Current

Current

Introduction

As we noted in our voltage project, the fundamental unit in any electrical circuit is electric charge. We also noted that, as engineers, we are primarily interested in charge movement rather than the charges themselves. (Transferring energy to perform some useful work is our primary goal —electrical energy transfer is accomplished by moving charges around.)

In our voltage section, we noted that an important parameter associated with energy transfer is voltage, which is an energy difference that a charge experiences when it moves from one point in a circuit to another. Of equal importance is the rate at which charge passes a particular point; current quantifies this rate. Since voltages and currents in a circuit are directly related to one another, it is recommended that you read the voltage section before continuing with this section. A link to the voltage section is provided to the right.

In electrical circuits, charge is always conserved—our circuit can only move charges around; they are never created or destroyed. This means that any charge entering a circuit must be balanced by the same amount of charge leaving the circuit, so the rate at which charge enters the circuit is the same as the rate at which charge leaves a circuit. Therefore, the current entering a circuit or circuit element must be the same as the current leaving the circuit element. This is illustrated in Fig. 1; if a current I enters terminal A of our circuit, the same amount of current must leave terminal B.

Figure 1. Charge conservation implies that currents entering and leaving a circuit are the same.

Units of current are Amperes, abbreviated A. A current of two amperes is thus typically denoted as 2A. An ampere is a rather large amount of current. Current is commonly measured in thousandths of amperes, or milliamperes (abbreviated mA). Thus, a 0.08A current is the same as a 80mA current, and a 0.3A current is the same as a 300mA current. These units are used as appropriate when the Analog Discovery displays currents.

Current Direction

Current has both a numerical value and a direction, as indicated in Fig. 1. The current direction is indicated by an arrow which shows the direction corresponding to a positive value of current. The current value (I, in Fig. 1) is shown next to the arrow. Please note that the arrow is not necessarily in the same direction as current flow; if the current value is negative, this means the current is opposite the direction shown.

Very Important Point

Switching the direction of the arrow simply changes the sign on the current value. The two currents shown in Fig. 2 are identical.

Figure 2. Reversing arrow direction changes the sign of the current.

Current Measurement

Currents are usually measured with an ammeter. An ammeter has two terminals, or leads, which are connected to the circuit at the point where current is to be measured. The current through the ammeter results in a voltage difference in the ammeter. This voltage difference is used to infer the current.

Since the current measured by an ammeter has to pass through the ammeter, some re-wiring of a circuit may be required when measuring current. For example, let's say that we want to measure the current provided by circuit 1 to circuit 2 in the system shown in Fig. 3(a). To measure the current, we need it to pass through the ammeter, so we disconnect the two circuits at terminal A and connect the ammeter as shown in Fig. 3(b).

Figure 3. Current measurement.

Like voltmeters, ammeters are implemented as one function of a digital multimeter, or DMM. When using a DMM to measure constant currents, the appropriate setting is indicated by the letter “A” with a bar over it, and the terminals are plugged into ports labeled as “A” and “COM” (for common). Positive current enters the “A” terminal and leaves the “COM” terminal. If the current is actually going in the other direction (from the “COM” terminal to the “A” terminal) the number displayed by the DMM will be negative. It is customary to use a red lead for the “A” terminal and a black lead for the “COM” terminal.

Current and Voltage Relationship

Current and voltage are closely related in any electrical circuit. Remember that we are interested in moving charges around to perform some useful work. Voltage provides an energy level difference between two points which causes charges to move around. The resulting rate of charge motion is the current.

So now our goal has become a bit more specific than to just “move charges around”. We want to create our circuit so that we provide the voltage differences necessary to create the currents required to perform the desired task. We do this by appropriate choice of the components in an electrical circuit. Typical circuit components such as batteries, resistors, capacitors, and transistors all implement some relationship between voltage and current at their terminals. When we design a circuit, we are choosing and interconnecting components which will provide the voltage-current relationships we need to accomplish our task.

Important Points

  • Current is a rate of charge motion at a point in a circuit. Units of current are amperes; abbreviated A. Amperes are often referred to as “amps”.
  • For any circuit element, the amount of current entering the element must equal the current leaving the element.
  • To specify a current, you have to include not only a magnitude (the number of amperes) but also a direction. The direction of a current is indicated by an arrow on a circuit diagram. The arrow does not necessarily correspond to the actual current direction, but it shows the direction associated with a positive current.
  • The magnitude of a current can be either positive or negative. If the current magnitude is positive, the current is in the same direction as the arrow defining the current. If the current magnitude is negative, the current is in the opposite direction as the arrow defining the current. Switching the direction of the arrow simply changes the sign on the current and vice-versa.
  • Current is measured using an ammeter. In order to measure a current in a circuit, the circuit must be broken apart at the point where the measurement is to be made and the ammeter placed in-line with that section of the circuit.
  • Current and voltage depend on one another. The components in a circuit set the relation between current and voltage. When we design circuits, our goal is to choose and interconnect circuit components to set voltages and direct currents in the way we want.

Test Your Knowledge!

  1. What is the current I in the circuits below?
  2. For the circuit elements below, the current value and direction are provided. State which terminal the current is entering, and what the current value is.

Selected Answers

    1. I = 3A. (The current leaving node b must equal the current entering node a.)
    2. I = -2A. (The current entering node b is 2A. Therefore, we must have 2A leaving node a. However, the sign convention associated with I is that positive current is entering node a, so the current I must be a negative 2A)
    3. I = -3A. (The current leaving node b is equal to the current entering node a.)

    1. 3A is entering node a.
    2. 2A is entering node b. (Since 2A is leaving node a.)
    3. 4A is entering node b. (Since negative 4A is leaving node b.)


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