# Chapter 21: Electric Current and Direct-Current Circuits

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### 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

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)

-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)

-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

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

V = IR

-ohm's law

-volts

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

-"ohmic" in their behavior

-a straight line

1/R

R = V/I

-volts per amp

1 ohm (Ω)

-an ohmmeter

-zigzag line

### What do the straight lines in a circuit indicate?

-ideal wires of zero resistance

R = ρ(L/A)

### What is ρ?

-a measure of the resistivity of a given material

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

-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

-conductors

-insulators

-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!

-yes

### As current moves through a resistor, does current change?

-no, it is constant

-no

-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.

-yes

-power decreases

-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

-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...

-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

-on its own

-on its own

-resistor 2

### Why?

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

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

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

-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

-increases

### Why?

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

-yes

-no

-no

### As resistance increases, what happens to voltage?

-voltage increases

-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

-yes

-no

### As R increases, what happens to current?

-current decreases

-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

-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

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