Voltage indicates the power level of a point. Voltage is measured in volts. If we continue the river comparison, a point at the top of a hill would be at a high voltage level and a point at the bottom of a hill would be at a low voltage level. Then, just as water flows from a high point to a low point, current flows from a point of high voltage to a point of low voltage. If one point is at 5 volts and another point is at 0 volts then when a wire is connected between them, current will flow from the point at 5 volts to the point at 0 volts.
A measurement of voltage is much like a measurement of height. It gives you the difference in voltage between those two points. If point A is at 10 volts and point B is at 2 volts then the voltage measured between A and B is 8 volts (10 -2). This is similar to measuring height. We measure the height of hills the same way. We say the sea level is at zero feet and then compare other points to that level. On top of Mary’s Peak you are 4000 ft high (compared to sea level). In the same way we call the lowest voltage in a circuit zero volts and give it the name ground. Then all other points in the circuit are compared to that ground point. Rivers always flow towards sea level and currents always flow towards ground.
A battery is similar to a dam. On one side is a lot of stored up energy. When a path is formed from that side to the other side then current flows. If there is no path then current does not flow and the energy just stays there waiting for a path to form to the other side. The path can be a big path with lots of current flowing or a small path with just a little bit of current flowing. With a dam, a little bit of water flow could go on for a long time, but flow through a big path that lets all the water go at once would only last a short while. A battery is the same. If there is big path from the high voltage side to the low voltage side then the battery will not last long.
There are two special cases that we give names. One is when the current is zero (open circuit) and the other is when the voltage is zero (short circuit).
