warm‐up - santa monica high school...
TRANSCRIPT
Warm‐UpAn electrical force of 972 N exists between two charges separated by a distance of 4 m. At what distance would the force be reduced to 12 N?
FlowofCharge Charge flows when there is a potential difference
When the ends of an electric conductor are at different electric potentials, charge flows from one end to the other
The flow of charge will continue until both ends reach a common potential
ElectricCurrent(I) Electric current = the flow of electric charge
e‐ carry the charge through the circuit because they are free to move throughout the atomic network
Electric current is measured in amperes (A) ampere = the flow of 1 coulomb of charge per second
current = charge time
I = Δq Δt
A current‐carrying wire has a net electric charge of zero because the number of electrons entering one end is the same as the number leaving the other.
What is the net charge of a current carrying wire (as pictured above)?
Current(I)vs.Voltage(V) Charges flow when there is a potential difference (V) A voltage source provides a potential difference
Ex: batteries, generators Charges flow through a circuit because of an applied voltage across the circuit
Voltage (V) causes current (I)
If I increase the potential difference (voltage), will the current… (a) increase or (b) decrease? Increase! V α I
Resistance(R) We now know:
Amount of charge that flows in a circuit depends on the voltage provided by the voltage source (V α I)
But… The current also depends on the resistance (R) that the conductor offers to the flow of charge—the electric resistance
Resistance is measured in ohms (Ω)
Resistance (R) of a wire depends on:
Conductivity of the material used in the wire Better conductors have less resistance
Thickness of the wire Thick wires have less resistance than thin wires.
Length of the wire Longer wires have more resistance than short wires
Temperature For most conductors, increased temperature means increased
resistance
Why will an electric drill operating on a very long extension cord not rotate as fast as one operated on a short cord?
The longer an extension cord, the greater its resistance, and the more energy is dissipated in it, with less available for the electric drill.
Whatdoweknow?How is current (I) related to voltage (V)?
How is current (I) related to resistance (R)?
Combining these findings…
I α V
I α 1/R
I = V/R
Ohm’sLaw The current in a circuit is directly proportional to the voltage impressed across the circuit, and is inversely proportional to the resistance of the circuit
I = V/R V = I*R
Units: V = volts (V) I = amps (A) R = ohms (Ω)
Examples If the voltage impressed across a circuit is constant but the resistance doubles, what change occurs in the current?
V = IR V = 2R*_I V = 2R * ½ I The current is halved
How much voltage is required to make 2 amperes flow through a resistance of 8 ohms?
V = IR V = 2 A * 8 Ω V = 16 volts
Ohm’sLawandElectricShock The damaging effects of electric shock are the result of current passing through the body
Body’s resistance = 100 ohms if you’re soaked with salt water to about 500,000 ohms if your skin is very dry
Whydon’tbirdsgetfried? Every part of the bird’s body is at the same high potential as the wire, so the bird feels no effects
For the bird to receive a shock, there must be a difference in potential between one part of its body and another part
Jane falls from a bridge and manages to grab onto a high‐voltage power line, halting her fall. Will she be ok?? As long as Jane touches nothing else of different potential, she will receive no shock at all. Even if the wire is thousands of volts above ground potential and even if she hangs by it with two hands, no charge will flow from one hand to the other. If, however, you reach over with one hand and grab onto a wire of different potential, ZAP!!
Sourceofe‐inacircuit The source of electrons in a circuit is the conducting circuit material itself
Electrons DO NOT come from: electric outlets in the walls of homes power lines
Power utilities do not sell electrons. They sell energy. You supply the electrons.
When you are jolted by an AC electric shock, the electrons making up the current in your body originate in your body. Electrons do not come out of the wire and through your body and into the ground; energy does.
ElectricPower A charge moving in a circuit expends energy
This may result in heating the circuit or in turning a motor
Electric power (P) = the rate at which electrical energy is converted into another form such as mechanical energy, heat, or light Measured in Watts (W)
Electric power is equal to the product of current and voltage
P = V*I P = I2*R
Example Calculate the power supplied to an electric blanket that carries 1.20 A when connected to a 120‐V outlet.
P = V*I
P = 120 V*1.20 A
P = 144 W
Warm‐UpThe Garcias like to keep their 40 W front porch light on at night to welcome visitors. If the light is on from 6 p.m. until 7 a.m., and the Garcias pay 8.00¢ per kilowatt‐hour (kWh), how much does it cost to run the light for this amount of time each year?
ElectricCircuits Circuit = Any path along which electrons can flow
For a continuous flow of electrons, there must be a complete circuit with no gaps
A gap is usually provided by an electric switch that can be opened or closed to either cut off or allow electron flow
In circuits, current flows through resistors and voltage is established across resistors.
A short circuit occurs when current flows along a path without no (or very little) resistance.
Light bulbs are examples of resistors. They take electrical energy and convert it to heat (light). The brightness of the bulb depends on the amount of electrical power the bulb dissipates.
InSeries Total resistance to current in the circuit is the sum of the individual resistances along the circuit path
Rtotal = R1 + R2 + R3
Find the total resistance of the three resistors connected in series.
InSeries Current passing through each electric device is the same
I1 = I2 = I3
Ohm’s Law applies across each individual device V1 = I*R1 V2 = I*R2 V3 = I*R3
Sum of the voltage drops across the individual devices is equal to the total voltage supplied by the source Vtotal = V1 + V2 + V3
Vtotal = I*Requivalent
Example2:Series Find the resistance of the unknown resistor, R.
The (A) is an ammeter. It is a device that measures current flow.
0.5 A
Example3:Series Two 4 Ω resistors and two 3 Ω resistors are connected in series to a 28 V battery. What is the equivalent resistance? What is the current in the circuit? What is the voltage drop across each resistor?
Warm‐UpA 10 Ω resistor, a 15 Ω resistor, and a 5 Ω resistor are connected in series across a 90 V battery.
a) What is the equivalent resistance of the circuit? b) What is the current in the circuit? c) What is the voltage drop across the 10 Ω resistor? d) What is the voltage drop across the 15 Ω resistor?
InParallel Overall resistance of the circuit is less than the resistance of any one of the branches
Find the total resistance of the three resistors connected in parallel.
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1Req
=1R1
+1R2
+1R3
+ ...
InParallel Overall resistance of the circuit is less than the resistance of any one of the branches 1/Rtotal = 1/R1 + 1/R2 + 1/R3
Voltage is the same across each device V1 = V2 = V3
Total current in the circuit equals the sum of the currents in its parallel branches Itotal = I1 + I2 + I3
Ohm’s Law applies across each individual device V = I1*R1 V = I2*R2 V = I3*R3
Example2:Parallel Three resistors are connected in parallel. If placed in a circuit with a 12‐volt power supply. Determine the equivalent resistance, the total circuit current, and the voltage drop across and current in each resistor.
Warm‐UpThree resistors of the same unknown resistance are connected in series with a 20 V battery. 5 A of current flows through the battery. What is the value of each of the resistors?