Chapter 21: Electric Current and Direct-Current Circuits

Created by kmcdaniel223 

Upgrade to
remove ads

What is electric current?

-a flow of electric charge from one place to another

Often, the charge is carried by...?

-electrons moving through a metal wire

What is the equation that defines electric current?

I = ΔQ/Δt

What are the units of current?

C/s or ampere (A)

What is an electric circuit?

-a system in which charge flows through a closed path and returns to its starting point

What are direct current (DC) circuits?

-circuits in which current always flows in the same direction

What are alternating current (AC) circuits?

-circuits with currents that periodically reverse their direction

What is an open circuit?

-a circuit in which there is no closed path through which electrons can flow

When a battery is disconnected from a circuit and carries no current, the difference in electric potential between its terminals is referred to as its...?

-electromotive force (emf)

What is a battery's electromotive force?

-the difference in electric potential between its terminals when it is disconnected from a circuit and carries no current

What are the units of emf?

-the same as electric potential-volts (J/C)

The emf of a battery is the ________ _________ it can produce between its terminals under _______ _________.

-The emf of a battery is the POTENTIAL DIFFERENCE it can produce between its terminals under IDEAL CONDITIONS

Is emf a force?

-no, it is a potential different (voltage)

Do electrons travel quickly or slowly through a typical wire?

-slowly

Why?

-they suffer numerous collisions with atoms in the wire, and hence their path is torturous and roundabout

By convention, the direction of current is given in terms of...?

-a positive test charge

Describe the direction in which a positive charge will flow in a circuit?

-a positive test charge will flow from a region of high electric potential (near the positive terminal of the circuit), to a region of low electric potential (near the negative terminal of the circuit)

And in doing so...?

-reduce their electric potential energy

Describe the direction in which a negative charge will flow in a circuit?

-a negative test charge will flow from a region of low electric potential (near the negative terminal of a circuit), to a region of high electric potential (near the positive terminal of the circuit)

What is the voltage difference across any contiguous piece of conductor?

-zero

Therefore, is an electric field present?

-no
-electrons are mobile but have no desire to do anything/go anywhere

Suppose a material has a higher voltage on one ened, and a lower voltage on the other. In what direction does current actually flow?

-electrons flow from lower potential/voltage to higher potential/voltage (seeking lower potential energy)

What do we actually pretend?

-that positive charges move from higher potential/voltage to lower potential/voltage

What is equation that describes electric current?

I = ΔQ/Δt

What are the units of current?

coulombs/second; aka ampere, "amp", or A

What is a significant notational change that occurs as we move into the circuits unit?

ΔV becomes simply V

What two key components does every battery have?

-a higher voltage plate and a lower voltage plate

In order to cause electrons to move against the resistance of a wire, it is necessary to...?

-apply a potential difference between its ends

For a wire with a constant resistance R, the potential difference V necessary to create a current I is given by what equation?

V = IR

What is this called?

-ohm's law

What are the SI units of V?

-volts

Materials that are well approximated by Ohm's law are said to be...?

-"ohmic" in their behavior

If one plots current versus voltage for an ohmic material, what is the result?

-a straight line

What is the slow of the line equal to?

1/R

Solving Ohm's law for resistance results in...?

R = V/I

From this expression, it is clear that the units of resistance are...?

-volts per amp

What is 1 volt per amp defined to be?

1 ohm (Ω)

What is a device for measuring resistance called?

-an ohmmeter

In an electrical circuit, a resistor is signified by a...?

-zigzag line

What do the straight lines in a circuit indicate?

-ideal wires of zero resistance

What is the equation that describes resistance in a given material of a given length and area?

R = ρ(L/A)

What is ρ?

-a measure of the resistivity of a given material

What are the units of ρ?

Ω * meter

As ρ increases, what is the effect on resistance?

-resistance increases

What is L?

-length of the material

As L increases, what is the effect on resistance?

-resistance increases

What is A?

-the area of the material

As A increases, what is the effect on resistance?

-resistance decreases

As a wire is heated, what is the impact on its resistivity?

-resistivity tends to increase

Why?

-because atoms that display increased thermal oscillation (due to a temperature increase) are more likely to colide with electrons and slow their progress through a wire

What is one type of materials that actually show a drop in resistivity as temperature is increased?

-semiconductors

SInce resistivity typically increases with temperature, what happens when a wire is cooled below room temperature?

-resistivity will decrease

What are superconducting materials?

-those materials that allow current to flow through them with zero resistance

When cooled below their critical temperature, superconductors not only lose their resistance to current flow but also exhibit _________ _________ _________ , such as the ability to _________ ____ _________ _________ _________ .

When cooled below their critical temperature, superconductors not only lose their resistance to current flow but also exhibit NEW MAGNETIC PROPERTIES, such as the ability to REPEL AN EXTERNAL MAGNETIC FIELD.

What is resistance?

-the opposition within any material to the movement of flow and charge

What are materials that offer low resistance called?

-conductors

What are those materials that offer very high resistance called?

-insulators

What are conductive materials that offer medium amounts of resistance called?

-resistors

What four factors is the resistance of a resistor dependent upon?

-resistivity of the conductive material
-length
-cross-sectional area
-temperature

What is the number that characterized the intrinsic resistance to current flow in a material called?

-the resistivity (ρ)

Does Ohm's law apply to a single resistor with a circuit, any part of a circuit, or to an entire circuit?

-all of the above!

As current moves through a resistor, does the voltage change?

-yes

As current moves through a resistor, does current change?

-no, it is constant

Is any charged gained or lost through the resistors?

-no

When a cell is not actually driving any current, what is its internal resistance equal to?

-zero

The longer that a material is, the ________ resistance it will display.

-The long that a material is, the MORE resistance it will display.

If we increase the cross-sectional area of a material, resistance will ____________.

-If we increase the cross-sectional area of a material, resistance will DECREASE.

True or false: All components of a circuit display resistance (excluding materials that are superconductors).

-true
-(its not just the resistor that displays resistance!)

If resistance exists, particles will not move unless...?

-there is a reward in the form of a drop in potential energy

This fact presents complications. As a result, what do we do?

-we consider all parts of a circuit that are not resistors to be ideal conductors

Therefore, the only location in which voltage will drop is...?

-through the resistor

A battery that produces a potential difference V is connected to a 5-W lightbulb. Later, the 5-W lightbulb is replaced with a 10-W lightbulb. In which case does the battery supply the greatest current? Why?

-according to the the equation P = IV, the battery delivers twice as much current when it is connected to the 10-W bulb

Which lightbulb has the greatest resistance? Why?

-according to the equation P = V^2/R, the resistance of the 5-W bulb is twice that of the 10W bulb

On a microscopic level, what is the power dissipated by a resistor the result of?

-incessant collisions between electrons moving through the circuit and the atoms making up the resistor

Power is proportional to a lightbulb's ___________.

Power is proportional to a lightbulb's BRIGHTNESS.

A 100 W lightbulb outputs _________ of potential electric energy _________.

A 100W lightbulb outputs 100J of potential electric energy per second.

A 50 W lightbulb outputs _________ of potential electric energy _________.

A 50W lightbulb outputs 50J of potential electric energy per second.

Is the resistance of a lightbulb inherent?

-yes

What is the effect on power if resistance is increased?

-power decreases

What is the effect on power if resistance is decreased?

-power increases

What are the three most basic types of circuits?

1. simple (with only one resistor)
2. series (with multiple resistors)
3. parallel (also with multiple resistors)

At what location within a circuit does a voltage drop occur?

-all voltage drop occurs across the resistor

Suppose you have a circuit with a resistor of 1 ohm attached to a lightbulb. Then, you replace the original resistor with a resistor of 2 ohms. Which resistor glowed brightest? Why?

-the first resistor glowed the brightest because it had the lowest resistance

Suppose you have a series circuit with two resistors. Describe the current at each resistor.

-the current at each resistor is equivalent

Suppose you have a circuit where V=12 V. Would the electric current increase or decrease if a 3 ohm resistor was replaced with a 2 ohm resistor?

-the electric current would increase
-->if resistance decreases, electric current will increase

When resistors are connected to each other end to end, what are they said to be in?

-series

Describe the current drawn by three resistors in series vs the current drawn by each independently (within the same circuit).

-they draw the same current

What is the equation that describes the equivalent resistance for any number of resistors in a series...?

Req = R1 + R2 + R3 +R4...

As resistance increases, what happens to V?

-V increases

The sum of individual potential differences in a circuit is equal to...?

-the total potential difference for the entire circuit/the emf value

When are resistors in parallel?

-when they are connected across the same potential difference

What equation describes the equivalent resistance for resistors in parallel?

1/Req = 1/R1 + 1/R2 + 1/R3...

Describe the current through a series resistor when resistance is large

-current never changes through a series circuit, it is always the same

Describe the current through a series resistor when resistance is small

-current never changes through a series circuit, it is always the same

Describe the voltage drop across a series resistor when resistance is large

-the voltage drop is large

Describe the voltage drop across a series resistor when resistance is small

-the voltage drop is small

Describe the power produced by a series resistor when resistance is large

-a large amount of power is produced

Describe the power produced by a series resistor when resistance is small

-a small amount of power is produced

Suppose resistor 1 has a resistance of 1 ohm. Resistor 2 has a resistance of 2 ohms. Does resistor 1 glow more brightly in series, or on its own?

-on its own

Suppose resistor 1 has a resistance of 1 ohm. Resistor 2 has a resistance of 2 ohms. Does resistor 1 glow more brightly in series, or on its own?

-on its own

Suppose resistors 1 and 2 are in series. Will resistor 1 or resistor 2 glow more brightly?

-resistor 2

Why?

-because it has a resistance of 2 ohms, whereas resistor 1 only has a resistance of 1 ohm

What equation should be used to calculate power in a series circuit?

P = (I^2)(R)

Why should this equation be used?

-it is harder to go wrong using this equation because the currents will always be the same in a series circuit

What does a parallel circuit provide?

-two or more distinct routes for current to pass through

Compare the voltage across any resistors in a parallel circuit

-the voltage across any resistors in a parallel circuit will always be the same

Suppose resistor 1 has a resistance of 1 ohm. Resistor 2 has a resistance of 2 ohms. If the resistors are in a parallel circuit, will more current pass through resistor 1 or resistor 2?

-more current will pass through resistor 1

Why?

-because it is the path of least resistance

What will the total current in the circuit be equal to?

I total = I1 + I2

Suppose resistor 1 has a resistance of 1 ohm. Resistor 2 has a resistance of 2 ohms. If the resistors are in a parallel circuit, will more power be generated by resistor 1 or 2?

-more power will be generated by resistor 1

Why?

-because resistance is less

What equation should be used to calculate power within a parallel circuit?

P = V^2/R

For a pure parallel circuit, each resistor has...?

-the same behavior as if connected alone

For a pure parallel circuit, each _________ has the same behavior as if connected alone.

-for a pure parallel circuit, each RESISTOR has the same behavior as if connected alone

Suppose you have a parallel circuit with two resistors, R1 and R2. Initially, the circuit is set up in such a way that current does not flow to R2. Then, a trap door closes which allows current to flow to R2. What happens to the current running through R1?

-remains the same

Why?

-because opening up another branch does not change the resistance offered by this resistor

What happens to the current running through R2?

-the current running through R2 increases

What happens to the total current running through the circuit?

-the total current running through the circuit increases

Where does all of this current increase go to?

-R2

What happens to the voltage of R1?

-it remains the same

Why?

-a result of Ohm's law, V=IR
-since I and R remain the same, V will remain the same

What happens to the power produced by R1?

-it remains the same

Why?

-a result of Ohm's law, P = V^2/R
-since voltage and resistance stay the same, power is constant

Suppose you have a series circuit with two resistors, R1 and R2. Initially, the circuit is set up in such a way that current flows from R1 to R2. Then, a short circuit is created after R1. What happens to I2, V2, and P2?

-decreases to zero

Why?

-because there is no longer any incentive for current to move through R2

What is the impact on I1?

-increases

Why?

-the equivalent resistance of the circuit has decreased (keep in mind that we are discussing a series circuit)

Is current (I) the same for each resistor in series?

-yes

Is current (I) the same for each resistor in parallel?

-no

Is voltage (V) the same fore each resistor in series?

-no

As resistance increases, what happens to voltage?

-voltage increases

As resistance increases, what happens to power?

-power increases

What is the total resistance across several resistors in series equal to?

Req = R1 + R2 + R3 + ...

What is the relationship between the total resistance and the resistance of one individual resistor in series?

total resistance > resistance of an individual resistor in parallel

Is voltage (V) the same for each resistor in parallel?

-yes

Is current (I) the saem for each resistor in parallel?

-no

As R increases, what happens to current?

-current decreases

As resistance increases, what happens to power?

-power decreases

What is the total resistance across several resistors in series equal to?

1/Req = 1/R1 + 1/R2 + ...

What is the relationship between the total resistance and the resistance of one individual resistor in series?

total resistance < resistance of an individual resistor in parallel

Are objects connected to a power strip (such as a hair dryer and straightener) connected in series or in parallel?

-in parallel

What is often used to protect against a surge in power?

-fuse or circuit breaker

What does a fuse or circuit breaker do when there is a surge in power?

-fuse will blow when power is too high
-circuit breaker will mechanically cause a break in a circuit

What does an ammeter measure?

-current through itself

Using a high or low resistance?

-using a low resistance

What does a voltmeter measure?

-a voltage drop across itself

Using a low or high resistance?

-using a high resistance

What does a voltmeter take advantage of?

-the fact that in parallel, the voltage across two resistors is the same

How should a voltmeter be connected to a resistor that you are interested in measuring?

-voltmeter should be connected in parallel with the resistor you are interested in

See More

Please allow access to your computer’s microphone to use Voice Recording.

Having trouble? Click here for help.

We can’t access your microphone!

Click the icon above to update your browser permissions above and try again

Example:

Reload the page to try again!

Reload

Press Cmd-0 to reset your zoom

Press Ctrl-0 to reset your zoom

It looks like your browser might be zoomed in or out. Your browser needs to be zoomed to a normal size to record audio.

Please upgrade Flash or install Chrome
to use Voice Recording.

For more help, see our troubleshooting page.

Your microphone is muted

For help fixing this issue, see this FAQ.

Star this term

You can study starred terms together

NEW! Voice Recording

Create Set