Electrical Power

Physics ACurrent #4

Learning Targets

I can predict and calculate power in a circuit.I can explain energy transfer in a circuit.I can explain the difference between electric

power and electric energy.

Power: Putting Charges to Work

Electric circuits are designed to serve a useful function.

Short Circuit: a circuit with a high rate of charge flow

With charge flowing rapidly between terminals, the rate at which energy would be consumed would be high.

Such a circuit would heat the wires to a high temperature and drain the battery of its energy rather

quickly.

Power: Putting Charges to Work

When a circuit is equipped with a light bulb, beeper, or motor, the electrical energy supplied to the charge by the battery is transformed into other forms in the electrical device.

A light bulb, beeper and motor are generally referred to as a load.

Power: Putting Charges to Work

An electrical circuit is simply an energy transformation tool.

Energy is provided to the circuit by an electrical energy source.

Energy is delivered by the circuit to the load at the location of the load.

Power is the rate at which electrical energy is supplied to a circuit or consumed by a load.

Power: Putting Charges to Work

Like current, power is a rate quantity. Its

mathematical formula is expressed on a per

time basis.

P =Wt

Power: Putting Charges to Work

The unit of electrical power is the watt,

abbreviated W.

A watt of power is equivalent to the delivery of

1 joule of energy every second. In other words:

1 watt = 1 joule / second

Power: Putting Charges to Work

When it is observed that a light bulb is rated at 60 watts, then there are 60 joules of energy delivered to the light bulb every second.

A 120-watt light bulbs draws 120 joules of energy every second.

The ratio of the energy delivered or expended by the device to time is equal to the wattage of the device.

The kilowatt-hour (kWhr)

Electrical utility companies who provide

energy for homes provide a monthly bill

charging those homes for the electrical

energy that they used.

Somewhere on the bill will be a charge for the

number of kilowatt-hours of electricity that

were consumed.

Exactly what is a kilowatt-hour?

The kilowatt-hour (kWhr)

The kilowatt • hour is a unit of energy. A utility company in the United States is

responsible for assuring that the electric potential difference across the two main wires of the house is 110 to 120 volts.

Maintaining this difference in potential requires energy.

Energy Bills

Check for Understanding

The purpose of every circuit is to supply the energy to operate various electrical devices. These devices are constructed to convert the energy of flowing charge into other forms of energy (e.g., light, thermal, sound, mechanical, etc.). Use complete sentences to describe the energy conversions that occur in the following devices.

a. Windshield wipers on a carb. Defrosting circuit on a carc. Hair dryer

Check for Understanding

Determine the ...

a. ... current in a 60-watt bulb plugged into a 120-volt outlet.

b. ... current in a 120-watt bulb plugged into a 120-volt outlet.

Check for Understanding

Determine the ...a. ... power of a saw that draws 12 amps of

current when plugged into a 120-volt outlet.

b. ... power of a toaster that draws 6 amps of current when plugged into a 120-volt outlet.

c. ... current in a 1000-watt microwave when plugged into a 120-volt outlet.

Check for Understanding

Your 60-watt light bulb is plugged into a 110-volt household outlet and left on for 3 hours. The utility company charges you $0.11 per kiloWatt-hour. Explain how you can calculate the cost of such a mistake.

Check for Understanding

Alfredo deDarke often leaves lights on for no good reason. The deDarke family pays 10¢/kilowatt-hour for their electrical energy. Express your understanding of the relationship between power, electrical energy, time, and costs by filling in the table below.

Power(Watts)

Time(hrs)

Energy Used(kW-hr)

Cost(cents)

Cost($)

60W light bulb 1

60W light bulb 4

100W light bulb 10 kW-hr

Power

The equation for calculating the power delivered to the circuit or consumed by a load is derived to be

P = I•ΔV

Power

P = I2 • R

P = ΔV2 / R

Check for Understanding

Calculate the resistance and the current of a 7.5-Watt night light bulb plugged into a US household outlet (120 V).

Check for Understanding

Calculate the resistance and the current of a 1500-Watt electric hair dryer plugged into a US household outlet (120 V).

Check for Understanding

The box on a table saw indicates that the amperage at startup is 15 Amps. Determine the resistance and the power of the motor during this time.