electrical energy and currents

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Electrical Energy and CurrentsChapter 17Pg. 592-625

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17.1 Electric Potential Pg. 594-601

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What do you think?

• You may have purchased batteries for radios, watches, CD players, and other electronic devices. Batteries come in a variety of different sizes and voltages. You probably have 1.5 volt, 3 volt, and 12 volt batteries in your home.• What do volts measure?• Is the number of volts related to the size of the

battery?• How is a 3 volt battery different from a 1.5 volt

battery?

+Electric Potential Energy

Potential energy associated with a charge due to its position in an electric field

Electrical potential energy is a component of mechanical energyM. E. is conserved so long as friction and

radiation are not present

Electrical potential energy can be associated with a charge in a uniform field

+Electrical Potential Energy

A uniform electric field exerts a force on a charged particle moving it from A to B.

Will the particle shown gain or lose PEelectricas it moves to the right? Lose energy (because it is moving with the

force, not against it) Similar to a falling object losing PEg

PEelectric = Wdone = Fd = -qED

+Electrical Potential Energy

PEelectric is positive if the charge is negative and moves with the field.

PEelectric is positive if the charge is positive and moves against the field.

+Classroom Practice Problem

A uniform electric field strength of 1.0 x 106 N/C exists between a cloud at a height of 1.5 km and the ground. A lightning bolt transfers 25 C of charge to the ground. What is the change in PEelectric for this lightning bolt?

d= 1,500m q= 25C

E= 1.0 X 106 N/C PEelectric= ??

+Classrom Practice Problem

PEelectric = -qEd

PEe= (-25)(1.0 X 106)(1500)

Answer: -3.75 x 1010 J of energy

+Potential Difference

Potential difference(DV) is the change in electrical potential energy per coulomb of charge between two points. Depends on the electric field and on the

initial and final positions Does not depend on the amount of charge SI unit: joules/coulomb (J/C) or Volts (V)

+Potential Difference

The potential difference is calculated between two points, A and B. The field must be uniform.

+Batteries

A battery maintains a constant potential difference between the terminals. 1.5 V (AAA, AA, C and D cell) or 9.0 V or 12 V (car)

In 1.5 V batteries, the electrons use chemical energy to move from the positive to the negative terminal. They gain 1.5 joules of energy per coulomb of charge

When connected to a flashlight, the electrons move through the bulb and lose 1.5 joules of energy per coulomb of charge.

Sort of like a concentration gradient.

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Now what do you think?

You may have purchased batteries for radios, watches, CD players, and other electronic devices. Batteries come in a variety of different sizes and voltages. You probably have 1.5 volt, 3 volt, and 12 volt batteries in your home.What do volts measure? Is the number of volts related to the size of the

battery?How is a 3 volt battery different from a 1.5 volt

battery?

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17.2 Capacitance Pg 602- 607

+What do you think?

• If a light bulb replaced the two metal plates and the battery was connected, electrons would flow out of the negative and into the positive terminal. Will this also occur with the two metal plates?• If not, why not?• If so, is the flow similar or different from that with the

light bulb? Explain.• The battery shown has a

potential difference of 6.0 volts. It has just been connected to two metal plates separated by an air gap. There is no electrical connection between the two plates and air is a very poor conductor.

+Capacitors

A device that is used to store PEelectric

The two metal plates are electrically neutral before the switch is closed. What will happen when the switch is closed if the left plate is connected to the negative terminal of the battery? Electrons will flow toward lower PE.

From the battery to the left plate From the right plate to the battery

+Parallel Plate Capacitors

Electrons build up on the left plate, giving it a net negative charge. The right plate has a net positive charge. Capacitors can store charge or electrical PE.

+Capacitance

Capacitance measures the ability to store charge.

SI unit: coulombs/volt (C/V) or farads (F)

In what way(s) is a capacitor like a battery?

In what way(s) is it different?

+Capacitance

How would capacitance change if the metal plates had more surface area? Capacitance would increase.

How would it change if they were closer together? Capacitance would increase.

+Dielectrics

The space between the plates is filled with a dielectric. Rubber, waxed paper, air

The dielectric increases the capacitance. The induced charge on the dielectric allows

more charge to build up on the plates.

+Capacitor Applications

Connecting the two plates of a charged capacitor will discharge it.Flash attachments on cameras use a charged

capacitor to produce a rapid flow of charge.

Some computer keyboards use capacitors under the keys to sense the pressure.Pushing down on the key changes the

capacitance, and circuits sense the change.

+Energy and Capacitors

As the charge builds, it requires more and more work to add electrons to the plate due to the electrical repulsion. The average work or PE stored in the capacitor is

(1/2)QV. Derive equivalent equations for PEelectric by

substituting:Q= CV and V = Q/C

+???? Classroom Practice Problem

A 225 F is capacitor connected to a 6.00 V battery and charged. How much charge is stored on the capacitor? How much electrical potential energy is stored on the capacitor?Answers: 1.35 x 10-3 C , 4.05 x 10-3 J

+Now what do you think?

If a light bulb replaced the two metal plates and the battery was connected, electrons would flow out of the negative and into the positive terminal. Will this also occur with the two metal plates?If not, why not?If so, is the flow similar or different

from that with the light bulb? Explain.

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17.3 Current and ResistancePg 608-617

+What do you think?

• The term resistance is often used when describing components of electric circuits.• What behavior of the

components does this term describe?

• Do conductors have resistance?• If so, are all conductors the

same? Explain.• What effect would increasing or

decreasing the resistance in a circuit have on the circuit?

+Electric Current

Electric current (I) is rate at which charges flow through an area.

SI unit: coulombs/second (C/s) or amperes (A) 1 A = 6.25 1018 electrons/second

+Conventional Current

Conventional current (I) is defined as the flow of positive charge. The flow of negative charge as shown would

be equivalent to an equal amount of positive charge in the opposite direction.

In conducting wires, I is opposite the direction of electron flow.

+Resistance to Current

Resistance is opposition to the flow of charge. SI unit: volts/ampere (V/A) or ohms ()

Ohm’s Law : V = IR Valid only for certain materials whose resistance

is constant over a wide range of potential differences

+Classroom Practice Problems

A typical 100 W light bulb has a current of 0.83 A. How much charge flows through the bulb filament in 1.0 h? How many electrons would flow through in the same time period?

Given:

I= 0.83A t= 1 hour= 3600 seconds

Q= ?? C electrons= ??

+Classroom Practice Problems

I= Q/t or Q= It

Q= (0.83)(3600)

2988 C

We know that 1 A = 6.25 1018 electrons/second

2988 C x (6.25 x 1018 electrons/C)

1.87 x 1022electrons

+Classroom Practice Problems

This same 100 watt bulb (from the previous question) is connected across a 120 V potential difference. Find the resistance of the bulb.

Given:

V= 120V I= 0.83A R= ?? Ω

R = V/I

120 V / 0.83 A144.6

+Resistance of a Wire

On the next slide, predict the change necessary to increase the resistance of a piece of wire with respect to:Length of wireCross sectional area or thickness of

the wireType of wireTemperature of the wire

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+Applications

Resistors in a circuit can change the current.Variable resistors (potentiometers) are used in

dimmer switches and volume controls.Resistors on circuit boards control the current

to components.

The human body’s resistance ranges from 500 000 (dry) to 100 (soaked with salt water).Currents under 0.01 A cause tingling.Currents greater than 0.15 A disrupt the

heart’s electrical activity.

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Now what do you think?

• The term resistance is often used when describing components of electric circuits.• What behavior of the

components does this term describe?

• Do conductors have resistance?• If so, are all conductors the

same? Explain.• What effect would increasing or

decreasing the resistance in a circuit have on the circuit?

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17.4 Electric Power Pg. 618-623

+What do you think?

• Hair dryers, microwaves, stereos, and other appliances use electric power when plugged into your outlets.• What is electric power?• Is electric power the same as the power

discussed in the chapter “Work and Energy?”• Do the utility companies bill your household for

power, current, potential difference, energy, or something else?

• What do you think is meant by the terms alternating current (AC) and direct current (DC)?• Which do you have in your home?

+Types of Current - Direct

Batteries use chemical energy to give electrons potential energy. There is a potential difference across the terminals Chemical energy is eventually depleted.

Electrons always flow in one direction. Called direct current (DC)

+Types of Current - Alternating

Generators change mechanical energy into electrical energy. Falling water or moving

steam

Electrons vibrate back and forth. Terminals switch signs 60

times per second (60 Hz). Called alternating current

(AC) AC is better for transferring

electrical energy to your home.

+Energy Transfer

Is the electrical potential energy gained, lost, or unchanged as the electrons flow through the following portions of the circuit shown: A to B B to C C to D D to A

Explain your answers.

+Energy Transfer

A to B (unchanged) B to C (lost in bulb) C to D (unchanged) D to A (gained in

battery)

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Click below to watch the Visual Concept.

Visual Concept

Electric Power

+Electric Power

Power is the rate of energy consumption (PE/t ). For electric power, this is equivalent to the equation shown below. SI unit: joules/second (J/S) or watts (W) Current (I) is measured in amperes (C/s). Potential difference (V) is measured in volts (J/C).

Substitute using Ohm’s law (V = IR) to write two other equations for electric power.

+Classroom Practice Problems

A toaster is connected across a 120 V kitchen outlet. The power rating of the toaster is 925 W.What current flows through the toaster?

Given:

V= 120v P= 925W I= ??A

I = P/V

925 W / 120 V

7.7 A

+Classroom Practice Problems

What is the resistance of the toaster?

V= 120v I= 7.7A R= ??

R = V/I

120 V/ 7.7 A

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+Classroom Practice Problems

How much energy is consumed in 75.0 s?

Energy = Pt

P= 925 W t= 75 secEnergy=??

(925 W)(75.0 s)

6.94 104 J

+Household Energy Consumption

Power companies charge for energy, not power.Energy consumption is measured in

kilowatt•hours ( kw•h).The joule is too small.

A kw•h is one kilowatt of power for one hour.Examples of 1 kw•h:

10 light bulbs of 100 W each on for 1 h 1 light bulb of 100 W on for 10 h

1 kw•hr = 3 600 000 J or 3.6 x 106 J

+Electrical Energy Transfer

Transfer of energy from power plants to your neighborhood must be done at high voltage and low current.Power lost in electrical lines is significant.

P = I2RPower lines are good conductors but

they are very long.Since power companies can’t control the

resistance (R), they control the current (I) by transferring at high voltage.

+Now what do you think?

Hair dryers, microwaves, stereos, and other appliances use electric power when plugged into your outlets.What is electric power?

Is electric power the same as the power discussed in the chapter “Work and Energy?”

Do the utility companies bill your household for power, current, potential difference, energy, or something else?

What do you think is meant by the terms alternating current (AC) and direct current (DC)?Which do you have in your home?

+NOT NEEDED??? Gravitational Potential Difference

Suppose a mass of 2.00 kg is moved from point A straight up to point B a distance of 3.00 m. Find the PEg for the mass if g = 9.81 m/s2. Repeat for a mass of 5.00 kg. Answer: 58.9 J and 147 J

What is the PEg per kg for each? Answer: 29.4 J/kg for both

The change per kg does not depend on the mass. It depends only on points A and B and the field strength.

There is an analogous concept for electrical potential energy, as shown on the next slide.