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Electrical Current (5)
There are electrical currents in your phone, in your house, and in the
hearts of your loved ones.
Mr. KlapholzShaker Heights
High School
The basic quantities:
• Charge• Current• Potential Difference (“Voltage”)• Resistance
Charge (1 of 2)• Charge can be _ _ _ _ _ _ _ _ or negative. • Symbol: Q or q.• Units: Coulomb (C)• Like charges _ _ _ _ _.• Opposite charges _ _ _ _ _ _ _.• An object will be neutral if it has _ _ _ _ _ numbers
of positive and negative charges.
Charge (1 of 2)• Charge can be positive or negative. • Symbol: Q or q.• Units: Coulomb (C)• Like charges _ _ _ _ _.• Opposite charges _ _ _ _ _ _ _.• An object will be neutral if it has _ _ _ _ _ numbers
of positive and negative charges.
Charge (1 of 2)• Charge can be positive or negative. • Symbol: Q or q.• Units: Coulomb (C)• Like charges repel.• Opposite charges _ _ _ _ _ _ _.• An object will be neutral if it has _ _ _ _ _ numbers
of positive and negative charges.
Charge (1 of 2)• Charge can be positive or negative. • Symbol: Q or q.• Units: Coulomb (C)• Like charges repel.• Opposite charges attract.• An object will be neutral if it has _ _ _ _ _ numbers
of positive and negative charges.
Charge (1 of 2)• Charge can be positive or negative. • Symbol: Q or q.• Units: Coulomb (C)• Like charges repel.• Opposite charges attract.• An object will be neutral if it has equal numbers of
positive and negative charges.
Charge (2 of 2)• If like charges are near each other, there is a _ _ _ of
electrical potential energy. [Example: lightning.]• If unlike charges are far from each other, there is a
_ _ _ of electrical potential energy. [Example: charging a rechargeable battery.]
• The charge on an electron is -1.602 x 10-19 C .• The charge on a proton is C.• There are 6.25 x 1018 C protons in 1 C.
Charge (2 of 2)• If like charges are near each other, there is a lot of
electrical potential energy. [Example: lightning.]• If unlike charges are far from each other, there is a
_ _ _ of electrical potential energy. [Example: charging a rechargeable battery.]
• The charge on an electron is -1.602 x 10-19 C .• The charge on a proton is _______________ C.• There are 6.25 x 1018 C protons in 1 C.
Charge (2 of 2)• If like charges are near each other, there is a lot of
electrical potential energy. [Example: lightning.]• If unlike charges are far from each other, there is a
lot of electrical potential energy. [Example: charging a rechargeable battery.]
• The charge on an electron is -1.602 x 10-19 C .• The charge on a proton is ______________ C.• There are 6.25 x 1018 C protons in 1 C.
Charge (2 of 2)• If like charges are near each other, there is a lot of
electrical potential energy. [Example: lightning.]• If unlike charges are far from each other, there is a
lot of electrical potential energy. [Example: charging a rechargeable battery.]
• The charge on an electron is -1.602 x 10-19 C .• The charge on a proton is +1.602 x 10-19 C.• There are 6.25 x 1018 C protons in 1 C.
Current• Current is the rate at which charge flows.• Symbol: I or i.
I = Q / T• Units: Ampere (A) • 1 Amp = 1 _ _ _ _ _ _ _ / _ _ _ _ _ _ .• Surprisingly, in the International System the Amp is
a fundamental unit: m k s A.• An object with a current passing through it does not
gain charge: just as much charge leaves as arrives.
Current• Current is the rate at which charge flows.• Symbol: I or i.
I = Q / T• Units: Ampere (A) • 1 Amp = 1 Coulomb/second.• Surprisingly, in the International System the Amp is
a fundamental unit: m k s A.• An object with a current passing through it does not
gain charge: just as much charge leaves as arrives.
Potential Difference (“Voltage”)• This is the single most abstract idea in the physics
curriculum. Roughly speaking, potential difference is the energy per charge.
• More exactly, potential difference is the electrical potential energy difference per charge.
• Symbol: V• V = DE / Q.• Units: Volts (V). 1 Volt = 1 _ _ _ _ _ / _ _ _ _ _ _ _ .• Voltage causes current.
Potential Difference (“Voltage”)• This is the single most abstract idea in the physics
curriculum. Roughly speaking, potential difference is the energy per charge.
• More exactly, potential difference is the electrical potential energy difference per charge.
• Symbol: V• V = DE / Q.• Units: Volts (V). 1 Volt = 1 Joule / Coulomb.• Voltage causes current.
Electrical Potential Energy• Electrons flow away from the negative place, and
toward the positive place.• Protons have a force on them too. Protons are
forced away from positive charge and toward negative charge.
• Positive locations have high PE, and negative locations have low PE.
• If current were made of positive charges, then in a circuit they would flow from the positive end of a battery, through wires, to the negative end of the battery. Electrons flow the other way.
Resistance (1 of 2)• This is a measure of how tough it is to make charge
flow through an object. • Symbol: R• Units: _ _ _ (W) • Long object have a _ _ _ of resistance. • Narrow objects have a _ _ _ of resistance.• Ceramics have a _ _ _ of resistance.• Metals do _ _ _ have much resistance.
Resistance (1 of 2)• This is a measure of how tough it is to make charge
flow through an object. • Symbol: R• Units: Ohm (W) • Long object have a _ _ _ of resistance. • Narrow objects have a _ _ _ of resistance.• Ceramics have a _ _ _ of resistance.• Metals do _ _ _ have much resistance.
Resistance (1 of 2)• This is a measure of how tough it is to make charge
flow through an object. • Symbol: R• Units: Ohm (W) • Long object have a lot of resistance. • Narrow objects have a _ _ _ of resistance.• Ceramics have a _ _ _ of resistance.• Metals do _ _ _ have much resistance.
Resistance (1 of 2)• This is a measure of how tough it is to make charge
flow through an object. • Symbol: R• Units: Ohm (W) • Long object have a lot of resistance. • Narrow objects have a lot of resistance.• Ceramics have a _ _ _ of resistance.• Metals do _ _ _ have much resistance.
Resistance (1 of 2)• This is a measure of how tough it is to make charge
flow through an object. • Symbol: R• Units: Ohm (W) • Long object have a lot of resistance. • Narrow objects have a lot of resistance.• Ceramics have a lot of resistance.• Metals do _ _ _ have much resistance.
Resistance (1 of 2)• This is a measure of how tough it is to make charge
flow through an object. • Symbol: R• Units: Ohm (W) • Long object have a lot of resistance. • Narrow objects have a lot of resistance.• Ceramics have a lot of resistance.• Metals do not have much resistance.
Resistance (2 of 2)• When a potential difference is put across a wire, the
electrons move toward the positive end. This is a current.
• While flowing, electrons slam into the particles in the solid; this _ _ _ _ _ the electrons; their motion is resisted. The current is limited.
Resistance (2 of 2)• When a potential difference is put across a wire, the
electrons move toward the positive end. This is a current.
• While flowing, electrons slam into the particles in the solid; this slows the electrons; their motion is resisted. The current is limited.
The essential hardware:
• Wire• Bulb• Cell and Battery• Resistor• Circuit
Wire
• Wires are conductors.• Wires do not have a lot
of resistance.
Insulators
• Insulators have high resistance.
• Why are the insulators in the photo just as vital as the conductors?
Bulb
• The filament is a resistor.
• Why does it glow?
Battery
• Inside a battery are chemicals ‘cells’.
• The reaction pushes electrons toward the negative pole.
• The reaction pulls electrons away from the positive pole.
• This produces a potential difference of about 1.5 V.
Resistor
• Although every object resists current, a resistor has a measured value for resistance.
• There are resistors in every electronic device you own.
http://www.jestineyong.com/?p=2340
Circuits are loops
http://www.linkwitzlab.com/Pluto/woofer-asp.htm
http://www.teachengineering.org/view_activity.php?url=http://www.teachengineering.org/collection/cub_/activities/cub_electricity/cub_electricity_lesson03_activity1.xml
The most important equation:
Ohm’s Law:I = V / R
What does it mean?...
What is Ohm’s law trying to tell us?I = V / R
• In simple objects, if you put twice as much potential difference across an object, then you will twice as much current in the object. [We call these objects “ohmic”.]
• If the object has twice as much resistance, then there will be half the current.
• This is a cause-effect pattern. Much like:a = SF / M and DT = Q / Mc
Resistance depends on at least 3 things.
• Longer (‘L’ for length) objects have more resistance.
• More narrow objects have more resistance (‘A’ for area).
• Some materials resist flow more than others ( ‘r’ for resistivity). See the table…
R = rL/A
Resistivity for most materials is either very high (insulators) or very low (conductors).
Material Resistivity r(Ohm meter)
Glass 1 x 1012
Copper 1 x 10-8
Series vs. Parallel
Series Circuit
http://www.petervaldivia.com/technology/electricity/types_of_currents.php
Three resistors in series
R1
R3
R2
Parallel Circuit
http://pzweb.harvard.edu/ucp/curriculum/circuits/s5_background.htm
Three resistors in parallel
R1 R2 R3
What is the relationship between the currents?
R1
R3
R2
I3
I1
IPS
I2
I1 I2 I3 IPS
PS
What is the relationship between the currents?
R1
R3
R2
I3
I1
IPS
I2
I1 = I2 = I3 = IPS
This makes sense because the charge justgoes around and around the single loop.
PS
What is the relationship between the voltages?
V1
R1
R3
R2V2
V3
V1 V2 V3 VPS
PS VPS
What is the relationship between the voltages?
V1
R1
R3
R2V2
V3
V1 + V2 + V3 = VPS
This makes sense because the energy coming out of theresistors comes into the system from the power supply
VPSPS
What is the relationship between the Resistances?
R1
R3
R2
R1 R2 R3 REQ
What is the relationship between the Resistances?
R1
R3
R2
R1 + R2 + R3 = REQ
This makes sense because any one electronwill need to go through all of the resistors.
What is the relationship between the currents?
R1 R2 R3
I1 I2 I3
IPS
I1 I2 I3 IPS
What is the relationship between the currents?
R1 R2 R3
I1 I2 I3
IPS
I1 + I2 + I3 = IPSThis makes sense because the small streams make the big river.
What is the relationship between the voltages?
V V V V
R1 R2 R3
V1 V2 V3 VPS
What is the relationship between the voltages?
V V V V
R1 R2 R3
V1 = V2 = V3 = VPSThis makes sense because each resistor is connected to the power supply.
What is the relationship between the resistances?
R1 R2 R3
R1 R2 R3 REQ
What is the relationship between the resistances?
R1 R2 R3
REQ is less than the smallest resistance!
This makes sense becauseeach resistor is really a pathway !
What is the relationship between the resistances?
R1 R2 R3
REQ is less than the smallest resistance!
€
1
REQ=1
R1+1
R2+1
R3
Batteries in series (What would be the effect?)
http://pzweb.harvard.edu/ucp/curriculum/circuits/s5_background.htm
Measuring Current (Ammeters)
Ammeters go in the flow.
R1
R3
R2
I3
I1
IPS
I2
PS
Measuring Voltage (Voltmeters)
Voltmeters go outside of the flow.
V V V V
R1 R2 R3
The Voltage Divider (1 of 4)
• If you needed 3 V to run a device, but you only had a 9 V battery, there is an important device that would come in handy: the Voltage Divider.
• You can build a voltage divider using two resistors…
The Voltage Divider (2 of 4)
http://people.sinclair.edu/nickreeder/eet150/mod05.htm
The Voltage Divider (3 of 4)• The total resistance is:
2000 W + 1000 W = 3000 W.
• The current in the circuit is: I = V/R = 9V / 3000 W = 0.003
A• The voltage across the top resistor is
V = IR = (0.003 A)(2000 W) = 6 V• The voltage across the bottom resistor is
V = IR = (0.003 A)(1000 W) = 3 VHey, that’s exactly what we needed!
The Voltage Divider (4 of 4)
€
VOUT =VINR2
R1 + R2
Sensors (part of what makes electricity so useful)
• Sensors are devices that take a non-electrical _ _ _ _ _ and produce an electrical _ _ _ _ _ _.
• This lets us measure temperature, acceleration, intensity of light and sound, force, and much more.
Sensors (part of what makes electricity so useful)
• Sensors are devices that take a non-electrical input and produce an electrical output.
• This lets us measure temperature, acceleration, intensity of light and sound, force, and much more.
Sensors
Thermistor Strain (Force)
Sensors are often used as one of the resistors in a voltage divider.
• This allows the output voltage to be proportional to the quantity that we care about: temperature, weight, etc.
Internal Resistance (r) of a battery
• Charge moves through every element of a circuit (including batteries).
• Every object has atoms, so every object has resistance to current (including batteries).
• The resistance of a battery is handled with a special variable: r, the “Internal Resistance” of the battery.
Emf (E)
• We have a special name for the ideal potential difference made by an energy source: Electromagnetic Force (E = Emf).
• This is the amount of chemical energy that has been changed to electrical energy (per charge).
• This applies to cells, batteries, generators, anything that changes energy of one sort into electrical energy.
Why is the voltage of a battery less than its emf?
• The potential difference (voltage) across a battery is less than the energy per charge that the chemicals made.
• Current in any object makes heat.• V < E is due to the wasted energy of heating up the
battery. The current moves through the cells and heats them.
• V = E – Ir.
Power = VI
• The power that is delivered to the circuit is EI.• The power that is taken out of the circuit is VI.• If we use Ohm’s law we can see that power
also can be written as I2R.• If we use Ohm’s law again we can see that
power also can be written as V2 / R.