Electrical Household Appliances - Cal Poly Pomona printable pdf download

Electrical Household Appliances – Lesson 6

Appliance Physics, Tasks, Power, and Energy

Summer 2004

Cal Poly Pomona

Objectives: (i) Investigate the physical principles involved in the operation of common

electrical household appliances. (ii) Identify the tasks done by each appliance.

(iii) Determine the power ratings of each appliance. (iv) Estimate the energy each

appliance uses monthly and the total energy use for all appliances. (v) Compare the latter

estimate with actual electrical energy use, as shown on a utility bill.

Background: The electric power required by an electric appliance can be estimated as

(the appliance's electric CURRENT) × (the VOLTAGE across its terminals). For example

if the current through a household light bulb is 0.5 amperes and the voltage across its

filament is the common 120 volts, then its power is (0.5 amperes) × (120 volts) = 60

ampere volts = 60 watts. Note: 1 amp × 1 volt is defined to be 1 watt.

As we saw earlier, power represents the RATE at which electric energy is being fed to an

appliance. Note that the power is properly zero if the voltage across the appliance is zero

– which is true when it is switched OFF. Fortunately, most electric appliances have

labels that give their power ratings, and it is not necessary to know the electric currents

flowing through them to calculate their power requirements. Furthermore, the

VOLTAGE supplied to most household appliances is 120 volts, so Power = (120 volts) ×

(CURRENT in amps). From this equation, it is clear that higher power appliances

require more amps.

The electric energy, in kilowatt hours (kWh), used by an appliance over a time interval is

POWER (watts) × TIME (hr)

Energy (kWh) =

1000 (watt • hr/kWh)

Example: The electric energy used by a 100 watt light bulb operating for 24 hours is

(100 watts) × (24 hours) / (1000 watt hours/kWh) = 2.4 kWh. The typical cost of 1 kWh

of electric energy from Southern California Edison is approximately $.13. Thus 2.4 kWh

of electric energy costs (2.4 kWh) × (0.13 $/kWh) = $0.31.

Many electric appliances operate using one or more of the following three parts:

A. electric motor (e.g., a fan),

B. electric current flowing through a conductor that gets hot (e.g., a toaster),

C. an electromagnet (e.g., a solenoid valve that quickly opens and closes, controlling

water flow in a washing machine).

Electrical Household Appliances - Cal Poly Pomona