Energy and Power

1. What is energy?

Energy is the capacity to do work.

• According to this definition, light is energy because, for example, it can make the molecules in your black sweater " work". It makes them vibrate faster as they absorb light, and you feel the effect of all that work in the form of heat.
• Rushing water also has energy: it can be made to turn turbines. Moving turbines have energy because they can be made to spin the magnets of a generator. That work, in turn (no pun intended), gets electrons to move, and the electricity is delivered to your home. Electricity is also a form of energy because it can make motors spin, heat tungsten filaments in ordinary light bulbs or excite mercury vapour in fluorescent lights.

The amount of energy it takes to raise the temperature of 1 gram of water by just 1^o C is defined as a calorie. But in SI ( systeme internationale) the accepted unit of energy is the joule (J).

1 calorie = 4.19 J.

2. What is power?

Power is the amount of energy used or delivered in a given unit of time. It is measured in watts(W).

1 W = 1 J/s

If an appliance consumes 1000 J in 10s, its power rating becomes 1000/10 = 100 W.

A small microwave oven has a low power rating because it cannot convert electrical energy into heat as fast as a bigger microwave oven.

A digital watch consumes very little energy per second. Its battery lasts a long time. Its power rating is very low: only 1.2 X 10^-5 W. It would be unrealistic to cook an egg with the heat released by a digital watch. If you could conserve the heat released by a watch, it would still take 323 days just to get a cup of 20 C water to boil.

A clothes dryer on the other hand releases 6480 J of heat every second to dry your LaurenHill uniform in time for you to get to school in the morning. So with a power rating of 6480 W, it is 540 million times more powerful than your digital watch and consumes that many time more energy.

3. The Connection between Power, Voltage, Current and Resistance.

Recall that 1V = 1 J/C; in other words if a resistor's voltage drop is 1 V, then 1 J of energy has been lost for every coulomb of charge that goes through that resistor.

1 A = 1 C/s; in other words if a 1 A current goes through a light bulb, then it means that 1 C of charge is going by every second.

Now let's multiply the units of V by those of I.

(J/C) (C/s) = J/s. Wow! Those are the units for power! So that means:

P = VI

Since E = P*t, where if P is power in watts( J/S = W) and time is measured in seconds, then E will be in joules (J).

Substituting,

E = V I t

Example 1

A heating coil has a resistance of 10 W. It is designed to operate at 120 V.

a. What current flows through the coil?

Answer: V = IR

120 = I (10)

I = 12 A.

b. What energy, in joules, is supplied by the heater if it's on for 10 seconds?

E = VIt

= 120(12)(10)

= 14 400 kJ OR 14.4 kJ

c. What's the power rating of the heater?

P = VI

= 120(12) = 1440 W or 1.44 kW

d. What if the 1440 W heater was on for 200 hours, how much energy was consumed?

E = P t = 1440 (200)(3600) = 1 036 800 000 J = 1 036 800 kJ. Notice we had to convert hours to seconds (X 3600) in order to get an answer in joules.

Now the answer is still very big! We can therefore introduce a more practical unit for energy, the kWh ( kilowatt-hour).

In other words use E = P t again but with kW and hours instead of W and seconds.

E = Pt = 1.44 kW(200 h) = 288 kWh.

(The number is smaller because every 1 kWh equals 3600 kJ)