Unit 7: Electric Current & Circuits

Electric current is related to voltage that produces it, and resistance that opposes it.

Heat flows through a conductor only when a temperature difference exists.

Heat flows from higher to lower temperature.

Flow of Chargeh

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Flow of Charge

Charge flows from a potential difference (voltage), between the ends of a conductor.

The flow stops when both ends reach the same potential.

To continue the flow of charge, one end must remain at a higher potential than the other.

difference in potential of water pressure

no potential difference at equal water pressure

electric current:flow of charge

Electrons carry the charge through a conductor because they are free to move (loosely held).

Electric Current

The net charge of a wire carrying a current is zero.

WHY?

Electric Current

Electric Current

Current is measured in ___________

1 A = 1 C/s (1 coulomb of charge per second)

6 billion billion electrons (1 C) per second is a current of 1 A.

amperes (A).

1. Electric charge will flow in an electric circuit when…

A. electrical resistance is low enough.

B. a potential difference exists.

C. the circuit is grounded.

D. the circuit has resistance.

Quick Quiz!

2. The electric current in a copper wire is normally composed of…

A. electrons.

B. protons.

C. ions.

D. amperes.

Quick Quiz.

3. Which statement is correct?

A. Voltage flows in a circuit.

B. Charge flows in a circuit.

C.A battery is the source of electrons in a circuit.

D.A generator is the source of electrons in a circuit.

Quick Quiz.

voltage source:batteries and generators provide a potential difference (for a continuous flow of electrons)

Voltage Sources

Charge flows from a potential difference (voltage), between the ends of a conductor.

Batteries release electrical energy from a chemical reaction.

Generators—like alternators in cars or turbines in power plants—convert mechanical energy to electrical energy.

(a ski-lift provides grav. PE diff)

Consider a long pipe filled with water.• Water flows through a pipe if there is a difference

in pressure across the pipe between its ends. • Water flows from high pressure to low pressure.

Current (charges) flows through a circuit b/c of a voltage (difference) across a circuit.

• Voltage does NOT flow through a circuit. • Voltage doesn’t go anywhere, the charges move. • Voltage causes current.

Voltage Sources

The amount of charge (current) that flows depends on the voltage source, and on…

electric resistance:resistance a conductor offers to charge flow

Electric Resistance

Resistance of a wire depends on the conductivity, thickness, and length of a wire.

• Thick wires have less resistance.

• Longer wires have more resistance.

Electric Resistance

Certain metals acquire superconductivity (zero resistance to the flow of charge) at temperatures near absolute zero.

Resistance

SeriesRtotal = R1 + R2 + R3

Electric Resistance

Resistance

Parallel1/Rtotal = 1/R1 + 1/R2 + 1/R3

Rtotal = Less than any single R in parallel

Electric Circuit Symbols & Examples

1. How do batteries and generators cause current to flow through a wire?

A. by producing a charge

B. by adding electrons

C.by creating a potential difference

D.by increasing electrical forces

Quick Quiz!

2. Which of the following type of copper wire would you expect to have the least electric resistance?

A. a thick long wire

B. a thick short wire

C.a thin long wire

D.a thin short wire

Quick Quiz.

Ohm’s Law

Term Symbol Unit

charge q

voltage V

current I

resistance R ---

coulomb (C)

volt (V)

ampere (A)

ohm (Ω)

6 x 1018 e–

1 J/C

1 C/s

∆PEq

V =V = IRq

tI =

Ohm’s Law

Double the voltage to ________ the current.

more voltage, more current

Double the resistance to ________ the current.

more resistance, less current

Ohm’s Law

V = IR VR

I =

double

halve

VI

R =

The resistance of a typical lamp cord is much less than 1 ohm,

while a typical light bulb has a resistance of about 100 ohms.

Toasters have a resistance of 15-20 ohms.

Ohm’s Law

Resistance converts electrical energy into light and/or heat energy.

think!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?

Answer:

Ohm’s Law

V = IRVR

I =

Ohm’s Law and Electric Shock

V = IR

Voltage

Resistance

Current

The Body’s ResistanceYour body’s resistance ranges from about 100 Ω if soaked with salt water to about 500,000 Ω if your skin is very dry.

Touch a battery’s electrodes with dry fingers and your resistance would be about 100,000 Ω.

You would not feel 12 V, and 24 V would just barely tingle.

With moist skin, however, 24 V could be quite uncomfortable.

Ohm’s Law and Electric Shock

Ohm’s Law and Electric Shock

Many people are killed each year by current from common 120 V electric circuits.

Ohm’s Law and Electric Shock

think!If the resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12-volt battery?

Answer:

Ohm’s Law and Electric Shock

V = IR

think!If a current of 0.050 A flows through a circuit with resistance 10 Ω, what is the voltage?

Ohm’s Law and Electric Shock

V = IR V = (0.050)(10) V = 0.50 V

If a 1.5 V battery produces a current of 0.002 A through a circuit, what is the resistance?

VI

R =1.5

0.002R = R = 750 Ω

1. When you double the voltage in a simple electric circuit, you double the …

A. current.

B. resistance.

C.ohms.

D. resistors.

Quick Quiz!

2. To receive an electric shock there must be…

A. current in one direction.

B. moisture in an electrical device being used.

C.high voltage and low body resistance.

D.a difference in potential across part or all of your body.

Quick Quiz.

You can buy a water hose that is empty of water, but you can’t buy a piece of wire, an “electron pipe,” that is empty of electrons.

The source of electrons in a circuit is the conducting circuit material itself.

The Source of Electrons in a Circuit

When plug into an outlet, energy flows from the outlet, not electrons.

Energy is carried by the electric field which vibrates electrons already there.

Electrical energy is converted to mostly heat and some light.

Power utilities do not sell electrons. They sell energy.

When you get shocked, your electrons vibrate. Small vibrations tingle; large vibrations can be fatal.

The Source of Electrons in a Circuit

A charge moving in a circuit expends energy, which may cause motion, heat, or light.

electric power:

the rate at which electrical energy is converted into another form.

Electric Power

electric power = current × voltage

P = IV

watts (W)

Wt

P =

The power and voltage on a light bulb is read as “60 W 120 V.”

What is the current that would flow through the bulb?

Electric Power

P = IV

I = P V

= (60 W) (120 V)

= 0.5 A

The power is also equal to 2 other equations. These are found by simply substituting Ohm’s Law equation.

Electric Power

P = IV P = I2R

P = V2

R

V = IR

A kilowatt is 1000 watts, and a kilowatt-hour represents the amount of energy consumed in 1 hour at the rate of 1 kilowatt.

If electrical energy costs

10 cents per kilowatt-hour,

a 100-watt light bulb burns

for 10 hours for 10 cents.

A toaster or iron draws more

current and more power

costing more to operate for the same time.

Electric Power

think!How much power is used by a calculator that operates on 8 volts and 0.1 ampere?

Answer:

P = (0.1 A) × (8 V) =

Electric Power

P = IV

0.8 W

1. When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons…

A. must be supplied by you, just as water must be supplied for a water pipe.

B. are already in the wire.

C.may fall out, which is why wires are insulated.

D.enter it from the electric outlet.

Quick Quiz!

2. If you double both the current and the voltage in a circuit, the power…

A. remains unchanged if resistance remains constant.

B. halves.

C.doubles.

D.quadruples.

Quick Quiz.