Circuit Symbols, Laws and Measurement

Circuit symbols, laws and measurement

Before you can design or fix an electronic circuit, you have to read it, and that means knowing the standard symbols and the laws that govern how a circuit behaves. This module covers three things. First, the standard circuit symbols, the shorthand every engineer uses to draw a cell, a resistor, a switch, a lamp and more. Second, the basic laws: how current and voltage behave in series and parallel circuits, and the key relationships linking voltage, current, resistance and power. Third, measurement: how to use an ammeter, a voltmeter and a multimeter to check what a circuit is actually doing. Get comfortable with the symbols, the sums and the meters, and the rest of electronics has firm ground to stand on.

Words for circuits

Learn these terms before the symbols are matched up. They name the ideas used in circuits and their measurement.

Match each symbol to its component

Series against parallel circuits

Components can be joined in two main ways, and the current and voltage behave differently in each.

Work out the missing value

A component has a voltage of 12 volts across it and a resistance of 4 ohms. Using the idea that voltage is current times resistance, what is the current through it?

• 3 amperes
• 48 amperes
• One third of an ampere
• 12 amperes

Ohm law and power

Two relationships do most of the work in this topic. The first is that the voltage across a component equals the current through it times its resistance, so if you know any two of the three you can find the third. The second is that the power used, measured in watts, equals the voltage times the current. From these you can also show that the power equals the current squared times the resistance, and that the energy used equals the power times the time. Learn to rearrange them and to keep the units straight, in volts, amperes, ohms, watts and joules, and most circuit calculations become routine rather than frightening.

Pick the true circuit facts

Select every statement about circuits that is true.

• Voltage equals current times resistance
• Power equals voltage times current
• Current is a flow of electric charge
• Current flows with no power source
• Resistance is measured in volts

Order how to measure a resistance

Put the steps of finding a resistance by experiment into a sensible order.

• Build the circuit with the component
• Measure the voltage across the component
• Measure the current through the component
• Divide the voltage by the current
• Record the answer as the resistance

Complete the circuit facts

To measure the current you connect an _____ in series with the component. To measure the voltage across a component you connect a _____ in parallel with it. In a _____ circuit there is only one path and the current is the same everywhere. In a _____ circuit there are separate branches and each gets the full voltage.

Tap the two measuring instruments

Read these four items of electronics kit. Tap the TWO that measure electrical quantities.

• an ammeter
• a voltmeter
• a battery
• a breadboard

A crowd through a turnstile

Picture people leaving a stadium through a turnstile. The number of people passing each second is like the current, the flow around the circuit. How hard the crowd behind is pushing is like the voltage, the force that drives the flow. And a stiff, narrow turnstile that slows everyone down is like the resistance, which holds the flow back. Widen the turnstile, or push harder, and more people get through each second; the same is true of charge in a wire. Picture the crowd, the push and the squeeze, and a circuit stops being abstract and starts to make plain sense.

Work out the voltage

A current of three amperes flows through a resistor of four ohms. Using the idea that voltage is current times resistance, what is the voltage in volts? Give the number only.

Apply the circuit laws

Read each situation and choose the best answer.

• You want to measure the current flowing through a lamp. How do you connect the meter?
• You want the reading of the voltage across a resistor. How do you connect the meter?
• A component has 6 volts across it and a resistance of 2 ohms. What is the current?

Build a circuit-laws point

Choose the word for each gap to complete one point about circuits.

Explain circuit symbols, laws and measurement

A friend is starting electronics and asks how to read a circuit and work out what it does. Using what you have learned, explain the main ideas.

• Explain what circuit symbols are and give some examples
• Explain how current and voltage behave in series and parallel circuits
• Explain the relationship between voltage, current and resistance
• Explain how to measure current and voltage
• Explain what the power of a circuit tells you