Chapter 6Chapter 6

PowerPower

Chapter 6.1Chapter 6.1

Power in Mechanical Power in Mechanical SystemsSystems

PowerPower• PowerPower is the amount of work is the amount of work

done in a certain amount of done in a certain amount of time. In other words, power is time. In other words, power is the the raterate at which work is done.at which work is done.

Calculating PowerCalculating Power• Power can be calculated using the Power can be calculated using the

following formulas:following formulas:– Power (P) = Work (W)/Time (t) P = W/tPower (P) = Work (W)/Time (t) P = W/t– Power (P) = Energy (E)/Time(t) P = E/tPower (P) = Energy (E)/Time(t) P = E/t

– Units:Units:• Energy or Work – Joules (J)Energy or Work – Joules (J)• Time – seconds (s)Time – seconds (s)• Power – Watts (W) 1 WPower – Watts (W) 1 W• 1 W is Equal to 1 J/s1 W is Equal to 1 J/s NOTE:NOTE: J/s is J/s is NOTNOT

the unit for power!the unit for power!

Calculating PowerCalculating Power• PowerPower can also be measured in can also be measured in

kilowatts. The watt is such a small kilowatts. The watt is such a small amount of power that many times amount of power that many times kilowatts are used instead.kilowatts are used instead.

• 1 kW = 1000 watts1 kW = 1000 watts• Another common measurement for Another common measurement for

power is power is horsepowerhorsepower (hp). (hp).• 1 hp = 746 W1 hp = 746 W

Chapter 6.3Chapter 6.3

Power in Electrical Power in Electrical SystemsSystems

Electrical PowerElectrical Power

• ElectricalElectrical powerpower is the rate at which is the rate at which electrical energy is converted into other electrical energy is converted into other forms of energy. P = I x Vforms of energy. P = I x V

• UnitsUnits::– Power – watts (W)Power – watts (W)– Current – Amperes (A)Current – Amperes (A)– Voltage difference – Volts (V)Voltage difference – Volts (V)

Energy and AppliancesEnergy and Appliances

• To calculate the amount of electrical energy To calculate the amount of electrical energy used by an appliance, multiply the used by an appliance, multiply the powerpower used used times the times the timetime the appliance is used. the appliance is used.

E = P x tE = P x t

Units:Units:• Electrical energy – kilowatt-hour (kWh)Electrical energy – kilowatt-hour (kWh)• Power – watts (W)Power – watts (W)• Time – hours (h)Time – hours (h)

Energy and Appliances - 1Energy and Appliances - 1

• A A kilowatt-hourkilowatt-hour (kWh) is the energy (kWh) is the energy consumed by a 1 kW device operated consumed by a 1 kW device operated continuously for 1 hour.continuously for 1 hour.

• The equation E = P x t is part of the The equation E = P x t is part of the method the method the electricelectric companycompany uses to uses to calculate your electric bill.calculate your electric bill.

• To calculate your bill, the number of To calculate your bill, the number of kWh used is kWh used is multipliedmultiplied by how much the by how much the power company is charging per kWh.power company is charging per kWh.

Energy and Appliances - 2Energy and Appliances - 2

• For example, if your electric company charges For example, if your electric company charges 12 cents ($0.12) per kWh and you used 1,375 12 cents ($0.12) per kWh and you used 1,375 kW of electricity during the month, then your kW of electricity during the month, then your electricity would cost you $165.00.electricity would cost you $165.00.

• Total Cost = Energy (E) x Cost per kWhTotal Cost = Energy (E) x Cost per kWh = 1,375 x $0.12= 1,375 x $0.12 = $165.00= $165.00

• • Power companies add other Power companies add other feesfees such as fuel such as fuel cost to this total, so your bill would actually becost to this total, so your bill would actually be moremore than just the cost of the electricity. than just the cost of the electricity.