Physics 102-002 Announcements

• WebAssign – – Chapter 22 due this Wednesday– Chapter 23 due next Wednesday

• Exam #2 graded .. Pick them up.

• Exam #3 is on April 9 (next Monday)

Picture: Aurora Borealis (courtesy Jan Curtis, University of Alaska). The electric currents that flow in the Earth’s vicinity from its interaction with the solar wind drive the aurora. In the process, they can create overloads on electric power distribution grids, creating massive power blackouts.

Class Schedule

3/19 Chapter 10 Projectile and Satellite Motion (Pg 184-191)

3/21 Chapter 11 Atomic Nature of Matter (Pg 211-223)

3/26 Chapter 22 Electrostatics, Part 1 (Pg 410-419)

3/28 Chapter 22 Electrostatics, Part 2 (Pg 419-430)

4/2 Chapter 23 Electric Current, Part 1 (Pg 436-443)

4/4 Chapter 23 Electric Current, Part 2 (Pg 444-452)

4/9 Midterm Exam #3

Chapter 23Electric Current, Part 1

• Flow of Charge• Electric Current• Voltage Sources• Electrical Resistance• Ohm’s Law• Direct Current and Alternating Current• Speed and Source of Electrons in a Circuit• Electric Power• Electric Circuits

Next time

Flow of ChargeIn order for a charge to flow, it needs a push (a force) and it is supplied by voltage, or potential difference. The charge flows from high potential energy to low potential energy.

Water will flow from a high reservoir to a lower one (high potential to low potential). Water will flow as long as the difference in levels is maintained – like with a pump.

Suppose A has a potential of 12 V and B has a potential of 2 V. There is a potential difference. A has higher potential energy than B, and it means there is voltage. The potential difference is VA - VB = 12 - 2 = 10 V.

What will happen if something pushes the charge from the bottom plate to the upper plate E in the diagram? This will generate a potential difference, and hence, there will be continuous flow of charge. This is how a battery works; it takes "+" charge from bottom and push it to top.

Electric CurrentElectric current is the flow of electric charge.

         

                    

    

Why can a bird sit on the power line?

Therefore, there are no potential difference between these two feet.

Let's say the power line is very low, almost touching the ground, and a chicken is trying to cross it. If one leg is on the ground and the other one is on the power line, then there are potential difference between these two legs.                                                                              Therefore, there is a flow of charge and eventually the chicken will be barbequed like the

Electric current is the rate of charge flow past a given point in an electric circuit, measured in coulombs/second which is named Amperes.

Remember the conduction electrons … the electron sea in metals … are free to move around. They’re the charges that move in a current.

Be careful, though, because in liquids (like in a car battery), the moving charges are the positive ions.

Note that although charge moves in a wire while a current flows, the net charge on the wire is always zero.

Physics Place video.

Question 1

Is a current-carrying wire electrically charged?– Yes– No– Maybe

Question 1 Answer

Is a current-carrying wire electrically charged?– Yes– No– Maybe

Voltage SourcesWe need more than just a continuous path (circuit) before a continuous flow of electrons will occur: we also need some means to “push” these electrons around the circuit. Just like marbles in a tube or water in a pipe, it takes some kind of influencing force to initiate flow. With electrons, this force is the same force at work in static electricity: the force produced by an imbalance of electric charge – or a difference in the “electric potential”.

Examples of voltage sources:

• Batteries• Electric Generators• Automobile battery = 12 V• Fuel Cells• Solar Cells• Piezo-electrics• Electrets• Lightning• Biological voltage generators• Thermo-electrics.• Capacitors

Electrical ResistanceVoltage can be thought of as the pressure pushing charges along a conductor, while the electrical resistance of a conductor is a measure of how difficult it is to push the charges along.

Using the flow analogy, electrical resistance is similar to friction. For water flowing through a pipe, a long narrow pipe provides more resistance to the flow than does a short fat pipe.

The same applies for flowing currents: long thin wires provide more resistance than do short thick wires.

In a hydraulic circuit, a narrow pipe resists the flow of water.

In an electric circuit, a lamp or other device (for example, a motor or resistor) resists the flow of electrons.

More water flows through a wide hose than through a narrow one. Similarly, more electric current flows through a thick wire than through a thin one.

Ohm’s LawIn many materials, the voltage and resistance are connected by Ohm's Law:

Ohms

VoltsAmperesor

Resistance

VoltageCurrent (in units form)

The unit for resistance is the “Ohm” and the symbol is “” (the Greek letter “Omega”)

Higher resistance IMPEDES the flow of current.

The greater the voltage, the greater the current.

The greater the resistance, the SMALLER the current!

Different types of resistors.

High current can cause injury. High Voltage + High Resistance = Low CurrentHigh Voltage + Low Resistance = HIGH CURRENT(The moral: Insulate yourself if you’re around high voltage)

Direct Current and Alternating Current

Physics Place video.

DC – Charges move in the same direction all the time.

AC – Charges move in alternating direction. Done using an electric generator arranged as shown on the right.

AC current is commonly used for residential and commercial power circuits.

In North America, the voltage is 120V – an average (called the RMS average). It oscillates at about 60 Hz (or 60 cycles per second), which you get by making the generator go around at that speed.

RMS

To convert AC current to DC current, you use a diode (to let through only one direction of current) combined with a capacitor (to store some energy and release it while the current isn’t going through the diode) …. See the figure below.

Some Diodes Effect of a capacitor

Diode lets through current in only one direction

Capacitor stores energy and releases it over the “gap” in the AC current

Using several diodes/capacitors makes the current more constant