Simple Circuits

Challenge Questions 1. Why can a bird be perched on a high voltage wire?

No potential difference between bird’s feet, therefore no current. http://www.youtube.com/watch?v=GLW6MEZ9Dcs 2. If a parachutist grabs onto a wire, what happens?

What if it breaks? Why should the parachutist let go as it falls to the ground? No potential difference in the first situation, so no current. If they

hold on and their feet touch the ground, there will be a current due to potential difference between the wire and the ground.

http://www.youtube.com/watch?v=jleAxuFGknk http://www.youtube.com/watch?

v=BtQtRGI0F2Q&feature=related

Battery and Light Bulb

Consider the diagram of the circuit you created to light the light bulb.

Light Bulb How does a light bulb make a full conducting

path?

Electric Circuits

Electric Circuit A set of electrical components connected so that

they provide one or more complete paths for the movement of charges

Ex. Light Bulb Filament is a resistor. When wire connects battery

to the bulb, charges built up on one terminal of battery have a path to reach the opposite charges on the other terminal. Charges move creating a current. Current causes filament to heat and glow.

Electric Circuits Circuit

The path where electrons flow. Current

The rate at which the charge flows past a point. Voltage

The amount of “push” behind electrons. Resistance

Equal to potential difference divided by current. EMF

The energy per unit charge supplied by a source of electric current Load – any element in a circuit that dissipates energy (ex. Bulb) Closed circuit – a complete path from one battery terminal to

another. Open circuit – no complete path, therefore no current

Wire

Resistor

Bulb

Plug

Battery

Switch

Capacitor

Schematic Diagram Symbols

Think of Christmas lights. What happens when one light burns out?

The circuit is no longer closed and all the bulbs go dark. So why use this? - It decreases the current needed. Several lesser resistances can add up to a single greater

resistance. Important to have no current if something fails (ex.

Burglar alarm)

Resistors in Parallel Parallel

Describes two or more components in a circuit that are connected across common points or junctions, providing separate conducting paths for the current.

Ex. Christmas Lights In series, if a single burns out, they all go dark. In parallel, they have an

alternative path. Current varies, potential difference remains the same.

Resistors in Series Req = R1 + R2 + R3…

Equivalent resistance equals the total of individual resistances in series. I = ΔV/Req ΔV = IR1 and ΔV = IR2 VT = V1 + V2 + V3…

Resistors in Parallel 1/Req = 1/R1 + 1/R2 + 1/R3…

Equivalent resistance of resistors in parallel can be calculated using a reciprocal relationship.

IT = I1 + I2 + I3… I = ΔV/Req ΔV =IReq

Series and Parallel Resistors

Series Parallel

Current same as total add to find total

Potential Difference add to find total same as total

Each Resistor =10 Ω

Req = 10+10+10

Req = 30 Ω

I = V/R

I = 12V / 30 Ω

I = 0.4A

12V

V = IR

V = (0.4A)(10 Ω)

V = 4 volts

VT = 4V + 4V + 4V = 12V

I = V/R

I = 4V / 10 Ω

I = 0.4A

P = IV

P = (0.4A)(4V)

P = 1.6 W

Each Resistor =10 Ω

1/ Req = (1/10)+(1/10)+(1/10) = 3/10

Req = 3.33 Ω

I = V/R

I = 12V / 3.33 Ω

I = 3.6A

V = 12V

I = V/R

I = 12V / 10 Ω

I = 1.2A

IT = 1.2A + 1.2A + 1.2A = 3.6A

P = IV

P = (1.2A)(12V)

P = 14.4W

12V