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Lesson 3 of 8Principles & Applications of Electricity
Unit 4 · Lesson 3 of 8

Current Electricity and Circuits

Current electricity is charge moving steadily around a closed loop called a circuit. Learn the parts of a circuit and their symbols, the three key quantities (current, potential difference and resistance) with their symbols and SI units, how to connect ammeters and voltmeters, and the difference between conventional current and electron flow.

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Key things to remember

  1. 1Current is the rate of flow of charge: , measured in amperes (A); 1 A = 1 C/s.
  2. 2Potential difference is energy per unit charge: , measured in volts (V); 1 V = 1 J/C.
  3. 3Resistance is opposition to current, measured in ohms ().
  4. 4Ammeters go in series (current through); voltmeters go in parallel (potential difference across).
  5. 5Conventional current flows from + to −; electrons flow from − to +. Diagrams in this unit show conventional current.

Static electricity is charge sitting still on an object until it escapes all at once in a spark. That is not much use for running a phone. To do useful work, we need charge to flow steadily through a path that leads back to where it started. That flow is current electricity, and the path is an electric circuit. Press the switch on a flashlight and you close a loop: battery, wire, switch, bulb and back to the battery. Open the switch and the loop is broken, so the light goes out. In this lesson you will learn the parts of a circuit and the three quantities that describe what is happening inside it.

Electric current

Electric current is the rate at which charge flows past a point in a circuit. Its symbol is and its SI unit is the ampere (A), often shortened to "amp". A current of 1 A means 1 coulomb of charge passes a point every second. Small currents are often given in milliamperes (mA), where 1000 mA = 1 A. Charge only flows when the circuit is a closed circuit: a complete, unbroken loop. A gap anywhere, such as an open switch or a broken wire, makes an open circuit and the current stops everywhere in that loop.

Current is charge per second. Time must be in seconds.

The parts of a circuit

PartJob in the circuitExamples
Energy sourceGives electrical energy to the charges and pushes them around the circuitCell, battery (two or more cells), solar cell, generator, wall outlet
ConductorProvides a path for the chargesCopper wires
LoadTransforms electrical energy into other useful forms (light, heat, motion, sound)Lamp, resistor, motor, heater, speaker
Control deviceOpens and closes the circuit to turn the current off and onSwitch, push button, dimmer
Protective deviceBreaks the circuit if the current gets dangerously highFuse, circuit breaker
Every working circuit needs at least a source, a conductor and a load in a closed loop.
Schematic symbols you must be able to read and draw.battery / DCresistorcapacitorLED (anode left)switch (SPST)CEBNPN transistorground (0 V)lamp / loadIn a schematic, a dot on a crossing means the wires join; no dot means they simply cross over.
Fig 1Schematic symbols for common circuit parts: a battery (DC source), resistor, capacitor, LED, switch, transistor, ground and a lamp or other load. For this unit, focus on the battery, resistor, lamp and switch; the others appear in electronics courses.

Potential difference and resistance

A battery gives energy to each coulomb of charge that passes through it, and loads take that energy and transform it. The potential difference between two points is the amount of electrical energy each coulomb of charge gains or loses between those points. People often call it voltage. Its symbol is and its unit is the volt (V): 1 V means 1 joule of energy per coulomb, so a 9 V battery gives each coulomb 9 J of energy. In formula form, . Resistance is how strongly a material or device opposes the flow of current. Its symbol is and its unit is the ohm (). A long, thin wire or a heating element has more resistance than a short, thick copper wire.

Electric circuitWater loop+−BatteryResistorcurrentWirePumpNarrow pipewater flowWide pipeBatteryis likePumpPotential difference (volts)is likePressure differenceCurrent (amperes)is likeRate of water flowResistoris likeNarrow pipeWireis likeWide pipeLimits of the analogyA wire is never empty of charge, but a pipe can be empty of water.Charges are not used up in a resistor; it is energy that is transferred.
Fig 2A circuit compared to a water loop. The battery is like a pump (potential difference is like a pressure difference), current is like the rate of water flow, a resistor is like a narrow pipe and a wire is like a wide pipe. The comparison has limits: for example, a wire is never empty of charge the way a pipe can be empty of water.
QuantitySymbolSI unitUnit symbolMeasured with
CurrentIampereAAmmeter (connected in series)
Potential differenceVvoltVVoltmeter (connected in parallel)
ResistanceRohmΩOhmmeter (with the circuit turned off)
ChargeQcoulombCCalculated from current and time
EnergyEjouleJCalculated
TimetsecondsTimer
Careful: V is both the symbol for potential difference and the symbol for its unit, the volt. A multimeter can work as an ammeter, voltmeter or ohmmeter.

Measuring current and potential difference

+−CellSwitch (closed)A+−Ammeter: in SERIESall the current flows through itLampV+−Voltmeter:in PARALLELacross the lampResistore⁻Conventional current: from the + terminal, around the circuit, back to −Electron flow: the opposite way, from − to +Connect each meter's + terminal toward the + side of the cell.
Fig 3A simple circuit with a cell, switch, lamp and resistor. The ammeter is connected in series so all the current passes through it, and the voltmeter is connected in parallel across the lamp. Each meter's + terminal goes toward the + side of the cell. The arrow shows conventional current; electrons actually flow the opposite way.
  • Ammeter: in series. Break the circuit and put the ammeter into the gap so that all the current must flow through it. An ammeter has very low resistance, so it barely changes the current it measures.
  • Voltmeter: in parallel. Connect its two leads across the component, one on each side, without breaking the circuit. It compares the energy per coulomb before and after the component. A voltmeter has very high resistance, so almost no current goes through it.
  • Polarity. Connect the meter's + (red) terminal to the side closer to the + terminal of the battery, and the − (black) terminal to the side closer to the − terminal.
  • Memory trick. Current goes through a component; potential difference is across a component.
Conventional current vs electron flow
Scientists decided current flows from + to − around a circuit long before anyone knew about electrons. This is conventional current, and it is what the arrows on circuit diagrams in this unit show. In a metal wire, the charges that actually move are electrons, and they flow the other way, from − to +. This is electron flow. Both describe the same circuit, so the math does not change; just say which one you are using.
DC and AC
Batteries and solar cells give direct current (DC): charge flows in one direction only. The outlets in your home give alternating current (AC): the charge moves back and forth, going through 60 complete back-and-forth cycles each second in Canada (60 hertz). In this unit, the circuits you analyze are DC circuits.

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
A charge of 12 C flows past a point in a wire in 4.0 s. What is the current?
Example 2
A phone charger delivers a current of 0.50 A for 2.0 minutes. How much charge flows into the phone?
Example 3
A 9.0 V battery pushes 3.0 C of charge through a circuit. How much electrical energy does the battery give to that charge?
Example 4
You want to measure the current through a lamp and the potential difference across it. Describe how to connect each meter.

Where marks get lost

  • Thinking current gets "used up" in a bulb, so less comes out than goes in. The current leaving a bulb equals the current entering it; what the bulb uses is energy, which it transforms into light and heat.
  • Connecting an ammeter in parallel or a voltmeter in series. An ammeter in parallel creates a near short circuit that can blow its fuse; a voltmeter in series blocks almost all the current. Ammeter in series, voltmeter in parallel.
  • Saying voltage "flows through" a component. Current flows through a component; potential difference is measured across it.
  • Forgetting to change minutes to seconds in . The ampere is coulombs per second, so time must be in seconds.
  • Thinking electrons race from the battery to the bulb in an instant. Electrons drift slowly; the light comes on at once because the wire is already full of charges that all start moving together.

Flashcards 9

Electric circuit
Electric current (I)
Potential difference (V)
Resistance (R)
+ 5 more

Practice

1
An ammeter reads 0.30 A in the wire going into a lamp. What will an ammeter read in the wire coming out of the lamp?
2
A charge of 30 C passes through a toaster in 1.0 minute. What is the current, in amperes?
A
3
A current of 2.0 A flows through a motor for 5.0 minutes. How much charge passes through the motor, in coulombs?
C
4
A 1.5 V cell moves 40 C of charge through a flashlight bulb. How much electrical energy does the cell give that charge, in joules?
J
5
Match each quantity to its SI unit.
Current
Potential difference
Resistance
Charge
Energy
6
How should a voltmeter be connected to measure the potential difference of a resistor?
7
What instrument measures electric current and is connected in series?
8
On the circuit diagrams in this unit, current arrows point from the + terminal of the battery, around the circuit, to the − terminal. What actually moves in the copper wires?
9
A flashlight has a good battery and a good bulb but does not light. Which is the most likely reason?

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