Current is the flow of electrically charged particles, defined by [mathjax]I=\dfrac{Q}{t}[/mathjax], measured in the SI units amperes ([mathjax]A[/mathjax]), or [mathjax]C/s[/mathjax]. Note that although charge flows by means of electrons in metals (as a conductor of electricity, the most typical scenario), conventional current is defined as the flow of positive charge, due to Ben Franklin’s incorrect conjecture that electrons were positive. Current flow through a wire is analogous to fluid flow (rate) through a pipe (discussed ), in various respects, including that just like fluid which has random translational motion and uniform translational motion, current also has random motion and drift velocity respectively. Drift velocity is the net flow of charge.Batteries are represented by a repeated set of two parallel lines of unequal length. The longer side is the positive terminal, and the shorter side is the negative terminal.

Elements either oppose electric current, known as resistors; or do not oppose electric current, known as conductors. Semiconductors lie between these two extremes. Resistors are represented by a single jagged line.

Capacitoris an element which has the ability to store an electric charge (whilst maintaining a low voltage drop) for delivery when required, or alternatively, [mathjax]C=\dfrac{Q}{V}[/mathjax]. Capacitors are represented by two parallel lines of equal length.

When a node is reached, current in each respective node can be determined using Kirchhoff’s laws. Node is the junction of wires coming together, or splitting away. Kirchhoff’s 1st law is that the currents flowing into a node is equal to the currents flowing out of a node. The equivalent hydraulic analogy, is that the rate of fluid flow into a pipe, is equal to the rate of fluid flow out of a pipe. Kirchoff’s 2nd law is that the sum of potential differences [or voltage drop] around any closed network is 0. The equivalent gravitational analogy, is that if a mass is moved up and down but eventually returns to its original height, the sum of the change in gravitational potential energy is 0.

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Maps to RK9.B

What are circuits?

3 Alternating currents

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Although the circuits assume direct current (DC), which is where electricity flows only in one direction, this is not always the case. Alternating current (AC) is where electrical charge periodically reverses direction. The usual waveform of AC is a sine wave. Note that if current is sinusoidal, that the average current is 0. Because this doesn’t provide sufficient information on current, root mean square (RMS) current is used instead. There is also a RMS voltage. Root mean square refers to the square root of the mean of the squares of the values. Note that because the mean is found of the squares of the values (rather than the mean of the values per se), the RMS is slightly higher than the average of the positive of the values. For a sinusoidal current, [mathjax]I_{rms}=\dfrac{I_{peak}}{\sqrt{2}}[/mathjax]; and for a sinusoidal voltage, [mathjax]V_{rms}=\dfrac{V_{peak}}{\sqrt{2}}[/mathjax]. For example, the typical voltage is 230V, meaning the related peak voltage is [mathjax]V_{peak}=230\sqrt{2}\approx 325[/mathjax].

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Maps to RK9.C

What is alternating current?

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