Review: Static Electricity
• The build up of charge on objects.
• Charges build up until they are
discharged.
Review: Static Electricity Examples
:
• Charge accumulated on a
balloon after it is rubbed
on someone’s hair.
• Electric shocks
• Van de Graff generator
• Lightning
Electric Field
• Electric charges affect other
charges in the space around
them.
• Electric field depends on:
1. amount of charge that
produces the electric field
and
• 2. the distance from the
charge.
Observing an Electric Field
Write the following on your handout:
1. Google: Phet Circuit Construction Kit (do this 1st)
2. Google: Phet electric field hockey (do this 2nd)
http://phet.colorado.edu/simulations/sims.php?si
m=Electric_Field_Hockey
• What affect do like charges have on each
other?
• What affect do unlike charges have on each
other?
If you have time to do electric field
hockey, format your paper from our
homework site as follows:
Electric Field Hockey
Level 1: sign off________
Level 2: sign off________
Level 3: sign off________
• Note: If there is more than one barrier, you must move the
charge around every one.
• http://public.rcas.org/hs/chs/chshomework/Pages/default.
aspx
Chapter 7:
Current Electricity: 3 Variables:
Ohm’s Law:
V=Voltage (Volts)
I=Current (Amps)
R=Resistance (Ohms)
Ohm’s Law
V=IR
(Volts)= (Amps) x (Ohms)
What does Ohm’s Law mean?
The water and pump analogy
analogy for Ohm's Law:
V=IR
Voltage: Water pump that exerts pressure.
Amps: Rate of flow of molecules of water.
Ohms: Resistance provided by the tubing in
the well.
Voltage
• Voltage is the electrical potential energy and is
measured in volts.
• We measure volts with a voltmeter. You will
use an online voltmeter for this lab.
• Once you open the faucet, the pressure
causes the water to rush through the hose.
Voltage (refer to p. 2 of lab hand out)
• Voltage is like a ________ from a battery that
makes a current flow.
• It’s also called __________difference, or
“____” for short.
• We measure voltage with a _______ meter,
and the units are called “Volts”.
• We put our ___________ in parallel with the
thing we’re measuring.
Current (I)
1. Current indicates the number of electrons
passing through the wire.
2. Current is measured in amperes, or amps for
short.
3. The symbol I is used to indicate current.
4. The unit symbol for amps is A, as in 2.0 A.
Current (I)
Electrical
current is
similar to the
rate of water
flowing
through a
hose.
Lab Hand out: Page 1
1. Current is the “flow” of electricity.
2. We measure current with an _________ and
the units are called __________, or ______
for short.
3. We put our ammeter in _____________ with
the thing we’re measuring. (demonstrate on
Phet website).
Resistance
• Electrical resistance can be thought of as the
"friction" on the movement of electrons in a
wire.
• Resistance is measured in ohms. (symbol Ω)
• Thus 3 ohms is written as 3 Ω.
Resistance
• Most devices in an electrical circuit can be
considered resistors, including light bulbs and
electric motors.
• Even the wire itself provides some resistance.
Just as you get some heat from friction,
electrical resistance also results in heat. That
is why the light bulb filament gets hot and
glows.
Resistance
• Following the water hose analogy, resistance is similar to
the friction inside the hose.
• Project on Elmo:
– 1x5 cm2 rectangle,
– 8x5 rectangle.
– Which would you expect to have more resistance?
– Short rectangle
– Long rectangle
– Which would you expect to have more resistance?
• http://www.school-forchampions.com/science/elect_ohms_law.htm
Ohm’s Law: Sample Problem
• What is the voltage of a cell if
• I=4 amps
• R=3 ohms?
• V=IR
• V= 4 amps x 3 ohms
• V= 12 volts
Series Circuits:
• Electrons move from the (-) pole to the (+)
pole.
• If the circuit is broken at any point, the current
will not flow.
• Demonstrate.
Parallel Circuit
• A parallel circuit has more than one pathway
for electrons to flow.
• If any circuit is broken to any branch, electrons
will continue to flow through the remaining
branches.
• Demonstrate.
Label Lab hand out
• You may use schematic view or lifelike view.
• Label batteries, ammeter, resistors (x), light
bulbs.
• Demonstrate: Click to show values.
• Demonstrate: Click for ammeter, voltmeter.
• If your circuit doesn’t work or starts on fire, x
out and start over.
Measuring amps
• Use ammeter in series.
• Demonstrate at Phet website:
• http://phet.colorado.edu/simulations/sims.ph
p?sim=circuit_construction_kit_dc_only
Measuring volts
• Use Voltmeter in parallel.
• Demonstrate.
• http://phet.colorado.edu/simulations/sims.ph
p?sim=circuit_construction_kit_dc_only
Phet Lab
• Build the circuits on the hand out.
• Use an ammeter to measure current in series
and parallel circuits. Move it into each
position and connect it in series.
• Use a voltmeter to measure voltage in series
and parallel circuits. Move the electrodes to
measure voltage across any 2 points.
• Record your values.
Conclusion:
• Determine how current and voltage vary in both
series and parallel circuits.
• Do this by taking measurements at designated
points in the circuits you build at Phet and
analyzing your results.
• If you finish series and parallel circuit lab, you
may do Phet: Electric Field Hockey for extra
credit. Follow the format on our homework site.
Get sign offs from Mr. Purcell and myself in class.
If you need help building circuits from
the schematic diagram:
• Get your lap top and log on.
• If you are ready to start, do so.
• If you want to build a circuit with me, I will
demonstrate again on the projector.
Electric Circuit
•
•
•
•
In an electric circuit, electric charges continuously travel though a closed loop. This flow of
charges is known as electric current. A basic battery-powered electric circuit uses a battery to
drive the current through a loop of wire.
The battery provides a difference in electrical potential energy, or a voltage difference, often
simply referred to as voltage. You can think of it as a difference in pressure: high pressure at
one end of the battery and low pressure at the other. In the same way that a difference in
water pressure pushes water through a pipe, creating a flow of water, a difference in voltage
pushes electric charge through the wire, creating a flow of current through the circuit. The
greater the voltage, the greater the current.
When electric current flows through a material, it is likely to encounter resistance. Instead of
traveling a direct path through the closed loop, charges may be scattered as they collide with
the atoms of the material. Electrical resistance is a measure of how much the flow of electric
charge is opposed in this way. The greater the resistance, the more difficult it is for current to
flow.
http://www.teachersdomain.org/resource/hew06.sci.phys.maf.ohmslaw/
If the pressure stays the same and the
resistance increases (making it more
difficult for the water to flow), then the
flow rate must decrease: