charged, no current will flow

- Loop Rule
- Solve for I
- I is rate of change of q
- Differential Calculus says

Charging a Capacitor

- RC = (time constant – The time required to charge the capacitor to 63.2%)
- CV = Q (maximum charge)

- Where
- V is voltage across the capacitor
- is emf
- t is time
- R is resistance of circuit
- C is capacitance

Discharging a Capacitor

- The battery is disconnected
- The capacitor acts like a battery supplying current to the circuit
- Loop Rule

- Often capacitors are used to charge slowly, then discharge quickly like in camera flash.
- Done by have different values for R in charging and discharging.

- Most cars have intermittent windshield wipers. How do capacitors help with that?
- The wipers are part of an RC circuit the time constant of which can be varied by selecting different R’s. The brief time that the wipers remain on and the time they are off are determined by the value of the time constant.

- An uncharged capacitor and a resistor are connected in series to a battery. If V = 12 V, C = 5 F, and R = 8x105 . Find the time constant, max charge, max current, and charge as a function of time.

- Discharge your knowledge by completing these problems
- 21P63-65, 68, 70

- Answers
- 63) 4.00 to 30.0 M
- 64)
- 65) 2.50 F, 2.00 s
- 68) 12.3 mA, s, 4.53 mA, 3.89 V
- 70) F, No