Electrical Systems

Types of Circuits

• Series Circuit: a circuit where current only follows only one path

• Parallel Circuit: a circuit where current “branches off” in different directions

Types of Circuits

• Short Circuit: a circuit made with a parallel branch using only a wire

Series & Parallel Circuits

• Series Circuit– Current is the same at all

points– All voltage drops add up

to voltage of battery– Total resistance increases

with more resistors

• Parallel Circuit– All currents add up to the

total current of battery– Voltage is the same at all

points– Total resistance decreases

with more resistors

...1111

321

RRRRP

Series Circuit• Series Circuit: current can only follow

one path.

• All voltage drops add up to the voltage of the battery.

• The current in a series circuit is the same at any point.

• The total resistance increases as more resistors are added.

R I V1 Ω 0.25 A 0.25 V2 Ω 0.25 A 0.50 V3 Ω 0.25 A 0.75 VTotal = 6 Ω

Total = 0.25 A

Total = 1.5 V

R I V

Total = Total = Total =

R I V

Total = Total = Total =

R I V

Total = Total = Total =

• Parallel Circuits: a circuit where current ‘branches off’ in different directions.

• All voltage drops are the same

• The current across each load adds up to the current supplied by the battery.

• Resistance decreases as more resistors are added.

Parallel Circuits

R I V1 Ω 3 A 3 V2 Ω 1.5 A 3 V3 Ω 1 A 3 VTotal = 6/11 Ω

Total = 5.5 A

Total = 3 V

...1111

321

RRRRP

R I V

Total = Total = Total =

R I V

Total = Total = Total =

R I V

Total = Total = Total =

Combination Circuits

• A combination of series and parallel.– Simplify resistors just in series or just in parallel– Redraw simplified circuits until there is a single

resistor

• Determine the total resistance

Find the Total Resistance

Find the Total Resistance

Find the Total Resistance

Short Circuits• Short Circuit (Short): a circuit made with a

parallel branch using only a wire– Bypasses a part of the circuit– Increases the current through a circuit

(less resistance)

• Electricity always follows the path of least resistance.

• Short circuits in homes can– Waste energy– Heat wires (melt the insulation or

cause fires)

• Which circuit below has a short in it?

Fuses

• A fuse prevents too much current from flowing through a circuit

• It “breaks” if the current is too high.

What Is Power?

Power

• Power: the amount of energy (work done) each second by a circuit.– Measured in watts

Determine the amount of power the circuit below has.

• How much current flows through a 70 watt guitar amp that runs on a voltage of 120 volts?

Additional Problems1. Determine the power used by a hair dryer

that draws a current of 8 amps and runs on 120 volts of electricity.

2. Determine the current used by an iPod that runs on a 3.7 V battery and has a power of 2 watts.

3. Convert 200 cV to kV4. Convert 4.5 in/min to m/hr

Electrical Power at Home

• Electric meters measure kilowatt hours (kW·hr)

• A 1 kW appliance ran for 1 hour = 1 kW·hr

• Energy = P·t

• An electric stove uses 3,000 watts of power. It is used to bake for 30 minutes.– How many kilowatts of power does it use?– How many kilowatt hours does it use while baking?– If electricity costs $0.15 per kilowatt hour, how much

does this cost?

• A 100 watts light bulb is left on for 5.5 hours.– How many kilowatts of power does it use?– How many kilowatt hours does it use?– If electricity costs $0.15 per kilowatt hour, how much

does this cost?

DC and AC

• DC current– Direct current – Current or voltage keeps

the same direction

• AC current– Alternating current– Current or voltage reverses

(60 times / sec in US)

DC and AC• DC current is good for short

distances

• AC can be converted to lower voltages or DC

• AC current can travel over large distances (from power plant)

• Transformers convert high voltage to lower voltage

Capacitors

• Capacitors: devices that store electrical energy by separating charges

• Can supply power for a short amount of time

• Measured in farads (F)– 1 F = 1 coulomb of charge stored

using 1 V

Capacitors

• Capacitance depends on 3 things– Surface area of plates– Type of insulator– Distance between the plates • Smaller distance = greater capacitance

• Capacitors discharge over time

Capacitors

• Defibrillators• • Subwoofers

• Crosswalk lights