CURRENT ELECTRICITY AND OHM

I. Review of cause of current electricity and advantages

A. Static electricity only discharges its energy in one quick release. Once it is released the electrons cease to flow. This is not very useful for work since you cant get constant energy flow.

B. People began to play around with making a source of constant energy flow – current electricity—that could do work at a constant rate.

C. In order for electricity to flow there must be a difference in electric potential difference to make the electrons move (voltage)

D. There must also be a conductor available for the electrons to flow through. There cannot be a break in the conductor for then the electrons do not experience a electric potential difference.\

E. Lastly, there must be an electron source for electricity cannot exist without electrons

F.

6V difference

Battery contains chemicals with electrons, battery provides difference in potential

G. For most electrical appliance there are two wires insulated from each other that allow the electrons to move away from the source to the appliance and the back to the source. They must not break or touch or electrons cannot flow. If they touch a short circuit occurs( electrons flow wires with no resistance and burn out energy source)

II. Check Question: Why don’t birds on a wire get electrocuted?

III. This is the reason electricians keep one hand behind their back while working on circuits. They don’t want to provide a potential difference between their hands so the electricity can go through their heart

IV. What should you do if a person is holding onto a bare wire being electrocuted?

V. Electricity will always flow to the ground. It will find the easiest path to get there. If it is through you, you are electrocuted. That is why many plugs contain a round prong , this connects the circuit right to the ground with a path of little resistance so there is less chance of it going through you.

VI. Ohm’s Law

A. Ohm discovered a relationship between the amount of electrons, the push applied to the electrons, and the resistance provided by the conductor.

B. V= IR

Where

V= voltage (push) – in volts (J/C- work per unit charge)

I= current ( amount of electrons) – in amperes- A (C/s – charge per unit time)

R= resistance (provided by appliances or conductor) – in ohms-

*** current is what will kill you. If you have as little as 2mA you can be dead yet your body can withstand 120 V with little problems

VII. The OHM triangle!!!

If you cover the value you want you get the eqution you need. EX. V= IR, I= V/R, R=V/I

VIII. Current speed

A. Demonstration of marchers—

Have students line up. Push back person.

Repeat saying one, two , three march!

Which is a better analogy for your hot water getting to your shower?

Which is a better analogy for electricity getting to your light bulb?

It only takes a brief time for the electric fields of electrons to travel through the wire ( nearly the speed of light) electrons at the far end of the circuit respond immediately.

B. Realize, however, that the velocity of the electric signal (nearly the speed of light) is quite a bit different than the drift velocity (a snail’s pace) of electrons in a circuit

C. CHECK QUESTION: When you turn the ignition key to start your car, electrons migrate from the negative battery terminal through the electric network to the starter motor and back to the positive batter terminal. About ho much time is required for an electron to leave the negative terminal and go through the circuit and return? Less than a millisecond? Less than a second? About a second or two? Or about a day?

IX. AC vs DC

A. DC means direct current

1. batteries supply dc current

2. electrons constantly flow from the negative terminal to the positive terminal

3. this provides a portable inexpensive type of energy.

B. AC means alternating current

1. Outlets in the US provide this type

2. The electrons are pushed back and forth along the line but never move very far. (kind of like your washing machine agitator moving back and forth)

3. This is more expensive but can provide energy at further distances for little loss. This is why we use it in the US.

4. Many European countries don’t need to move electricity that far so they use DC.

C. Converting AC to DC

Battery driven devices can use AC by using a device called a diode. A diode allows the passage of charge in only one direction. A capacitor smoothes the flow.

X. How many electrons have been pumped by PG&E into your home during the past year? [ZERO!] PG&E supplies the energy to move the electrons you supply.

XI. Power

A. Power is the amount of energy produced during a given time. The higher the power the faster the energy is used up or produced.

B. Units are J/s = Watt. This is small for electrical appliances so usually they use the kilowatt. Your electric bill is in kilowatt-hours. That si the actual energy you have consumed.

C. Formula – P = VI

(energy) (charge) = energy

(charge) (time) time

more power = more light = more energy

XII. Sample Problems

A. How much current will a person experience if the resistance of their body across 120 volts is 100,000 ohms?

G: V = 120 V,R = 100,000 ohms

F: I = ?

R: V = IR

S: I = V/R

C: I = 120 V/ 100,000 ohms = 0.0012 A

B. How much power went through the person above?

G: V = 120 V, I= 0.0012A

F: P = ?

R: P = VI

C: P = (120V)(.0012 A) = .144 W

I. Millikan

a) first person to calculate the mass of a singe electron.

b) shot oil drops horizontally into the air and tried to get them to fall right in between two charge plates. As the drop rubbed against the mistifier they usually gained electrons and became negatively charged.

c) He thought that if the force of E-field between the plates was equal to the weight of the electron then the drop would not fall down.

+

-

Fg = Fe

mg = Eq

so…

q = mg/E

d) Since Millikan knew the charge on an electron was 1.6 x 10^-19 C, and g was 10 m/s², he only needed to measure the distance between the plates and the voltage between the plates to find E since:

E= V/d.

II. Sample problem:

An oil drop weighs 1.9x 10 ^-14 N. It is suspended in an electric field of intensity 4x 10⁴ N/C.

a) what is the charge on the oil drop?

b) If the drop is attracted toward the positive plate, how many excess electrons does it have?

a) G: Fg= 1.9 x 10^-14 N, E= 4x 10⁴ N/C

F: q=?

S,R: q = mg/E

C: q= 1.9x10 ^-14/4 X 10⁴ N/C

Q= 4.8 x 10^-19 C

b) G: qe= 1.6 x 10^-19 C, q = 4.8 x 10^-19 C

F: #e=?

S,R: #e=q/qe

C: #e= 4.8 x 10^-19 C / 1.6 X 10^-19 C

#e= 3

III. because oil drops are difficult to get to work without hours of lab work, we will do this using a computer simulation that will allow you to adjust the voltage between the plates as oil drops shoot out on the screen. You need only record the voltages that work to find to get your data.

IV. Now, just like Millikan, some drops will have 1 electron, some will have 2 electrons, some will have 3, etc. But you graph your results from lowest voltage to highest voltage, the difference between one bar and the next will always be the same representing one additional electron. This type of graph is called a histogram.

histogram

Each step = 1 more electron = charge of one electron, SMALLEST step = charge of one electron

charge

trial

ELECTRIC CIRCUITS

I. Student discovery activity

A. provide students with three light bulbs, 6 wires stripped on each end, and 1 battery.

B. Teacher demonstrates how to hook up one light bulb wo it lights up.

C. Students are to hook up all light bulbs so that they all light in three different ways. That is they are all bright or they are all dim or some are bright and some are dim.

D. Have student disconnect each bulb after the have set up the circuit and record what happens.

E. They need to draw how they hooked up the light bulbs to the best

of their ability and label bulbs as dim or bright.

II. Simple circuits

A. you should have noticed that is was not easy to show how you hooked up your light bulbs. Electrician have come up with a short hand notation to make communication easier.

B. Here are a few simple symbols:

= battery

= resistor

= conductor (wire)

labels can be added such as 6 ohms or 6 V to make things more accurate

C. Here is a simple circuit:

III. Series VS Parallel circuits

A. Series circuit is when you had the current flowing through one path.

1. If one light bulb is disconnected the rest will go out because there is no unbroken path for the electrons to flow through.

2. Since there is only one path the amount of current flowing through each resistor is the same. That is if 2 C of electrons go through R1 then 2C of electrons must then go through R2. The total voltage drop from start to end must equal the total voltage drop for the circuit so the voltage and R’s are not necessarily the same.