Circuits and Power

Series Circuit

• What happens to the resistance of the circuit as more bulbs are added?– The resistance will

increase (light bulbs are resistors)

• How do you know?– The bulbs are not as

bright … ↑ R = ↓ Ihttp://iss.cet.edu/electricity/

Drawing a Series Circuit

www.bbc.co.uk

Series Circuit & Broken Bulbs

• Why do all the lights turn off when one is unscrewed?– The path is broken –

the current needs to go through each light bulb in order to flow (there is only 1 path for the current to take)

www.bbc.co.uk

Parallel Circuit

• What happens to the resistance of the circuit as more “branches” are added?– The resistance decreases – there are several paths

for the current to follow (think about the difference in the flow of water out of a bucket with 1 hole in it verses several holes)

• How do you know?– The bulbs are bright … ↓R = ↑I

http://iss.cet.edu/electricity/

Drawing a Parallel Circuit

• There are different “branches” of the circuit

• Each bulb is connected to the battery by a separate path

www.bbc.co.uk

Parallel Circuits and Broken Bulbs

• Why does one bulb stay lit when the other is unscrewed?– The current from the

battery “splits” up to go through the different branches

– The path to the other bulb is not broken – the current can take another path

www.bbc.co.uk

Decorative Strings of Lights

• Which type of circuit would you want your decorative strings of light to have?– Parallel

• Why?– So the whole string

does not go out when one bulb breaks

Series and Parallel Combo

Are all the bulbs the same brightness? Why?

Which light bulb can you unscrew to have 1 bulb lit… 2 bulbs lit?

A B

C

Series and Parallel Combo

Which bulb would be the brightest? Why?

B

A C

Which light bulb can you unscrew to have 1 bulb lit… 2 bulbs lit?

More Series/Parallel Circuits

Which bulb would be the brightest? Why?

C

B

A

Which light bulb can you unscrew to have 2 bulbs lit… 0 bulbs lit?

Ohm’s Law

• Current – the flow of electrical charge» Unit: Amp» Abbreviation: I

• Resistance – the measurement of how hard it is for current to flow

» Unit: Ohm» Abbreviation: R

• Voltage – the “force” that causes current to flow

» Unit: Volt» Abbreviation: V

Relationships

• As Resistance goes up, current goes down

• As voltage goes up, current goes up

Formula

• V = IR

• Voltage = current x resistance

V = IR

I = V÷R

R = V÷I

V

I R

Power

• Power = The rate at which energy is transformed from one form to another– electrical energy to thermal energy – electrical energy to mechanical energy– electrical energy to light energy

Power

• Potential Energy for a circuit?Voltage

• Rate of flow of energy?Current (remember this means # charges

flowing past a point per second)

• So what does power equal?Power = Voltage x CurrentP = VIUnit = Watt

Formula

• P = VI

• Power = Voltage x Current

P = VI

V = P÷I

I = P÷V

P

V I

Do Just Light Bulbs have a Wattage?

Typical Wattages of Various Appliances• Here are some examples of the range

of nameplate wattages for various household appliances:

• Aquarium = 50–1210 Watts • Clock radio = 10 • Coffee maker = 900–1200 • Clothes washer = 350–500 • Clothes dryer = 1800–5000 • Dishwasher = 1200–2400 (using the

drying feature greatly increases energy consumption)

• Dehumidifier = 785 • Fans

– Ceiling = 65–175 – Window = 55–250 – Furnace = 750 – Whole house = 240–750

• Hair dryer = 1200–1875 • Heater (portable) = 750–1500 • Clothes iron = 1000–1800 • Microwave oven = 750–1100 • Personal computer

– CPU - awake / asleep = 120 / 30 or less

– Monitor - awake / asleep = 150 / 30 or less

– Laptop = 50 • Radio (stereo) = 70–400 • Refrigerator (frost-free, 16 cubic feet) = 725 • Televisions (color) = 65–170• Toaster = 800–1400 • Toaster oven = 1225 • VCR/DVD = 17–21 / 20–25 • Vacuum cleaner = 1000–1440 • Water heater (40 gallon) = 4500–5500

http://www.eere.energy.gov/

Electric Bills - FYI

• The cost of electricity is based on:– Power use of the building (kW)– Amount of time power was used (hr)– Power x Time = Energy use (kWh)

• The power company then multiplies your energy use by a generation rate (and adds various fees as well)