Measuring Electrical Energy

Ch. 11

Energy: the ability to do work

Electrical Energy: energy transferred to an electrical load by moving electric charges. (symbol is E)

Joule (J): unit for measuring energy.

Because a joule is so small, energy is also measured in much larger units such as the watt hour and the kilowatt hour.

Calculation

Electrical Energy = Voltage x Electrical Current x Time Interval

E = V x I (amps) x t (sec)

E = V x I x tE = V x I x t

Question to try

Calculate the energy released from a battery in a flashlight bulb that was switched on for 4.5 hours, in which the voltage drop was 6V and the current flowing through the bulb was 0.35A.

Question to try

Calculate the energy released from a battery in a flashlight bulb that was switched on for 4.5 hours, in which

the voltage drop was 6V and the current flowing through the bulb was 0.35A.

Convert hours to seconds

1 hour has 60 minutes1 minute has 60 seconds

60 x 60 = 3600

So 1 hour has 3600 seconds (1 x 3600). 2 hours would have 7200 sec (2 x 3600).

E = ?JV = 6VI = 0.35AT = 4.5

hoursx 60 270

x 6016200 sec

E = V x I x tE = 6V x 0.35A x 16200secE = 34020 joules

Rate at which Energy is Used!

• Electrical power is used by us every day – washer, mp3, toaster, discman, etc…

• When you think about power – it means, how rapidly an appliance is using up electrical energy.

Ex. A 300w bulb uses up 3 times as much electrical energy as a 100w bulb.

• The symbol for electrical power is P and the unit for power is the watt (W).

Power = Voltage Drop x Current

P = V x IP = V x I

P = V x I

V = P I

I = P V

P

V I

Calculate the power of a vacuum cleaner if the operating voltage is

120v, and the current flowing through it when it is used is 7.90A.

P = ?WV = 120vI = 7.9A

P = V x IP = 120V x 7.9AP = 948 W

Calculate the voltage of a computer that has 600W of power and 1.9A

flowing into the monitor?

P = 600WV = ?I = 1.9A

The voltage would be 316V.

V = P I

V = 600W

1.9A

V = 316V

Joules to Watt hours

E = ?JV = 6VI = 0.35AT = 16200

sec

*16200sec ÷ 3600 =

4.5 hours

E = V x I x tE = 6V x 0.35A x 16200

secE = 34020 joules

The energy released would be 34020 joules.

or

E = V x I x tE = 6V x 0.35A x 4.5 hours E = 9.45 Whours

The energy released would be 9.45 watt hours.

P. 336 – 1, 2, 3a, 3b

1. Electrical energy is the energy transferred to an electrical load by moving electrons.

SI Unit – Joulesymbol – J

2. Voltage drop (potential difference), electric current, time interval

3a

E = ?JV = 6VI = 0.30AT = 45 sec

E = V x I x tE = 6V x 0.30A x 45 secE = 81 joules

The energy released would be 81 joules.

3b

E = ?JV = 9VI = 0.50AT = 2.5

hours

E = V x I x tE = 9V x 0.50A x 2.5 hoursE = 11.25 watt hoursThe energy released would be

11.25 Whours.or

E = V x I x tE = 9V x 0.50A x 2.5 (3600) E = 40500 joules

The energy released would be 40500 joules.