 Chapter 18 Electric Currents.

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Chapter 18 Electric Currents

Units of Chapter 18 The Electric Battery Electric Current
Ohm’s Law: Resistance and Resistors Resistivity Electric Power

Units of Chapter 18 Power in Household Circuits Alternating Current
Microscopic View of Electric Current Superconductivity Electrical Conduction in the Human Nervous System

Objectives After studying the material of this chapter, the student should be able to: 1. Explain how a simple battery can produce an electrical current. 2. Define current, ampere, emf, voltage, resistance, resistivity, and temperature coefficient of resistance. 3. Write the symbols used for electromotive force, electric current, resistance, resistivity, temperature coefficient of resistance and power and state the unit associated with each quantity. 4. Distinguish between a) conventional current and electron current and b) direct current and alternating current. 5. Know the symbols used to represent a source of emf, resistor, voltmeter, and ammeter and how to interpret a simple circuit diagram. 6. Given the length, cross sectional area, resistivity, and temperature coefficient of resistance, determine a wire's resistance at room temperature and some higher or lower temperature. 7. Solve simple dc circuit problems using Ohm's law. 8. Use the equations for electric power to determine the power and energy dissipated in a resistor and calculate the cost of this energy to the consumer. 9. Distinguish between the rms and peak values for current and voltage and apply these concepts in solving problems involving a simple ac circuit.

Explain how a simple battery can produce an electrical current.
Objectives After studying the material of this chapter, the student should be able to: Explain how a simple battery can produce an electrical current. Describe the concepts in an electrical circuit including electric potential energy, electric potential, voltage, current, and resistance. Describe conditions that create current in an electric current.

Recap In ch. 16, Electrostatics, electric field must be 0 inside a conductor and charges did not move. When charges move in a conductor, there is usually an electric field present. An electric field is needed to put charges in motion. The flow of charge can be controlled using electric fields and electric potential. Therefore, in order to have a current in a wire, a potential difference is needed. That difference can be provided by a battery.

18.1 The Electric Battery Mechanical Universe The Electric Battery
Volta discovered that electricity could be created if dissimilar metals were connected by a conductive solution called an electrolyte. This is a simple electric cell.

BATTERY BASICS INTRODUCTION
1800 – Alessandro Volta discovered the chemical battery by creating a portable electricity source known as a “Voltaic Pile”. A Voltaic Pile is a device using pieces of silver and zinc separated by moist cloth soaked in an electrolyte (in Volta’s case, sea water) solution. Humphry Davy later proved that the electricity from voltaic piles was caused by the chemical reaction, and not the different metals, as first assumed.

BATTERY BASICS Voltaic Pile
In the lemon experiment, the lemon juice allows the metal plates to gain or lose electrons. Then, those electrons travel over to the other plate (via the electrolyte solution, lemon juice), forming a redox reaction. The electrolyte is electrically the same on both sides, but the reaction creates a different electrical potential on the two different plates, so connecting them shows a voltage difference.

18.1 The Electric Battery A battery transforms chemical energy into electrical energy. Chemical reactions within the cell create a potential difference between the terminals by slowly dissolving them. This potential difference can be maintained even if a current is kept flowing, until one or the other terminal is completely dissolved.

18.1 The Electric Battery Several cells connected together make a battery, although now we refer to a single cell as a battery as well.

What does a battery do? Static discharge is the moving of electrons from one atom to another. In order to keep the electrons moving through the circuit, there has to be something that causes a push, or a voltage difference- a battery does just that…

How do batteries work? Batteries create an uneven level of electrons which causes the electrons to move from a high concentration to a low concentration… This is also known as the voltage difference..

Parts of the Dry Cell Positive End Plastic Insulator Moist Paste
Carbon Rod Zinc Container Negative Terminal

How does a Dry Cell Work? When the circuit is closed, and the battery is connected, a chemical reaction starts the process. The chemical reaction with zinc and several other chemicals occurs in the moist paste.

How does a Dry Cell Work? The carbon rod acts as a conductor and transfers electrons. The carbon rod is not part of the reaction happening in the moist paste. But…the chemical reaction in the moist paste does cause the carbon rod to become charged. This charge on the carbon rod creates a positive end. The negative end is made by the Zinc.

How does a Dry Cell Work? The voltage difference between the positive and negative ends causes the current to flow. By connecting more batteries you increase the voltage difference.

18.2 Electric Current In order for current to flow, there must be a path from one battery terminal, through the circuit, and back to the other battery terminal. Only one of these circuits will work:

18.2 Electric Current By convention, current is defined as flowing from + to -. Electrons actually flow in the opposite direction, but not all currents consist of electrons.

18.2 Electric Current Electric current is the rate of flow of charge through a conductor: (18-1) Unit of electric current: the ampere, A. 1 A = 1 C/s.

18.2 Electric Current A complete circuit is one where current can flow all the way around. Note that the schematic drawing doesn’t look much like the physical circuit!

Electric Circuit Electric Circuit – continuous conducting path
Electric Current – flow of charge from one terminal to the other

Diagram of Electric Circuit

Remember: Electric Potential Energy- Two Unlike Charges
+ Higher Potential Energy - Lower Potential Energy To cause movement of a charge, there must be a potential difference.

While the switch is open:
Free electrons (conducting electrons) are always moving in random motion. The random speeds are at an order of 106 m/s. There is no net movement of charge across a cross section of a wire.

What occurs in a wire when the circuit switch is closed?

What occurs in a wire when the circuit switch is closed?
An electric field is established instantaneously (at almost the speed of light, 3x108 m/s). Free electrons, while still randomly moving, immediately begin drifting due to the electric field, resulting in a net flow of charge. Average drift velocity is about 0.01cm/s.

Electrons flow in a net direction away from the (-) terminal
Closing the switch establishes a potential difference (voltage) and an electric field in the circuit. High Potential Low Potential Electrons flow in a net direction away from the (-) terminal towards the (+) terminal.

Conventional Current By tradition, direction in which “positive charges” would flow. Direction is opposite of electron flow.

Question: What is required in order to have an electric current flow in a circuit? Answer: A voltage source. The circuit must be closed.

Homework Questions p Due by Friday – collected and graded

Question 2 In the circuit (not in the battery), electrons flow from high potential energy (at the negative terminal) to low potential energy (at the positive terminal). Inside the battery, the chemical reaction does work on the electrons to take them from low potential energy to high potential energy (to the negative terminal). A chemical description could say that the chemical reaction taking place at the negative electrode leaves electrons behind on the terminal, and the positive ions created at the negative electrode pull electrons off the positive electrode.

Question 3 Battery energy is what is being “used up.” As charges leave the battery terminal, they have a relatively high potential energy. Then as the charges move through the flashlight bulb, they lose potential energy. The battery uses a chemical reaction to replace the potential energy of the charges, by lowering the battery’s chemical potential energy. When a battery is “used up,” it is unable to give potential energy to charges.

Question 4 The terminal of the battery (usually the negative one) is connected to the metal chassis, frame, and engine block of the car. This means that all voltages used for electrical devices in the car are measured with respect to the car’s frame. Also, since the frame is a large mass of metal, it can supply charges for current without significantly changing its electric potential.

Question 5 Water flows immediately from the spout because there is already water in the spout when you turn on the faucet. Since water is (essentially) incompressible, a push on the water from some distant location (at the valve or pump) causes all the water to move almost immediately, pushing the water out of the spout. In a battery, a similar phenomenon exists in that the wires are “full” of charges (free electrons). The voltage “push” from the battery pushes all the electrons along the wire almost immediately, and the first ones at the other end of the wire are the beginnings of the electric current.

What is ohms law? Ohm’s Law explains the relationship between voltage (V), current (I) and resistance (R) Used by electricians, automotive technicians, stereo installers

VOLTAGE (V) It is the push or pressure behind current flow through a circuit, and is measured in (V) volts.

CURRENT Current refers to the quantity/volume of electrical flow. Measured in Amps (A)

RESISTANCE  Resistance to the flow of the current. Measured in Ohms

Chart

18.3 Ohm’s Law: Resistance and Resistors
Experimentally, it is found that the current in a wire is proportional to the potential difference between its ends: Hewitt- Ohm’s Law

General Rule. 1. Assuming the resistance does not change:
As voltage increases, current increases. as voltage decreases, current decreases. 2.  Assuming the voltage does not change: As resistance increases, current decreases. As resistance decreases, current increases.

18.3 Ohm’s Law: Resistance and Resistors
The ratio of voltage to current is called the resistance: (18-2a) (18-2b)

V (E) = I x R I = V R R = V I

18.3 Ohm’s Law: Resistance and Resistors
In many conductors, the resistance is independent of the voltage; this relationship is called Ohm’s law (a). Materials that do not follow Ohm’s law are called nonohmic (b). (semiconductor diode) Unit of resistance: the ohm, Ω. 1 Ω = 1 V/A.

What can we use to measure?
Multi meters Measures voltage, current, and resistance.

Resistors Why do we need resistors
To decrease the amount of voltage applied to a component The value of the resistor is marked on the body using colored rings

Resistors

18.3 Ohm’s Law: Resistance and Resistors
Standard resistors are manufactured for use in electric circuits; they are color-coded to indicate their value and precision.

18.3 Ohm’s Law: Resistance and Resistors

Ohm’s Law

Putting It All Together…
a.) The gumballs in this demonstration represent the flow of ___________ through the pipe.

Putting It All Together…
a.) The gumballs in this demonstration represent the flow of electrons through the pipe.

Putting It All Together…
b.) The height the pipe is raised above the desk represents potential difference, or _____________.

Putting It All Together…
b.) The height the pipe is raised above the desk represents potential difference, or voltage.

Putting It All Together…
c.) The nails placed in the pipe represent _________ and slow down the flow of the gumballs.

Putting It All Together…
c.) The nails placed in the pipe represent resistance and slow down the flow of the gumballs.

Putting It All Together…
d.) The higher we raise the pipe, the ________ the gumballs flow; the more nails we place in the pipe, the ________ the gumballs flow.

Putting It All Together…
d.) The higher we raise the pipe, the faster the gumballs flow; the more nails we place in the pipe, the slower the gumballs flow.

18.3 Ohm’s Law: Resistance and Resistors
Some clarifications: Batteries maintain a (nearly) constant potential difference; the current varies. Resistance is a property of a material or device. Current is not a vector but it does have a direction. Current and charge do not get used up. Whatever charge goes in one end of a circuit comes out the other end.

Hewitt – Water and Electron Circuits

Question: Why is the bird on the wire safe?

Answer When a bird is perched on a single wire, its two feet are at the same electrical potential, so the electrons in the wires have no motivation to travel through the bird’s body.  No moving electrons means no electric current. If that bird stretches out a wing or a leg and touches a second wire, especially one with a different electrical potential, it will open a path for the electrons—right through the bird’s body.    Source:

Question Why do electricians work with one hand behind their back?

Answer Why do electricians work with one hand behind their back?
You're harmed only if current passes through part of your body. In order for that to happen, you must be touching two places that have a voltage difference between them. The experienced electrician is constantly aware that he has two big efficient electrodes hanging from his shoulders. To avoid setting up a conducting path through his body, he keeps one electrode behind his back, or in his pocket. Source:

Question: Why is the ground prong longer than the other two in a plug?

Question: Why is the ground prong longer than the other two in a plug?
If there is a fault current (leakage) inside the appliance, the whole metal case becomes live. If you happen to touch it, a current will flow through you to the earth (ie. electric shock). It is longer so it is connected before the live prongs. This way if there is a short, you know you are grounded before there is a circuit.

Practice Question 1 20 Coulombs of charge pass a given point in a conductor in 4.0 seconds. What is the current in the conductor?

Practice Question 1 20 Coulombs of charge pass a given point in a conductor in 4.0 seconds. What is the current in the conductor?

Practice Problem 2 In a circuit with a battery supplying 12 volts, the current is 10 amperes. How much charge flows through the circuit in 2.0 minutes? How many electrons were transferred during these 2.0 minutes?

Practice Problem 2 In a circuit with a battery supplying 12 volts, the current is 10 amperes. How much charge flows through the circuit in 2.0 minutes?

Practice Problem 2 In a circuit with a battery supplying 12 volts, the current is 10 amperes. (b) How many electrons were transferred during these 2.0 minutes?

Practice Problem 3 What voltage is used by an electrical appliance that draws 0.4 amps of current and has a resistance of 3 ohms?

Practice Problem 3 What voltage is used by an electrical appliance that draws 0.4 amps of current and has a resistance of 3 ohms?

Practice problem 4 Calculate the current used by a television that runs on 240 volts and has a resistance of 600 ohms.

Practice problem 4 Calculate the current used by a television that runs on 240 volts and has a resistance of 600 ohms.

Practice Problem 5 A 20 ohm resistor has 40 Coulombs of charge passing through it in 5 s. What is the potential difference across this resistor?

Practice Problem 5 A 20 ohm resistor has 40 Coulombs of charge passing through it in 5 s. What is the potential difference across this resistor?

Practice Problem 6 A wire carries a current of 2.0-amperes. How many electrons pass a given point in this wire in 1.0 second?

Practice Problem 6 A wire carries a current of 2.0-amperes. How many electrons pass a given point in this wire in 1.0 second?

phET activity Ohm’s Law

Homework Problems p , 2, 6, 7, 9

Kahoot 18-1 through 18-3