Electrical Current (5) There are electrical currents in your phone, in your house, and in the hearts...

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.