D. Physics: Principles and Applications of Electricity · Electricity: Charges, Circuits and Energy

DC Circuit Components, Symbols and SI Units

D2.3 A1.5About 65 minutes

Learning goals

I can identify the components of a DC circuit and explain the function of each.
I can read and draw circuit diagrams using standard symbols.
I can connect an ammeter in series and a voltmeter in parallel, and explain why.
I can name the electrical quantities, their symbols and their SI units.

Words to know

circuit diagram
A drawing of a circuit that uses standard symbols for each component, joined by straight lines for wires.

source
The part of a circuit that supplies electrical energy, such as a cell or battery.

load
A device that transforms electrical energy into another form, such as a lamp, resistor, motor or buzzer.

switch
A control device that opens (breaks) or closes (completes) a circuit.

fuse
A safety device with a thin wire that melts if the current gets too high, opening the circuit. A circuit breaker does the same job and can be reset.

ammeter
A meter that measures current. It is connected in series, so all of the current passes through it.

voltmeter
A meter that measures potential difference. It is connected in parallel, across the component being measured.

resistor
A component made to have a particular resistance, used to control the current in a circuit.

multimeter
One instrument that can be set to work as an ammeter, a voltmeter or an ohmmeter.

Remember from Grade 6

In Grade 6 you learned that a circuit has an energy source, conductors (wires), a load, and often a switch, and you drew circuit diagrams with symbols for cells, batteries, bulbs, switches and motors. In Grade 9 you add meters, resistors, fuses, and the SI units for every quantity you measure.

Why use symbols?

Imagine an electrician in Thunder Bay and an engineer in Halifax looking at the same plans. Drawings of real wires and bulbs would be messy and hard to read. Instead, everyone uses the same standard symbols and draws wires as straight lines with square corners. A circuit diagram shows how components are connected, not what they look like or where they sit.

Part of a circuit	Function	Examples
Source	Supplies electrical energy; creates the potential difference	Cell, battery, DC power supply, solar cell
Conductor	Provides a path for the charge	Copper wires, metal strips on a circuit board
Load	Transforms electrical energy into other forms	Lamp (light), resistor (thermal energy), motor (motion), buzzer (sound)
Control	Opens or closes the circuit	Switch, push button
Protection	Opens the circuit if the current is too high	Fuse, circuit breaker
Meters	Measure current or potential difference	Ammeter, voltmeter, multimeter

The components of a DC circuit and their jobs.

Standard circuit symbols. In a cell, the long thin line is the positive (+) terminal and the short thick line is the negative (−) terminal. A battery is two or more cells joined together. A dot means wires are joined; wires that cross without a dot are not connected. (Some books draw a resistor as a plain rectangle instead of a zigzag.)

Meters: where they go and why

Ammeter (measures current). Current is the charge flowing through a point, so the ammeter must be in the path: break the circuit and connect the ammeter in series. All of the current flows through it. An ammeter has a very low resistance so it does not slow the current.

Voltmeter (measures potential difference). Potential difference is the energy difference between two points, so the voltmeter connects across a component, in parallel, with one lead on each side. A voltmeter has a very high resistance so almost no current goes through it.

Connect the + terminal of each meter towards the + side of the source.

A battery, a closed switch, an ammeter in series and a lamp in one loop. The voltmeter is connected across the lamp, in parallel: the two dots show where its leads join the circuit on either side of the lamp.

Quantity	Symbol	SI unit	Unit symbol	Measured with
Electric charge	Q	coulomb	C	(calculated)
Electric current	I	ampere	A	ammeter
Potential difference	V	volt	V	voltmeter
Resistance	R	ohm	Ω	ohmmeter
Energy	E	joule	J	(calculated)
Power	P	watt	W	(calculated)
Time	t	second	s	stopwatch

Electrical quantities and their SI units. Careful: V is both the symbol for potential difference and the symbol for its unit, the volt (for example, V = 9 V).

The circuit for Example 1 below: a cell, then three mystery components.

Worked example

Example 1: Name the components

Identify the three components marked ? in this circuit, and give the job of each.

The rectangle with a line through it is a fuse: it protects the circuit by melting if the current is too high.
The two dots with a raised line form an open switch: it controls the circuit. While it is open, no current flows.
The circle with M is a motor: a load that transforms electrical energy into motion (kinetic energy).

Fuse (protection), open switch (control), motor (load). The source is a single cell.

Worked example

Example 2: Spot the mistake

Ryan connects a voltmeter in series with a lamp. The lamp goes out and the voltmeter reads almost the full battery voltage. Explain.

A voltmeter has a very high resistance.
In series, all the current must pass through it, so the current drops to almost zero and the lamp goes out.
Almost all of the battery's potential difference ends up across the voltmeter, so it reads nearly the battery voltage.

Voltmeters must go in parallel (across a component), never in series. (An ammeter connected in parallel is even worse: its very low resistance can let a large current flow and may damage the meter.)

Drawing circuit diagrams

Use a ruler; draw wires as straight lines with square corners.
Use the standard symbol for every part, and label values (for example, 9 V, 100 Ω).
Put a dot wherever wires join.
Show + and − on the source when it matters for meters.
Ammeter: in the loop. Voltmeter: across the part.

Hands-on activity

Flashlight and toy detective

You need: a battery-powered flashlight or toy with a battery compartment, paper, a ruler, a pencil.

Open only the battery compartment. Count the batteries and note their voltage (for example, 2 × 1.5 V). Note how they are lined up (end to end?).
Find the switch. Turn the device on and off and think about what the switch does to the path.
Draw a circuit diagram of the device with standard symbols: the cells (as a battery), the switch, and the load (lamp, LED or motor).
Add where you would put an ammeter to measure the current and a voltmeter to measure the potential difference across the load.

What to look for: a model flashlight diagram is one loop: battery (2 cells, + and − marked), switch, lamp. The ammeter goes anywhere in the loop; the voltmeter is drawn with its two leads on either side of the lamp.

Safety (grown-up note): Open only the battery compartment. Do not open sealed devices or rechargeable battery packs, and keep small batteries, especially coin cells, away from young children.

Just for fun

Why did the ammeter and the voltmeter never get along?

The ammeter only wanted a series relationship, but the voltmeter insisted on keeping things parallel.

Your turn

Name the four basic parts of every circuit and the job of each.

In the symbol for a cell, which line is the positive terminal?

How is an ammeter connected? Why?

How is a voltmeter connected? Why?

Give the symbol and SI unit for resistance.

What is the job of a fuse? How is a circuit breaker different?

In a diagram, two wires cross with no dot. Are they connected?

More practice: warm-up

What does the symbol of a circle with an M inside represent?

Which unit is J?

Which meter has a very high resistance?

Name two loads and the energy each produces.

What symbol is used for current?

More practice: on your own

Draw (or describe) a circuit diagram with a 6 V battery, an open switch, and two resistors in one loop, with an ammeter measuring the current.

A student puts an ammeter across (in parallel with) a lamp. What might happen and why?

Write the quantity, its symbol and its unit: (a) 3.0 A, (b) 1.5 V, (c) 220 Ω, (d) 45 J.

Why are circuit diagrams drawn with standard symbols rather than pictures?

A circuit has a battery, a lamp and a switch. The lamp is off, but the battery is new and the lamp works in another circuit. Give two possible reasons.

Challenge

Draw (or describe) a circuit in which one switch controls a motor and a lamp at the same time, with a fuse protecting the whole circuit and a voltmeter measuring the potential difference across the motor.

Explain why an ideal ammeter should have zero resistance and an ideal voltmeter should have infinite resistance.

In a home, why are circuit breakers placed at the start of each circuit (in the panel) rather than at the end, near the last outlet?

Check your understanding

Quiz

Which component transforms electrical energy into motion?

How should an ammeter be connected?

What is the SI unit of potential difference?

In a cell symbol, the short thick line is the:

Which symbol is used for resistance and what is its unit?

What does a fuse do?

For parents and teachers

Big idea: Expectation D2.3 asks students to identify the components of a DC circuit, explain their functions, and identify electrical quantities with their symbols and SI units. Clean, correct circuit diagrams are used in every later lesson.

Common difficulties:

Meter placement: ammeter in series (in the loop), voltmeter in parallel (across). Memory hook: "Ammeter along, Voltmeter via the sides."
V means two things: the quantity (potential difference) and its unit (volt).
Diagrams drawn freehand with curved wires and missing junction dots.

How to help: Ask your teen to draw the circuit of a flashlight or toy. Open only battery compartments; keep coin cells away from young children.