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Electrical Current
Electrical current consists of moving electrons
Conductors such as copper are filled with movable charge somewhat like a cloud of electrons. A net flow of these charges within the conductor constitutes electrical current flow. An external influence is required to cause the electrons to move through the conductor. This force is usually an applied electric field. When the electric field pushes against the electron cloud, the entire cloud, acting as one, moves. In this way electrons are caused to flow at the opposite end of the electron cloud.
pushing in
copper nuclei electron cloud
conductor or wire
electric field
electron or current flow
Figure 1: Current Flow in a Copper Conductor
Here is another way to think about current flow; the pipe and ball analogy. A conductor is like a pipe full of electrons. If an electron is pushed into one end of the pipe, another electron must fall out at the other end. Electrons flowing through a wire are like balls traveling through a pipe, not like an empty pipe which electrons fall through.
electron or current flow
electrical field pushing in
Figure 2: Last Electron In 6 = First One Out
Measuring current
Both water molecules and electrons are small. We don’t measure water flow in molecules per minute but by gallons (many, many molecules) per minute. We measure electron flow in much the same way.
Electron flow is measured in Coulombssec. One Coulomb (C) is equal to 6. 25 x 1018 electrons. The term that refers to one Coulomb per second of current flow is the Ampere (A). It is informally referred to as an ”Amp”. Thus,
1 A = 1
C
sec
To restate, the rate of electron movement that would cause one Coulomb of electrons to move across a plane surface bisecting a wire in one second is called 1 Ampere of electron flow.
Specifying electrical current flow in a conductor
To accurately specify a current flowing in a conductor, three bits of information must be known.
- The magnitude of the current flow
- The reference direction of the current flow
- The conductor the current is flowing through
All the above items are commonly conveyed by placing a arrow adjacent to the conductor of interest with the magnitude of the current given by a numerical value and an arrow indicating the reference direction to which the numerical value refers. See the example in figure 3.
3.5A
current reference direction
current magnitude
wire
3.5A
Relative to this arrow, a current of 3.5A flows towards the right −3.5A wire
Relative to this arrow, a current of −3.5A flows towards the left
3.5A 3.5A −3.5A −3.5A
current flows right current flows left current flows left current flows right To change the sign of the magnitude, flip the arrow’s direction To reverse the arrow’s direction, multiply the magnitude by −1.
Figure 3: Specifying Current: Arrow, Magnitude, and Conductor
The direction of the arrow alone does not necessarily indicate the actual direction of current flow. The arrow indicates the reference direction. When coupled with the sign of the current magnitude, the actual direction of current flow may be determined. See the examples in figure 4.
3.5A
current reference direction
current magnitude
wire
3.5A
Relative to this arrow, a current of 3.5A flows towards the right −3.5A wire
Relative to this arrow, a current of −3.5A flows towards the left
3.5A 3.5A −3.5A −3.5A
current flows right current flows left current flows left current flows right To change the sign of the magnitude, flip the arrow’s direction To reverse the arrow’s direction, multiply the magnitude by −1.
Figure 4: Specifying Current is Relative to the Current Arrow
Figure 6: Inside back of DMM showing current sense wire and fuse
Figure 7 shows how we would open the circuit up to measure the current flowing by inserting a DMM into the loop.
10ADC com
Digital Multimeter
Original Circuit (^) Measuring current in the original circuit
250.0 mA
Figure 7: Opening a circuit to make a current measurement
Note that the DMM has terminals marked as (+) and (−) or COM on it. These markings indicate the reference current direction for the meter. The meter expects positive current to flow into the positive terminal marked with the (+) for there to be a positive reading. In other words, if cur- rent actually flows into the positive terminal, the reading on the display will be positive. If the current is flowing out of the positive terminal, the current reading will be negative. The meters (+) and (−) signs make more sense when measuring voltage rather than current. As such, you
could imagine that an invisible arrow is on the meter terminals to indicate the expected reference direction as shown in figure 8.
Figure 8: Face of DMM showing imaginary reference current arrows.
For measuring larger currents (greater than about 200mA), the very low resistance wire is used inside the DMM. This necessitates the use of a second jack on the DMM. It is just used for high current measurements. This is seen in figure 8 as the ”10ADC” (10 Amps Direct Current) jack on the DMM. The ”COM” terminal is also known as the negative (−) terminal. The one marked with ΩmA is the positive or (+) terminal.
When magnitude and direction of current flowing in a circuit does not vary with time, the current is referred to as direct current (DC). If the current continuously varies amplitude and direction, it is referred to as alternating current (AC). See figure 9.
Table 1: Commonly Used Engineering Unit Prefixes
- tera T 1012 1,000,000,000, Prefix Abbreviation Value Multiplication Factor
- giga G 109 1,000,000,
- mega M 106 1,000,
- kilo k 103 1,
- none
- milli m 10 −^3.
- micro μ 10 −^6.
- nano n 10 −^9.
- pico p 10 −^12.
- femto f 10 −^15.
- atto a 10 −^18.
