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Flow Rate: How do you Measure the Flow of Electricity?

based on 7 ratings
Author: Janice VanCleave

Problem

How do you measure the flow of electricity?

Materials

  • 7-oz (210-ml) paper cup
  • masking tape
  • 7 oz (210 ml) table salt
  • small bowl
  • stopwatch
  • helper

Procedure

NOTE: This experiment works best on a dry day because salt tends to stick together when the air is humid.

  1. Use the pencil to punch a hole in the center of the bottom of the paper cup. The hole should be equal to the circumference (the distance around a circle) of the pencil.
  2. Place a piece of tape over the hole.
  3. Fill the cup with salt.
  4. Hold the cup of salt about 6 inches (15 cm) above the bowl, and ask your helper to start the stopwatch the second you remove the tape from over the hole in the cup.
  5. Tell your helper to stop the stopwatch as soon as the salt stops pouring out the hole.
  6. Calculate the flow rate of the salt by using the following equation:

Results

The time for the salt to pour from the cup can vary depending on the hole size made by the pencil. See the example for sample flow rate calculations:

Flow Rate

Flow Rate Example

Why?

The flow of salt passing through the hole can be described as the quantity of salt that passes a given point divided by the unit of time. In this experiment, 7 ounces (210 ml) of salt pass out of the cup in 35 seconds, so the flow rate is calculated to be 0.2 ounces (6 ml) per second. The flow rate of the salt simulates, but is not an exact model of, an electric current (the flow rate of electric charge), which is the amount of electric charges passing a given point per second. Electric charges are measured in the metric unit of coulombs; thus, the flow rate of a current is measured in coulombs per second. The amount of current flowing through a conductor is usually expressed in amperes (amps, for short), which is equal to one coulomb per second.

Let's Explore

  1. Does the amount of salt affect the results? Repeat the experiment twice: first fill a cup one fourth full, and then fill a second cup halfway. Be very careful to make the hole size the same in each cup. Use information about electron movement to determine if the results can be compared to current flow in shorter and longer pieces of wire of the same circumference.
  2.  
    1. How would the size of the hole affect the results? Repeat the experiment twice: first make a small hole in the cup by using only the pointed end of the pencil, and then enlarge the hole in the cup by hollowing it out with the pencil.
    2. Repeat the previous procedure, recording the information in a data chart similar to the one shown here. Hole size can be recorded as small, medium, or large. Science Fair Hint: Display the data chart for the salt experiment along with a chart displaying samples of different gauges (measurement of a wire's circumference) of wire and an indication of how the wire's gauge affects the flow rate of an electric current through the wire.

Flow Rate

Show Time!

  1. Ask an electrician to show you the size of wire coming into a house compared to the size of wire going to electrical outlets and the size of wire going to a lamp. Ask for an explanation about the gauging sizes of wire. Construct a display showing different gauges and uses of wire.
  2. Cars on a highway are good models to represent the flow rate of electrons through different gauges of wire. With model cars, arrange a display showing cars lined up on one, two, three, or more laned highways. Use a legend showing that each car represents a single electron and that each size highway indicates a wire gauge. Use your information about wire gauges when comparing highways to wire gauges.

Check it Out!

The cup of salt contains many individual grains of salt. Instead of counting each grain and determining the flow rate in number of salt grains per unit of time, a larger unit of measurement (cups or ml) was used. Coulomb is a unit used to express a large quantity of electric charges. A current of 1 amp is equal to 1 coulomb of charges per second flowing past a given point. Use a physics book to find out how many electric charges pass a point per second in a 1-amp current.

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