Measuring Current

Go with the Flow

Introduction

Ammeters measure the current flowing through them.

Material

Current meter

Solar cell

1.5 volt cell, e.g. a AA battery

10 ohm resistor

To Do and Notice

Measure the current produced by the solar cell.

For a meter with a dial scale:

Set the meter on the 250 mA DCA, direct current amperage, scale.

Read the amperage off the arc of numbers which ends on the right with 25.
Since this 25 corresponds to 250 mA remember to add a zero to the number you read.

For a digital scale just read the number off the display.

Current is defined to be positive when positive charges flow into the positive, or red, lead of the meter.

Observe how the current changes when you change the amount of light falling on the solar cell.

Measure the current produced when a 1.5 volt cell is attached to a 10 ohm resistor. Attach one end of the resistor (it doesn't matter which) to the positive side of the battery and the other end of the resistor to the negative end. You have just created a circuit, the current flows out of the battery and through the resistor. The mythical positive current flows out of the positive terminal of the battery and through the resistor to the negative terminal of the battery. To measure the current you will have to break the circuit at some point and attach the meter so that the current flows through the meter. For example open the circuit at the positive end of the battery, touch the red lead of the meter to the positive terminal of the battery and the black lead to the resistor. You should then be measuring the current flowing through the circuit.

If you insert the meter at the negative end of the battery it will read the same current flow. In fact we say that the electrical current is the same throughout the circuit.

The current is the same anywhere in a single loop circuit.

The current is the same in the wire, in the resistor, in the meter, and in the battery too. It has the same magnitude and flows around the loop in the same direction.

What's Going On?

Electric current

We know that electric current flows through a wire because when a compass needle is held above a wire so that the needle is parallel to the wire, when current flows through a wire it will deflect the compass needle.

Electric current also causes changes, when an electrical current flows, it can be made to deposit metal such as copper onto a piece of metal in a solution. In some solutions, each charge which flows through the circuit deposits one atom of copper on the metal. By measuring the number of atoms deposited the number of charges flowing through the wire can be found.

Until 1916 it was not known which charges, positive or negative, flowed in an electrical current in a metal. In that year Tolman and Stewart found that in copper the electrons, which are negative charges, flowed. Essentially if you slam a block of copper into a wall, the mobile electrons pile up on the wall side creating a negative voltage across the block where the rear of the block is defined to be zero volts. (Just as people who do not wear seat belts pile up against the front of a car which stops rapidly during an impact with a wall.) If the positive charges carried electric current in copper then they would have piled up at the wall creating a positive voltage.

The electric current is carried in different ways in different materials. The small electric current which flows in ice is carried almost entirely by the flow of positive ions (hydrogen ions). In salt water the current is carried by positive ions, Na+, flowing one way and negative ions, Cl-, flowing the other directions at the same time. When silver is deposited on an electrode in a solution, the current may be carried almost entirely by flowing positive silver ions in solution.

In any given material it is hard to determine exactly which charges are mobile. However, the net effect is known, and the net effect is indistinguishable in most cases from the convention that the entire electric current is carried by the flow of positive charges. (It isn't , but the model serves to explain what happens most of the time in electrical circuits. It is a lie but an effective one.

A flow of charge in a wire equivalent to the flow of 6.2 x 1018 positive charges per second is a flow of one coulomb per second and is called a 1 ampere current.

This mythical positive flow naturally flows from high voltages to low voltages, just like balls on a rough surface naturally flow from the high gravitational potential energy tops of hill to the lower valleys.

The people who manufactured the meter used this convention. When a flow of positive current goes into the positive terminal of the meter, red lead, then the meter reads positive current.

Solar Cells Convert light energy into electrical energy. In general, more light shining on the solar cells will produce more electrical current. Photons, particles of light, give energy to electrons in the solar cell, i.e. the electrons are boosted to a higher voltage. This voltage can drive a current around an electrical circuit.

Does adding a meter effect the current?

A perfect current meter would have no resistance. However real current meters do have resistance. So adding a current meter into a circuit to measure the current, adds resistance to the circuit and changes the current. To estimate the effect on the current measurement add a second current meter in series with the first. The percentage change in the reading of the first current meter when the second one is added is an estimate for the error introduced into your measurement by adding the current meter.

 Scientific Explorations with Paul Doherty © 2000 2 August 2000