16 Electricity: Coulomb's Law and Circuits

16 Electricity: Coulomb's Law and Circuits

• AP Physics 1 requires an understanding of direct-current circuits.
• The chapter shows you how to deal with circuits.

• Positive and negative charge can be found.

• One ampere is a coulomb of charge.

• The AP physics 1 exam requires you to learn about two aspects of elec tricity.
• When a balloon sticks to the wall, charged objects apply forces to each other in isolation.
• You need to know about circuits, in which lots of submicroscopic flowing charges produce effects that can be observed.
• The following picture shows a circuit in which the flow of charge causes two bulbs to light up.
• The meter is near one of the bulbs.

• All matter is made up of three particles.

• The coulomb is the unit of charge.

• The majority of objects that we encounter in our daily lives are neutral.
• There are many positive charges in these objects.
• They have the same number of protons as electrons.

• Coulomb's Law and Circuits 159 charged particles contain so many protons and electrons that they won't make much of a difference.
• Even though they might have a small electric charge, it would be too small to detect.

• Tiny objects, like atoms, carry a measurable electric charge because they have so few protons and electrons that an extra electron, for example, would make a big difference.
• You can have large charged objects.
• When walking across a carpeted floor in the winter, you pick up a lot of charges and become a charged object.

• Two posi tively charged particles will try to get as far away from each other as possible, while a positively charged particle and a negatively charged particle will try to get as close as possible.

• How much is determined by Coulomb's law.

• You won't be asked to calculate much with Coulomb's law, just like you won't be asked to calculate withNewton's law of universal gravitation.
• The questions will be qualitative and semiquantitative.

• Positive and negative charges can be equal, but they still exist on the object in the form of protons and electrons.
• Charge can be transferred from one object to another, for example by touching two metal spheres together, but the total amount of charge stays the same.

• In conjunction with circuits and Kirchoff's junction rule, conservativism of charge is discussed.

• Any wire path that allows charge to flow is a circuit.
• The coulomb of charge has a potential energy that is higher at one position in the wire than the other.
• The difference in potential energy per coulomb is called a "voltage," and a battery's job is to provide it.
• Current goes from the positive side of the battery to the negative side.

• Charge is difficult to flow through if resistance is used.
• The resistance of the wires connecting the "somethings" together is usually nothing compared to the resistance of the things.

• It's important to know the properties of a wire.
• The longer the wire is, the more resistance it has.
• The bigger the wire's cross-sectional area, the less resistance it has.
• If the wires are made of different materials, they can have different resistances.

Who cares about the AP exam?

• There is a battery and three identical 100 resistors in the preceding circuit diagram.

• Sometimes questions about circuits will ask for calculation of the voltage and current through the resistors.
• Qualitative questions, like which bulb takes the greatest current, or rank resistors from smallest to largest, are more often asked.

• First-year physics students are more comfortable with circuit problems than they are with explaining effects in words.
• If you're confused by a qualitative circuit question, try answering with a calculation: "Well, with a 150-V battery here's a calculation showing that I get 1 A of current in the circuit, but with a 75-V battery I only get 0.5 A."

• The missing value can be found on any row of the chart.

• Start by sketching a chart.
• We know the resistance of the resistors.
• The battery's voltage isn't given, so make it up.

• You're asked to rank the currents.
• Any reasonable values can be used to answer qualitative questions.

• Take the knowledge you need to score high and simplify the circuit by collapsing sets of parallel and series resistors into their equivalents.

• The parallel resistance is less than the individual resistance.

• The parallel combination of resistors has the same resist ance.
• Even with nonidentical parallel resistors, you can estimate their equivalent resistance to do qualitative problems.

• The equivalent resistance of the whole circuit is 150 if the other 100 is in series with the 50 equivalent resistance.
• We can put that resistance into the chart.

• The V-I-R chart is a tool for organizing your calculations for a complicated circuit.

• Two of the three entries in the total row are complete.
• We can use Ohm's law to calculate the total current in this circuit.

• We don't have any rows missing just one entry, so we can't use the law.
• We went back to the four key facts.

• The chart shows that it's 67 V.

• It's in series with the 100 resistors.
• To get the total voltage of 100 V and the total voltage across the 50 equivalent resistors is 33 V, the 50resistor must be added to 67 V.

• Use the facts.
• The total voltage across the combination is equal to the total voltage across the parallel resistors.
• 3 take 33 V.

• The chart can now be used to answer questions.

• The exam might ask you to rank the voltage across the resistors from the largest to the smallest.
• Give a verbal description of why you decided to use the calculations.
• The current through the two series of resistors must be the same as the current through the battery.

• The total current should be increased by the parallel resistors.
• We said the total current was 0.67 A.
• The whole point is to help answer conceptual questions.

• If 1 C of charge enters each second, the same amount must leave.

• This fact is a statement of energy efficiency because of the change in the elec trical potential energy of 1 C of charge.
• The sum of all the energy changes must be zero.

• The four key facts are restatements of the laws of Kirchoff.

• There's no junction so the series resistors take the samecur rent.
• The junction before and after the parallel combination adds to the total.

• The total voltage is increased because the resistors can only drop the potential energy of 1 C of charge as much as the battery raises the charge's potential energy.
• The loop rule applies to all loops of the circuit and the voltage across parallel resis tors must be the same.
• No matter which path you look at, the sum of voltage changes is still zero.

• In the chapter about energy, power was defined as energy per second.

• The amount of power used to convert electrical energy to other forms is what counts.

• You can use any of the power equations.

• The potential energy can be converted into internal or mechanical energy, such as in an electric motor, or into light or air, such as in a lamp.

• The knowledge you need to score high power doesn't obey the four key facts.
• The total power dissipated by a bunch of resistors is the sum of the power dissipated by each of them.

• When a real circuit is set up in the laboratory, it usually consists of more than just resis tors--light bulbs and motors are common devices to hook to a battery.
• For the purposes of computation, we can consider any electronic device to act like a resistor.

• You are often asked about observational and measurable effects on the AP exam.
• The measurement of current and voltage are not the only things that involve the brightness of light bulbs.

• The power dissipated by the bulb is what determines the brightness of a bulb.
• When you go to the store to buy a light bulb, you don't ask for a 400 watt bulb, but for a 100 watt bulb.
• A 100 watt bulb is brighter than a 25 watt bulb.
• A bulb's power can change depending on the current and voltage it's hooked up to.
• Consider the problem.

• Your first thought might be to say that the bulb is just as bright because it depends on power.
• The power of a bulb can change.

• The resistance of a lightbulb is a property of the bulb itself, and so it will not change no matter what the bulb is hooked to.

• When the bulb's temperature is very hot or very cold, the bulb's resistance can vary.
• Unless a problem clearly asks you to consider temperature variation, you can assume a bulb has constant resistance.

• Current and voltage are measured by ammeters.
• This is obvious because current is measured in Amps.

• It is the same for any resistors in parallel with each other.

• A voltmeter is used to measure voltage.

• If you're going to measure the current with a resistor, the ammeter must be in series with it.

• The ammeter is used to measure current.

• The battery's charge came directly from the battery.
• It's left over for the right hand branch of the circuit.

• A 9-V battery has four resistors connected to it.

• Justify your answers with short para graphs.

• The resistances current in the circuit must run through the add to give them 340.

• Each pair of parallel resistors takes the same thing.
• That's the total number of transistors.

• Each of the 120 and 220 resis has more power than the 150 resis.
• The tors are ranked.

• It's possible for the parallel combination to take more power than the 400.
• The equivalent resistors can be used to get the current through each by just dividing the voltage.
• Don't worry, there's no need to worry about a 1,000 Resistor.
• If you rounded differently than I did, the parallel combination would have an ance of 200 and take more power than the 100.

• There's no way to avoid having the largest 400 resistor, which must take all the (d) Start with the entire equivalent circuit.
• The 9-V battery is not affected by the current in the circuit.
• The bottom branch says that the parallel resistor that comes first had a resistance of 220 but now is just the diagram has to take the larger current.

• A larger total resistance causes a smaller total current to choose a parallel path.
• It would take this cuit because the 400 Resistor is in a different state than the Resistor.
• The 200 and 300 resistors would not split the current, but the total current would be less than before, so both would have the same voltage.

• Using takes less power.

• It's not the junction rule.

• The magnitude of a force should not have each other, which is equal to the total.
• The direction is repulsive.

• The force can be decreased.
• The magnitude of the force is reduced by the age to go across the Resistor.

• More charges will not change if the charge of each item and the distance between the two are small.

• The total amount of charge in a system is always the same.

• The sum of all of the individual resistors is the equivalent resistance.

• If you have to do the calculation, the equivalent resistance of parallel resistors is less than any individual one.

• The junction rule says that the current entering a wire junction equals the current leaving it.

• The sum of voltage changes around a circuit loop is zero according to the loop rule.