Calculate the missing quantities for the circuit below.

Calculate the missing quantities for the circuit below.

V1 = 7.5 VR1 = 5.0 R2 = 3.0

V1 = 40 VR1 = 2.0 V2 = 40 V

• Electrical power is the rate at which electric potential energy is being transformed.

• One watt (W) of power is one joule (J) of electric potential energy transformed in one second 1W = 1J/s

• Electrical power (P) be calculated by multiplying voltage (V) and current (I)

• P=V∙I

• The amount of electrical energy (E) used by a device is its power consumption (P) multiplied by the length of time (t) the device is turned on

• E = Pt

• Since the joule (J) is a very small amount of electrical energy, the kilowatt-hour (kW∙h) is used for devices that consume larger amounts of energy

How much energy is used to dry someone’s hair using a hair dryer?

the same amount of energy needed to lift an average student 1.5km into the air.

9.2 The Power of ElectricityPg. 320

• Electrical power (P) is the rate of change in electrical energy

and the rate at which work is done or energy is transformed.

• The unit for measuring energy is the joule (J), named for the British scientist James Prescott Joule (1818-1889).

• One watt (W) of power is one joule (J) of energy transformed in one second (s)

• 1W = 1J/s

named in honour of Scottish inventor James Watt (1736 – 1819)

• James Watt invented the unit of power called “horse power” to compare the power of his improved steam engine to the power of a horse, to help boost sales.

• One unit of horsepower is 746 W

CALCULATING ELECTRICAL POWER p.323

If a 6.0V battery supplies a current of 2.0A, what is the power output of the battery?

A current of 5.0 A flows through a flashlight bulb when it is connected to 6.0 V. What is the power of this bulb?

Q43. A 600 W electric heater is connected to a 120 V source. What current flows through the heater?

Example

Power (P) = voltage (V) x current (I)

P = V x I

P = 6.0V x 2.0A

P = 12.0W

A 2.5 A current flows through a 100 W lamp. What is the voltage across the lamp?

What is the current through a 6.0 W light bulb when it is connected to a 1.5 V battery?

A 40 W light bulb is connected to a power supply and draws a current of 0.75 A. What is the voltage of the power supply?

(b) If the 40 W light bulb is replaced by a 100 W light bulb, how much current will flow through the 100 W bulb?

A power rating is a measurement of how much electrical energy an electrical device consumes for every second it is in use

1.0 W is the transfer of 1.0 J of energy every second (W=J/s)

For example, a 60 W light bulb uses 60 J of electrical energy each second (60 W = 60 J/s)

Calculating Energy ConsumptionPg. 324

You can calculate the amount of electrical energy (E) a device consumes by multiplying the power rating (P) with the amount of time (t) the device is used for.

Time (t) is always calculated in seconds (s)

P = E/t

E = Pt

Example

How much electrical energy is consumed by a 1400W hair dryer it if is used for 5.0min?

E = Pt

E = 1400W x 300s (5 x 60)

E= 420 000 J or 4.2 x 105 J

How much energy, in joules, is consumed by a 75 W light bulb if it is left on for 30 min?

How much energy, in joules, is consumed by the typical triple strand xmas tree lights that use 25 W per strand and are left on for 6 hours?

How much energy, in joules, is consumed by a 500 W stereo if it is left on for 4.0 h?  

A kilowatt-hour (kW∙h) is the product of power in kilowatts and time in hours

1000 W = 1kW

As you might notice, most electrical devices consume many joules of electrical energy. In terms of electrical energy, the joule is a very small amount

The power company that supplies electricity to your home keeps track of the electrical energy you consume

Your home has a meter that keeps track of how much electricity is used, and there is a meter-reader that comes to read it to determine how much energy has been consumed since the last bill

A Larger Unit for EnergyPg. 325

The Cost of ElectricityPg.325

A meter reader determines that a business has used 6525 kWh of energy in two months. If electricity costs 10¢ per kWh, calculate the bill.

A 500 W stereo is used in a household. Electricity costs 8¢ per kWh. How much does it cost to operate the stereo 3.0 h a day for 30 days?

The average home will use more than 50 strings of xmas lights each season and each strand uses 25 W which are left on for roughly 8hrs a day. If electricity costs 8¢ per kWh, how much will it cost to have those lights on for a month?