SAT II Physics (SN) Episode 2 Part 2 potx

Given two resistors, and , in parallel, the equivalent resistance, , is: When a circuit splits in two, the current is divided between the two branches, though the current through each re

simply the product of the current and the resistance The voltage drop across the two resistors is:

Note that the voltage drop across the two resistors is 10 V + 20 V = 30 V, which is the total voltage drop across the circuit

Resistors in Parallel

Two resistors are in parallel when the circuit splits in two and one resistor is placed on

each of the two branches

In this circumstance, it is often useful to calculate the equivalent resistance as if there were only one resistor, rather than deal with each resistor individually Calculating the equivalent resistance of two or more resistors in parallel is a little more complicated than calculating the total resistance of two or more resistors in series Given two resistors,

and , in parallel, the equivalent resistance, , is:

When a circuit splits in two, the current is divided between the two branches, though the current through each resistor will not necessarily be the same The voltage drop must be the same across both resistors, so the current will be stronger for a weaker resistor, and vice versa

EXAMPLE

Two resistors, = 5 and = 20 , are set up in parallel, as in the diagram above The battery produces a potential difference of = 12 V What is the total resistance in the circuit? What is the current running through and ? What is the power dissipated in the resistors?

WHAT IS THE TOTAL RESISTANCE IN THE CIRCUIT?

Answering this question is just a matter of plugging numbers into the formula for

resistors in parallel

So = 4

WHAT IS THE CURRENT RUNNING THROUGH R1 AND R2?

We know that the total voltage drop is 12 V, and since the voltage drop is the same across all the branches of a set of resistors in parallel, we know that the voltage drop across both resistors will be 12 V That means we just need to apply Ohm’s Law twice, once to each resistor:

If we apply Ohm’s Law to the total resistance in the system, we find that = (12 V)/(4 )

= 3 A As we might expect, the total current through the system is the sum of the current through each of the two branches The current is split into two parts when it branches into the resistors in parallel, but the total current remains the same throughout the whole circuit This fact is captured in the junction rule we will examine when we look at

Kirchhoff’s Rules

WHAT IS THE POWER DISSIPATED IN THE RESISTORS?

Recalling that P = I2R, we can solve for the power dissipated through each resistor

individually, and in the circuit as a whole Let be the power dissipated in , the

power dissipated in , and the power dissipated in

Note that + =

Circuits with Resistors in Parallel and in Series

Now that you know how to deal with resistors in parallel and resistors in series, you have all the tools to approach a circuit that has resistors both in parallel and in series Let’s take a look at an example of such a circuit, and follow two important steps to determine the total resistance of the circuit

1 Determine the equivalent resistance of the resistors in parallel We’ve

already learned to make such calculations This one is no different:

So the equivalent resistance is 6 In effect, this means that the two resistors in parallel have the same resistance as if there were a single 6 resistor in their place We can redraw the diagram to capture this equivalence:

1 Treating the equivalent resistance of the resistors in parallel as a single resistor, calculate the total resistance by adding resistors in series The diagram above gives us two resistors in series Calculating the total

resistance of the circuit couldn’t be easier:

Now that you’ve looked at this two-step technique for dealing with circuits in parallel and

in series, you should have no problem answering a range of other questions

EXAMPLE

Consider again the circuit whose total resistance we have calculated What is the current through each resistor? What is the power dissipated in each resistor?

WHAT IS THE CURRENT RUNNING THROUGH EACH RESISTOR?

We know that resistors in series do not affect the current, so the current through is the same as the total current running through the circuit Knowing the total resistance of the circuit and the voltage drop through the circuit, we can calculate the circuit’s total current

by means of Ohm’s Law:

Therefore, the current through is 3 A

But be careful before you calculate the current through and : the voltage drop across these resistors is not the total voltage drop of 30 V The sum of the voltage drops across and the two resistors in parallel is 30 V, so the voltage drop across just the resistors in parallel is less than 30 V

If we treat the resistors in parallel as a single equivalent resistor of 6 , we can calculate the voltage drop across the resistors by means of Ohm’s Law:

Now, recalling that current is divided unevenly between the branches of a set of resistors

in parallel, we can calculate the current through and in the familiar way:

WHAT IS THE POWER DISSIPATED ACROSS EACH RESISTOR?

Now that we know the current across each resistor, calculating the power dissipated is a

straightforward application of the formula P = I2R:

Common Devices in Circuits

In real life (and on SAT II Physics) it is possible to hook devices up to a circuit that will read off the potential difference or current at a certain point in the circuit These devices provide SAT II Physics with a handy means of testing your knowledge of circuits

Voltmeters and Ammeters

A voltmeter, designated:

measures the voltage across a wire It is connected in parallel with the stretch of wire whose voltage is being measured, since an equal voltage crosses both branches of two wires connected in parallel

WHAT DOES THE AMMETER READ?

Since the ammeter is not connected in parallel with any other branch in the circuit, the reading on the ammeter will be the total current in the circuit We can use Ohm’s Law to determine the total current in the circuit, but only if we first determine the total

resistance in the circuit

This circuit consists of resistors in parallel and in series, an arrangement we have looked

at before Following the same two steps as we did last time, we can calculate the total resistance in the circuit:

1 Determine the equivalent resistance of the resistors in parallel.

We can conclude that = 4

1 Treating the equivalent resistance of the resistors in parallel as a single resistor, calculate the total resistance by adding resistors in series.

Given that the total resistance is 9 and the total voltage is 9 V, Ohm’s Law tells us that the total current is:

The ammeter will read 1 A

WHAT DOES THE VOLTMETER READ?

The voltmeter is connected in parallel with and , so it will register the voltage drop across these two resistors Recall that the voltage drop across resistors in parallel is the same for each resistor

We know that the total voltage drop across the circuit is 9 V Some of this voltage drop will take place across , and the rest of the voltage drop will take place across the

resistors in parallel By calculating the voltage drop across and subtracting from 9 V,

we will have the voltage drop across the resistors in parallel, which is what the voltmeter measures

If the voltage drop across is 5 V, then the voltage drop across the resistors in parallel is

9 V – 5 V = 4 V This is what the voltmeter reads

Fuses

A fuse burns out if the current in a circuit is too large This prevents the equipment

connected to the circuit from being damaged by the excess current For example, if the ammeter in the previous problem were replaced by a half-ampere fuse, the fuse would blow and the circuit would be interrupted

Fuses rarely come up on SAT II Physics If a question involving fuses appears, it will probably ask you whether or not the fuse in a given circuit will blow under certain

circumstances

Kirchhoff’s Rules

Gustav Robert Kirchhoff came up with two simple rules that simplify many complicated

circuit problems The junction rule helps us to calculate the current through resistors in parallel and other points where a circuit breaks into several branches, and the loop rule

helps us to calculate the voltage at any point in a circuit Let’s study Kirchhoff’s Rules in the context of the circuit represented below:

Before we can apply Kirchhoff’s Rules, we have to draw arrows on the diagram to denote the direction in which we will follow the current You can draw these arrows in any

direction you please—they don’t have to denote the actual direction of the current As you’ll see, so long as we apply Kirchhoff’s Rules correctly, it doesn’t matter in what

directions the arrows point Let’s draw in arrows and label the six vertices of the circuit:

We repeat, these arrows do not point in the actual direction of the current For instance,

we have drawn the current flowing into the positive terminal and out of the negative terminal of , contrary to how we know the current must flow

The Junction Rule

The junction rule deals with “junctions,” where a circuit splits into more than one branch,

or when several branches reunite to form a single wire The rule states:

The current coming into a junction equals the current coming out.

This rule comes from the conservation of charge: the charge per unit time going into the junction must equal the charge per unit time coming out In other words, when a circuit separates into more than one branch—as with resistors in parallel—then the total current

is split between the different branches

The junction rule tells us how to deal with resistors in series and other cases of circuits branching in two or more directions If we encounter three resistors in series, we know that the sum of the current through all three resistors is equal to the current in the wire before it divides into three parallel branches

Let’s apply the junction rule to the junction at B in the diagram we looked at earlier.

According to the arrows we’ve drawn, the current in the diagram flows from A into B

across and flows out of B in two branches: one across toward E and the other toward C According to the junction rule, the current flowing into B must equal the current flowing out of B If we label the current going into B as and the current going

out of B toward E as , we can conclude that the current going out of B toward C is –

That way, the current flowing into B is and the current flowing out of B is + ( –

) =

The Loop Rule

The loop rule addresses the voltage drop of any closed loop in the circuit It states:

The sum of the voltage drops around a closed loop is zero

This is actually a statement of conservation of energy: every increase in potential energy, such as from a battery, must be balanced by a decrease, such as across a resistor In other words, the voltage drop across all the resistors in a closed loop is equal to the voltage of the batteries in that loop

In a normal circuit, we know that when the current crosses a resistor, R, the voltage drops

by IR, and when the current crosses a battery, V, the voltage rises by V When we trace a

loop—we can choose to do so in the clockwise direction or the counterclockwise direction

—we may sometimes find ourselves tracing the loop against the direction of the arrows

we drew If we cross a resistor against the direction of the arrows, the voltage rises by IR

Further, if our loop crosses a battery in the wrong direction—entering in the positive

terminal and coming out the negative terminal—the voltage drops by V To summarize:

• Voltage drops by IR when the loop crosses a resistor in the direction of the

current arrows

• Voltage rises by IR when the loop crosses a resistor against the direction of the

current arrows

• Voltage rises by V when the loop crosses a battery from the negative terminal to

the positive terminal

• Voltage drops by V when the loop crosses a battery from the positive terminal to

the negative terminal

Let’s now put the loop rule to work in sorting out the current that passes through each of the three resistors in the diagram we looked at earlier When we looked at the junction

rule, we found that we could express the current from A to B—and hence the current from

E to D to A—as , the current from B to E as , and the current from B to C—and hence the current from C to F to E—as – We have two variables for describing the current,

so we need two equations in order to solve for these variables By applying the loop rule

to two different loops in the circuit, we should be able to come up with two different equations that include the variables we’re looking for Let’s begin by examining the loop

described by ABED.

Remember that we’ve labeled the current between A and B as and the current between

B and E as Because the current flowing from E to A is the same as that flowing from A

to B, we know this part of the circuit also has a current of

Tracing the loop clockwise from A, the current first crosses and the voltage drops by

Next it crosses and the voltage drops by Then the current crosses , and the voltage rises by 12 V The loop rule tells us that the net change in voltage is zero

across the loop We can express these changes in voltage as an equation, and then

substitute in the values we know for , , and :

Now let’s apply the loop rule to the loop described by BCFE

Tracing the loop clockwise from B, the arrows cross , but in the wrong direction, from

positive to negative, meaning that the voltage drops by 8 V Next, the current crosses ,

with an additional voltage drop of Finally, it crosses , but in the opposite

direction of the arrows, so the current goes up by Now we can construct a second equation:

Plugging this solution for into the earlier equation of 4 + 3 = 12, we get:

So the current across is 28/13 A With that in mind, we can determine the current

across and by plugging the value for into the equations we derived earlier:

The negative value for the current across means that the current actually flows in the opposite direction of the arrow we drew This makes perfect sense when we consider that current should normally flow out of the positive terminal and into the negative terminal

of battery

It doesn’t matter how you draw the current arrows on the diagram, because if you apply Kirchhoff’s Rules correctly, you will come up with negative values for current wherever your current arrows point in the opposite direction of the true current Once you have done all the math in accordance with Kirchhoff’s Rules, you will quickly be able to

determine the true direction of the current

Capacitors

Capacitors rarely come up on SAT II Physics, but they do sometimes make an

appearance Because capacitance is the most complicated thing you need to know about

DC circuits, questions on capacitors will usually reward you simply for knowing what’s going on So long as you understand the basic principles at work here, you’re likely to get

a right answer on a question most students will answer wrong

A capacitor is a device for storing charge, made up of two parallel plates with a space between them The plates have an equal and opposite charge on them, creating a

potential difference between the plates A capacitor can be made of conductors of any

shape, but the parallel-plate capacitor is the most common kind In circuit diagrams,

a capacitor is represented by two equal parallel lines

For any capacitor, the ratio of the charge to the potential difference is called the

capacitance is halved The proportionality constant between C and A/d is , called the

permittivity of free space, which we encountered in the previous chapter in relation to Coulomb’s constant In case you forgot, C2 /N · m2

The unit of capacitance is the farad (F) One farad is equal to one coulomb per volt Most

capacitors have very small capacitances, which are usually given in microfarads, where 1

µF = 10–6 F

Energy

To move a small amount of negative charge from the positive plate to the negative plate of

a capacitor, an external agent must do work This work is the origin of the energy stored

For two capacitors in series:

For two capacitors in parallel:

EXAMPLE