Ele ctric curr ent
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Ele ctric Curr ent. Chapter 34. Flow of Charge. When the ends of an electric conductor are at different potentials, charge flows from one end to another - just like heat due to temperature difference - just like water flows downhill (difference in gravitational potential energy).

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Ele ctric curr ent

Electric Current

Chapter 34


Flow of charge

Flow of Charge

When the ends of an electric conductor are at different potentials, charge flows from one end to another

- just like heat due to temperature difference

- just like water flows downhill (difference in gravitational potential energy)


Flow of charge1

Flow of Charge

  • This difference in potential is often referred to as potential difference or voltage

  • Voltage (V) is measured in volts (V).

  • You can think of the source of voltage like a pump.


Voltage sources

Voltage Sources

  • Supplies the voltage for any circuit.

  • Acts like an “electric pump”.

  • Examples:

    • Batteries

    • Generators

    • Wall plug


Types of current

Types of Current

There are two types of current:

AC – alternating current

- electrons flow first in one direction and then in the opposite direction

- this happens by alternating the polarity of the generator or voltage source

- the voltage of AC in America is normally 120 volts

DC – direct current

– electrons flow in a single direction

– a battery in a circuit produces DC because the terminals of the battery always have the same signs.


Electric current

Electric Current

  • Potential difference causes a flow of charge (electrons) in a conductor

  • We call this electric current (I)

  • Current is measured in amperes (A)

    Charge

    Current = time

    I = q/t


Ele ctric curr ent

A car’s starter motor draws 50 A. How much charge flows if the motor runs for 0.75 s?

Given:I = 50At = 0.75 s

Unknown:q (charge)

Equation:I = q/t

Substitute:50 A = q/(0.75 s)

Solve:q = 37.5 C


Ele ctric curr ent

How long does it take for 52 C to pass through a wire carrying a current of 8.0 A?

Given:q = 52 CI = 8.0 A

Unknown:t (time)

Equation:I = q/t

Substitute:8.0 A = 52 C/t

Solve:t = 52 C/8.0 A

t = 6.5 s


Activity 1

What will happen to bulbs 1 and 2 when you disconnect the wires at various points?

Activity 1

Consensus:

Current requires a closed loop


Activity 2

What type of object, when inserted into the loop, will allow the two test bulbs to light?

Activity 2

Consensus:

Current requires a closed loop made entirely of conductors.

New Term:

Continuous Conducting Path

Test Circuit


Activity 3

What parts of a socket and bulb are conductors and which are insulators? What is the conducting path through the bulb?

Clips

Base

Fig 2. Clip – side view

Plates

Activity 3

Consensus:

Trace the Continuous Conducting Path (CCP) through the bulb.


Activity 31

What parts of a socket and bulb are conductors and which are insulators? What is the conducting path through the bulb?

Clips

Base

Fig 2. Clip – side view

Plates

Activity 3

Consensus:

Trace the Continuous Conducting Path (CCP) through the bulb.


Worksheet 1

Worksheet 1


Worksheet 11

Worksheet 1


Worksheet 12

Worksheet 1


Worksheet 13

Worksheet 1


Worksheet 14

Worksheet 1


Worksheet 15

Worksheet 1


Worksheet 16

Worksheet 1


Electric power

Electric Power

Where have we seen “power” before?

It was the rate work was done (energy was transferred).

P = W/t

All power is measured in Watts (W).


Electric power1

Electric Power

Electric power is similar. It’s the rate electrical energy is transferred into another form.

P = IV

Electric power is often measured in the kilowatt, because the watt is so small.


Electric power2

Electric Power

What do you know of that is electrical and has a watt measurement on it?


Electric energy

Electric Energy

Electric energy can be calculated the same as any other form.

E = Pt

Electric energy, however, is measured in kilowatt-hours (kWh)

Your parents’ energy bill is measured in kilowatt-hours. Usually, electric companies charge 7-12 cents per kWh.


Ele ctric curr ent

How much power is used by a calculator that operates on 8 volts and 0.1 ampere? If it is used for two hours, how much energy does it use?

Given:V = 8 VI = 0.1 A

Unknown:P (power)

Equation:P = IV

Substitute:P = (0.1 A)(8 V)

Solve:P = 0.8 W


Ele ctric curr ent

How much power is used by a calculator that operates on 8 volts and 0.1 ampere? If it is used for two hours, how much energy does it use?

Given: V = 8 V I = 0.1 A P = 0.8 W t = 2 hr

Unknown:E (energy)

Equation:E = Pt

Substitute:E = (0.8 W)(2 hr)

Solve:E = 1.6 Wh

E = .0016 kWh


Ele ctric curr ent

Will a 1200-watt hair dryer operate on a 120-volt line if the current is limited to 15 amperes by a safety fuse? Can two hair dryers operate on this line?

Given:P = 1200 WV = 120 V

Unknown:I (current)

Equation:P = IV

Substitute:1200 W = I (120 V)

Solve:I = 1200 W/120 V

I = 10 A

Can you operate the hair dryer? What about two hair dryers?


Ele ctric curr ent

How much energy is expended in lighting a 100-watt bulb for 30 minutes?

Given:P = 100 Wt = 30 min

Unknown:E (energy)

Equation:E = Pt

Substitute:E = (100 W)(0.5 hr)

Solve:E = 50 Wh

E = 0.05 kWh


Ele ctric curr ent

How much does it cost to operate a 100-watt lamp continuously for one week if the power utility rate is 10 cents per kilowatt-hour?

Given:P = 100 Wt = 1 week

Unknown:E (energy)

Equation:E = Pt

Substitute:E = (100 W)(168 hr)

Solve:E = 16800 Wh

E = 16.8 kWh


Ele ctric curr ent

How much does it cost to operate a 100-watt lamp continuously for one week if the power utility rate is 10 cents per kilowatt-hour?

Given:E = 16.8 kWhcost = 10c/kWh

Unknown:T(total cost)

Equation:T = E(cost)

Substitute:T = (16.8 kWh)(10c/kWh)

Solve:T = 168 cents

T = $1.68


Electric resistance

Electric Resistance

  • Current depends not only on the amount of voltage impressed upon it but on the amount of resistance in the conductor.

  • Different conductors offer different amounts of resistance

  • Resistance is measured in ohms (W)


Ohm s law

Ohm’s Law

  • Georg Simon Ohm, a German physicist, tested different wire circuits to see what effect resistance had on the current.

  • He discovered that:

    Voltage

    Current = ---------------

    Resistance

    We call this Ohm’s Law!


Ohm s law1

Ohm’s Law

Ohm’s Law can also be written using symbols:

I = V/R

A triangle can help when solving problems with Ohm’s Law.


Ele ctric curr ent

What is the resistance of an electric frying pan that draws 12 amperes of current when connected to a 120-volt circuit?

Given:I = 12 AV = 120 V

Unknown:R (resistance)

Equation:R = V/I

Substitute:R = 120 V/12 A

Solve:R = 10 W


Ele ctric curr ent

How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?

Given:R = 100 WV = 50 V

Unknown:I (current)

Equation:I = V/R

Substitute:I = 50 V/100 W

Solve:I = 0.5 A


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