Watts are Volts times Amps when the you are talking about direct currents, and a few a.c. devices. Watts equals Volt•Amps for battery powered devices, light bulbs, heaters and old electronics -pre 1996. Technology and alternating circuits have changed the way power is measured. There is something called "REAL" power and "apparent" power. Apparent power is used by electrical engineers who have a much deeper understanding of what is happening electrically when electrical power is supplied or consumed. Suffice to say when talking about modern electronics, Watts does not equal Volt•Amps. Watts measures the real power. But for most electronic devices built after 1996, (0.60)•Apparent power = Real Power. In other words (0.60)(Volt•Amps) = Watts.

The device above is called a UPS, Uninterrupted Power Supply. It is battery that turns on when the power goes out. When connecting the UPS to a computer you need to look at the VA. If you bought a 500 VA uninterrupted power supply then you can hook up (0.60)550VA = 330 Watts worth of modern electronics.

For everything we do in the introductory physics class, a Watt equals a Volt•Amp ...always. (However, it's good to know how world outside of this course deals with Watts. That's we this topic is brought up.)

Recall that

Therefore by rearranging the equation

Charge = Current•Time

In terms of units this is

Coulombs = A•s

The number of Coulombs stored in a battery is equal to the Amps that can be drawn out of the battery times the length of time it takes to draw the Amps out. This gives the number of Coulomb's of charge stored in the battery. Battery manufacturers have given consumers a similar way to measure the amount of charge in a battery. On the package it lists the amount of charge in units called A•h or mAh. Recall that the "m" symbol is used to indicate milli units, x 10^-3. These units are pronounced AmpHours or milliAmpHours. to the right is an image of a package of rechargable batteries. The storage capacity of each battery is prominantly displayed on the package. It is 1000 mAh which converts to 1 Ah.

Note: Adding batteries to a device by placing them in a row, end-to-end, does not increase battery life.

Adding batteries side does increase battery life. Nearly all appliances do not add batteries like this. Even though they look side by side, they are still connected end to end.

All of the battery charging questions will use I=Q/t. An easier way to solve these battery charging problems is by using dimensional analysis. This could have been done for the example problem by taking AmpHours and dividing it by Amps.

Refer to the notes for an example problem (Click here)

The stuff that plugs into your walls does not need a specific voltage. They need a difference in voltages. Half of your house is wired with +120 V and the other half of your house is wired with -120 V. They are all compared to the neutral line of 0 Volts. This means that each plug has potential difference of 120 V between the small and large slots. Potential difference is the difference in voltages between two locations.(This was discussed earlier too.)

The electric company charges home owners for the work , electrical work, they do to supply your energy needs. Recall that work is he change in energy. Instead of charging your a rate like

and reporting the energy you use in 1/2 million units, the electric company charges you in units of

The actual rate of $0.10 will vary depending on where you live and the demand on the electric company versus the available supply. In a way the kW•h unit is a better unit. It allows you to do some quick dimensional analysis calculations. These dimensional analysis calculations are different than conversion type calculations. instead of keeping units and just changing them, you will be eliminating units. See the example below.