ELECTRIC CURRENT

Chapter 13.2

Voltage, Current and Resistance

The flow of electricity depends upon… Voltage Current Resistance

http://cache.eb.com/eb/image?id=100742&rendTypeId=4

Current (electron flow)

voltage

resistance -

+

Voltage

Force that causes charges to move (the push or the electric field)

Voltage exists if there is a potential difference in charges (negatively charged vs. positively charged)

SI unit is Volt (V)

Does the size of the cell matter?

D cells AA cells

1.5 volts 1.5 volts

6 volts? 9 volts?

120 Volts

110-120 V0 V

To the ground (the path of least resistance and easiest flow!)

There must be a voltage difference between the two terminals for current to flow.

Voltage Analogy (the push that causes charges to move)

0 V

0 V

0 V

0 V

0 V

1.5 V

9 V

120 V

Balanced charges

ball (electron)

Angle of slide (voltage)

Batteries

http://www.qrg.northwestern.edu/projects/vss/docs/Power/2-how-do-batteries-work.html

High voltage is at the positive terminal

Zero voltage is at the negative terminal

1.5 V

0 V

Batteries

http://www.explainthatstuff.com/batteries.html

Electric Current

The flow of charged particles. Charges move from a position of higher to

lower electric PE (or higher voltage to lower voltage).

SI unit is ampere (A or amps)

Resistance

Resistance restricts the flow of electric charges.

The cause: internal friction as moving charged particles collide.

Unit for resistance is ohm

Factors influencing theresistance of a wire length temperature cross-sectional area conductivity of the material

Increasing Resistance Which is more resistant? Long or short wire? long Thin or thick wire? thin Hot or cold wire? hot Less conductive or more conductive wire? Less conductive

Circuits

Series circuit only one path for electric current to flow bulbs dim if more bulbs are added in series other bulbs go out if one bulb burns out

Parallel circuit two or more paths for electric current to flow bulbs maintain same brightness if more bulbs are

added in parallel other bulbs stay lit if one bulb goes out

Series circuit Parallel circuit

http://iss.cet.edu/electricity/pages/a17.xml

Match statements with the correct circuit Series circuit vs. Parallel circuit

1. other bulbs go out if one bulb burns out 2. bulbs maintain same brightness if more bulbs

are added 3. two or more paths for electric current to flow4. bulbs dim if more bulbs are added 5. other bulbs stay lit if one bulb goes out 6. only one path for electric current to flow

12

4

3

6

5

Series or Parallel?

1 23

4

7

5

6

parallel

series

series

series

parallel

parallel

parallel

Hewitt and electric circuits

Series circuits __________ resistance and decrease in _________ and __________ as more light bulbs are added.

Adding more light bulbs to parallel circuits causes the resistance of the circuit to __________, the voltage across each bulb to ___________ the ________, and the current flowing through the entire circuit to ____________.

Draw these circuits using schematic symbols in your spiral notebook. Series, 1.5 V dry cell, three bulbs, one closed

switch Parallel, 1.5 V dry cell, three bulbs in parallel,

one open switch controlling only the second bulb.

What’s wrong with this picture?

http://www.fire-extinguisher101.com/article0705-electrical-fire-safety.html

In a parallel circuit, each device will receive the amount of electric current needed to operate. This can lead to wires overheating, which can cause fires.

http://www.animatedsoftware.com/environm/nukequiz/nukequiz_one/nuke_parts/reactor_parts.swf

http://www.eas.asu.edu/~holbert/flash.html

Fuses and Circuit Breakers

For overloaded current flow, the path will be broken fuse burns out breaker springs open.

Each parallel circuit in your home has a fuse or circuit breaker rated at a certain amount of amperes (15A, 20A, 30A, and so on)

As more electrical devices are added to the parallel circuit, the resistance of the circuit decreases. This means that more electric current can flow.

Circuit Breaker and Fuse Box

http://jimspearsfusebox.tripod.com/help/oldfusebox.jpg

http://www.smarthome.com/7103.html

Exit Question: Draw these two circuits using schematic symbols. Series, three 1.5 V dry cells, two bulbs, one

open switch Parallel, one 6 V battery, three bulbs, one

closed switch controlling one bulb, another open switch controlling all three at once.

Series or parallel?

http://www.pz.harvard.edu/ucp/curriculum/circuits/s5_lessonplan.htm

http://mrskingsbioweb.com/IntegratedScience.htm

http://www.fortunecity.com/greenfield/eagles/180/current.html

http://www.inclusive.net/exemplars/exemplar_b_4_1.shtml

http://www.school-for-champions.com/science/dc_circuits.htm

Circuits: Basic Questions

Series circuits How many paths can an electron follow from one end of the

terminal to the other? Do the bulbs get brighter, dimmer or stay the same if more

bulbs are added in series? What happens to the other bulbs if one bulb goes out?

Parallel circuits How many paths can an electron follow from one end of the

terminal to the other? Do the bulbs get brighter, dimmer or stay the same if more

bulbs are added in seriesseries What happens to the other bulbs if one bulb goes out?

parallel?

Ohm’s Law

Resistance = Voltage/Current R = V / I

R is resistance, unit is ohm V is voltage, unit is volt (V) I is current, unit is ampere (A)

Do practice problems 1-4 on page 443.

Using Ohm’s Law

Use to calculate electrical resistance, amperage or voltage.

Ohm’s Law predicts the degree of electric shock one can experience

Do practice problem #1 on page 443.

Adding load in parallel causes a decrease in resistance (more current is available, voltage stays the same)

Adding load causes an increase in resistance and decreases voltage across each device.

How batteries produce voltage A chemical reaction facilitated by an electrolyte causes electrons

to move internally from one terminal of the battery (one type of metal) to the other (a different type of metal). This difference in charges produces voltage.

Positive end (cathode) loses electrons Negative end (anode) gains electrons The reaction starts only after a conducting wire connects both

terminals. Unconnected batteries still react, but very slowly. This means they have a shelf life.

The speed of the reaction determines how many electrons will be transferred.

Batteries wear out when the reaction slows or is impeded and cannot keep up with the demand of the electric device (power!)

What’s A Coulomb? A coulomb is a measure of charge consisting

of a large group of electrons (6.24 x 1018 electrons)

One ampere contains one coulomb of charge passing by each second.

1 volt is one joule of energy per coulomb A 100 W bulb in a 120 V circuit needs A 100 W bulb needs 100 joules of energy per

second. (power = work or energy/time)

Electric Shock

0.001 amps is equal to 1 milliamp!

5 milliamps

70 milliamps

Electric Shock For you to receive an electric shock…

There must be a voltage difference between one part of your body and another.

The current will pass along the path of least electrical resistance. Average wet skin… 1000 ohms Dry skin… 100,000 ohms Range 100 to 500,000 ohms

Wet skin reduces your resistance to the flow of current.

DANGER!

How to Prevent Electric Shock Increase the resistance to the flow of charged

particles (electric current). Cut off the pathway for electric current. Re-route the pathway for electric current

(lightning rods and grounding wires)

Ground Fault Current Interrupter

The GFCI senses changes in current and will trip if the leakage current reaches 0.005 A (5 milliamps).

Lightning Rods

Lightning rods leak off charges to prevent build up of charges. Lightning rod systems act as a conductor, directing the electrical energy away from the house to the ground.