Electric charge and current as the rate of flow of charge, the charge equation, voltage (potential difference) as energy per unit charge, standard circuit symbols, and the test equipment used to measure electrical quantities (multimeter, oscilloscope, logic probe).

What this topic is asking

WJEC Eduqas wants you to define electric charge and current (current as the rate of flow of charge), use the charge equation, define voltage (potential difference) as energy transferred per unit charge, recognise and draw standard circuit symbols, and know the test equipment used to measure electrical quantities. These are the basic ideas every other Component 1 topic is built on.

Charge and current

In a metal conductor, charge is carried by free electrons that drift towards the positive terminal of the supply. Conventional current is taken to flow from positive to negative (the opposite direction to the electrons), and this is the direction shown on circuit diagrams. A larger current means more charge passes a point each second. Current is measured with an ammeter connected in series with the component, so the same current flows through both.

Voltage (potential difference)

A supply provides the potential difference that drives charge around a circuit. A higher voltage gives each coulomb more energy to deliver. Across a component, the potential difference tells you how much energy each coulomb gives up there. Voltage is measured with a voltmeter connected in parallel across the component, because both then experience the same potential difference.

Standard circuit symbols

Circuit diagrams are a shared language: the same symbol means the same component to every electronics engineer. The cell symbol is a long thin line (positive) and a short thick line (negative); a resistor is a plain rectangle; an LDR or thermistor is a resistor rectangle inside a circle with arrows or a temperature marker; an ammeter is a circle with an "A" and a voltmeter a circle with a "V". The specification prints the full symbol list, and questions expect you to read a circuit and to add a named component in the right place.

Test equipment

Choosing the right instrument is an examinable skill. Use an ammeter or multimeter (current range), in series to measure current, and a voltmeter or multimeter (voltage range), in parallel to measure voltage. Use an oscilloscope when you need to see the shape of a changing voltage, such as the output of a timing circuit, and read its frequency or amplitude. Use a logic probe for a quick check of the logic state at a point in a digital circuit. A multimeter on its resistance range checks a component or finds a break in a circuit (but only with the power off).

Try this

Q1. A charge of $90\,\text{C}$ flows through a resistor in $30\,\text{s}$. Calculate the current. [2 marks]

• Cue. Rearrange $Q = It$: $I = \dfrac{Q}{t} = \dfrac{90}{30} = 3.0\,\text{A}$.

Q2. State which instrument you would use to measure the period of a square wave, and why. [2 marks]

• Cue. An oscilloscope, because it displays how the voltage changes with time so you can read the period from the screen.