Principles & Applications of Electricity: Test Review

Everything for the Unit 4 test on one page: the key facts from all 8 lessons, the mistakes that cost the most marks, and a practice test that points you back to the lessons you need.

Key facts

Static Electricity

A neutral object has equal numbers of protons (+) and electrons (−). Gaining electrons makes it negative; losing electrons makes it positive.
In solids, only electrons move from object to object. Protons stay locked in the nucleus.
Laws of electric charges: opposite charges attract, like charges repel, and charged objects attract neutral objects.
Charging by contact gives the same sign as the charging object; charging by induction gives the opposite sign.
In the electrostatic series, the material lower in the list gains electrons and becomes negative.
Charge has the symbol $Q$ and is measured in coulombs (C); about $6.24 \times 1 0^{18}$ electrons make 1 C.

Conductors, Insulators and Grounding

Conductors (such as metals) have free electrons that can drift, so charge moves through them easily.
Insulators (such as rubber, glass, plastic and dry wood) hold their electrons tightly, so charge does not move through them easily.
Semiconductors such as silicon conduct a little and are used in chips, LEDs and solar cells.
Grounding connects an object to the Earth, which can give or take electrons, so the object becomes neutral.
Lightning is a huge static discharge between a charged cloud and the ground (or within a cloud); a lightning rod gives it a safe path to the ground.

Current Electricity and Circuits

Current is the rate of flow of charge: $I = \frac{Q}{t}$ , measured in amperes (A); 1 A = 1 C/s.
Potential difference is energy per unit charge: $V = \frac{E}{Q}$ , measured in volts (V); 1 V = 1 J/C.
Resistance $R$ is opposition to current, measured in ohms ( $Ω$ ).
Ammeters go in series (current through); voltmeters go in parallel (potential difference across).
Conventional current flows from + to −; electrons flow from − to +. Diagrams in this unit show conventional current.

Ohm's Law

Ohm's law: $V = I R$ , so $I = \frac{V}{R}$ and $R = \frac{V}{I}$ .
Use volts, amperes and ohms: 1000 mA = 1 A and 1 k $Ω$ = 1000 $Ω$ .
For an ohmic resistor, current is directly proportional to potential difference: double V, double I.
On a graph of V (vertical) against I (horizontal), the slope is the resistance; a steeper line means more resistance.
Lamps, LEDs and diodes are non-ohmic: their resistance changes as the current changes.

Series and Parallel Circuits

Series: one path; $I$ is the same everywhere; $V_{T} = V_{1} + V_{2} + \dots$ ; $R_{T} = R_{1} + R_{2} + \dots$
Parallel: separate branches; $V$ is the same across each branch; $I_{T} = I_{1} + I_{2} + \dots$
Adding a parallel branch decreases the total resistance and increases the total current.
Removing one load in series stops all the current; removing one branch in parallel leaves the others working.
Homes are wired in parallel; switches, fuses and breakers are in series with what they control.

Electrical Energy, Power and Cost

Electricity is the general term for effects of electric charges; electrical energy is the energy carried by moving charges and transformed by loads.
Power is the rate of energy transformation: $P = \frac{E}{t}$ and $P = V I$ , measured in watts (1 W = 1 J/s).
Energy: $E = Pt$ . In joules, use watts and seconds; in kilowatt-hours, use kilowatts and hours.
$1 kWh = 3.6 \times 1 0^{6} J$ ; kW measures power, kWh measures energy.
Cost = energy (kWh) × price per kWh.

Efficiency and Energy Transformations in Devices

Conservation of energy: energy is never created or destroyed, only transformed. Input energy = useful output + wasted energy.
Percent efficiency $= \frac{useful output energy}{total input energy} \times 100% .$
Wasted energy is usually thermal energy (and some sound) that spreads into the surroundings.
In a Sankey diagram, arrow widths are proportional to energy, and the output arrows add up to the input arrow.
LED bulbs are several times more efficient than incandescent bulbs; ENERGY STAR and EnerGuide labels help compare products.

Generating Electricity Sustainably

A generator transforms kinetic energy into electrical energy by spinning a magnet inside coils of wire (electromagnetic induction); a turbine usually does the spinning.
Ontario's electricity is roughly half nuclear and a fifth to a quarter hydroelectric, with natural gas, wind, solar and bioenergy making up the rest; coal was phased out in 2014.
Renewable sources (water, wind, sunlight, regrown biomass) are replaced naturally; non-renewable sources (uranium, natural gas) are not.
Every source has benefits and challenges; storage (batteries, pumped storage) and smart grids help balance supply and demand.
A plan of action: identify the issue, gather data, brainstorm conservation, efficiency and shifting actions, set a measurable goal, act and measure results.

Where marks get lost

Saying an object became positive because it gained protons. In solids, protons never leave the nucleus; a positive object has lost electrons. (Static Electricity)
Thinking attraction proves an object is charged. A charged object also attracts neutral objects, so only repulsion proves that both objects are charged. (Static Electricity)
Mixing up contact and induction. Contact gives the same sign as the charging object; induction gives the opposite sign. (Static Electricity)
Believing rubbing creates charge. Rubbing only moves electrons, so the two objects end up with equal and opposite charges (unless some charge leaks away). (Static Electricity)
In induction, taking the charged rod away before removing the ground. The electrons flow straight back and the object ends up neutral. (Static Electricity)
Saying a car is safe in a thunderstorm because of its rubber tires. The metal body carries the charge around you to the ground; a few centimetres of rubber cannot stop a bolt that just crossed kilometres of air. (Conductors, Insulators and Grounding)
Thinking pure water is an excellent conductor. Pure water conducts poorly; tap water, sweat and pool water conduct because of dissolved salts, which is why water and electricity are a dangerous mix. (Conductors, Insulators and Grounding)
Thinking only conductors can be charged. Insulators hold static charge very well because the charge cannot flow away; conductors lose their charge unless they are isolated. (Conductors, Insulators and Grounding)
Believing the ground can only accept electrons. Grounding works both ways: a positive object gains electrons from the Earth and a negative object gives electrons to it. (Conductors, Insulators and Grounding)
Cutting off or bypassing the third prong of a plug. That removes the safety path for fault current and leaves a metal case able to become live. (Conductors, Insulators and Grounding)
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. (Current Electricity and Circuits)
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. (Current Electricity and Circuits)
Saying voltage "flows through" a component. Current flows through a component; potential difference is measured across it. (Current Electricity and Circuits)
Forgetting to change minutes to seconds in $I = Q / t$ . The ampere is coulombs per second, so time must be in seconds. (Current Electricity and Circuits)
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. (Current Electricity and Circuits)
Plugging milliamperes straight into V = IR. Convert to amperes first (divide by 1000), or the answer is 1000 times off. (Ohm's Law)
Rearranging wrongly, for example writing I = R/V. Use the triangle: V on top, so I = V/R and R = V/I. (Ohm's Law)
Thinking that raising the voltage raises a resistor's resistance. For an ohmic resistor, R stays the same; the current goes up instead. (Ohm's Law)
Reading the slope of an I against V graph (current on the vertical axis) as the resistance. On that graph the slope is 1/R; only on a V against I graph is the slope R. Check which quantity is on each axis. (Ohm's Law)
Assuming every device obeys Ohm's law. A lamp filament's resistance rises as it heats up, so its graph curves. (Ohm's Law)
Thinking that adding another branch in parallel increases the total resistance. Each new branch is another path, so the total resistance goes down and the total current goes up. (Series and Parallel Circuits)
Believing the first bulb in a series circuit "uses up" current so later bulbs are dimmer. The current is the same through every part of a series circuit; identical bulbs in series are equally bright. (Series and Parallel Circuits)
Splitting the voltage between parallel branches. Each parallel branch gets the full source voltage; it is the current that splits. (Series and Parallel Circuits)
Giving every load in a series circuit the full battery voltage. In series, the potential differences add up to the source voltage. (Series and Parallel Circuits)
Adding resistances in parallel as if they were in series. Parallel total resistance is always smaller than the smallest branch; if your answer is bigger, something went wrong. (Series and Parallel Circuits)
Mixing up kW and kWh. A kilowatt is a rate (power); a kilowatt-hour is an amount (energy). Your bill charges for kWh. (Electrical Energy, Power and Cost)
Using minutes in E = Pt when the answer should be in kWh. Convert minutes to hours by dividing by 60 (30 min = 0.5 h). (Electrical Energy, Power and Cost)
Multiplying watts by hours and calling the answer kWh. 100 W × 3 h = 300 Wh, which is 0.3 kWh; divide watts by 1000 first. (Electrical Energy, Power and Cost)
Saying you pay the power company for electrons or for current. The charges stay in the circuit; you pay for the electrical energy they deliver. (Electrical Energy, Power and Cost)
Thinking a device with a high power rating always costs more than one with a low rating. Cost depends on power AND time: a 10 W device left on all month can cost more than a 1500 W kettle used for a few minutes a day. (Electrical Energy, Power and Cost)
Dividing the useful energy by the wasted energy instead of by the total input. Efficiency compares useful output to total input. (Efficiency and Energy Transformations in Devices)
Getting an efficiency above 100%. That usually means the numbers were flipped (input divided by output). Useful output always goes on top. (Efficiency and Energy Transformations in Devices)
Saying wasted energy is destroyed. It is still there, mostly as thermal energy spread through the surroundings; it is just no longer useful. (Efficiency and Energy Transformations in Devices)
Forgetting to multiply by 100%. An answer of 0.85 is the decimal; the percent efficiency is 85%. (Efficiency and Energy Transformations in Devices)
Calling a heat pump more than 100% efficient at making heat from electricity. It moves heat from outdoors; the electricity only runs the pump. (Efficiency and Energy Transformations in Devices)
Thinking renewable means no impact at all. Hydro dams flood land, wind turbines can harm birds and bats, and solar panels need mining and recycling; renewables are low-carbon, not impact-free. (Generating Electricity Sustainably)
Saying nuclear plants release lots of greenhouse gases. They release almost none while running; their main challenges are radioactive waste, cost and long build times. (Generating Electricity Sustainably)
Thinking batteries and pumped storage produce electricity. They store energy that was generated elsewhere and release it later, losing a little along the way. (Generating Electricity Sustainably)
Saying power plants create energy. They transform energy from one form (chemical, nuclear, gravitational, kinetic, light) into electrical energy. (Generating Electricity Sustainably)
Believing Ontario still burns coal for electricity. The last coal-fired generation ended in 2014. (Generating Electricity Sustainably)

Practice test

24 questions from across the unit. Each one shows the lesson it comes from.

Static Electricity
1
A plastic comb becomes negative after you run it through your hair. What happened?
Static Electricity
2
Match each charging method with what it does.
Friction
Contact
Induction
Static Electricity
3
A negatively charged electroscope has its leaves spread apart. An unknown object is brought near the knob and the leaves spread even farther. What can you conclude?
Conductors, Insulators and Grounding
4
Why do metals conduct electricity well?
Conductors, Insulators and Grounding
5
You see lightning and hear the thunder 6.0 seconds later. Sound travels about 340 m/s. How far away was the lightning, in kilometres? Give your answer to one decimal place.
km
Conductors, Insulators and Grounding
6
Why are GFCI outlets required near bathroom sinks?
Current Electricity and Circuits
7
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?
Current Electricity and Circuits
8
Match each quantity to its SI unit.
Current
Potential difference
Resistance
Charge
Energy
Current Electricity and Circuits
9
A flashlight has a good battery and a good bulb but does not light. Which is the most likely reason?
Ohm's Law
10
A 12 V battery is connected to a 4.0 $Ω$ resistor. What current flows, in amperes?
A
Ohm's Law
11
Two resistors are tested and their results plotted on the same V (vertical) against I (horizontal) graph. Line A is steeper than line B. What does this tell you?
Ohm's Law
12
As a rough example, a person with wet hands might have a body resistance of about 1000 $Ω$ . What current would 120 V push through them, in milliamperes?
mA
Series and Parallel Circuits
13
Three identical bulbs are connected in series to a battery. One bulb burns out. What happens to the other two?
Series and Parallel Circuits
14
Resistors of 10 $Ω$ , 15 $Ω$ and 25 $Ω$ are connected in series to a 10 V battery. What current flows, in amperes?
A
Series and Parallel Circuits
15
Are the outlets and lights in a house wired in series or in parallel?
Electrical Energy, Power and Cost
16
Which unit is used on an electricity bill to measure the amount of energy used?
Electrical Energy, Power and Cost
17
A 1500 W space heater runs for 4.0 hours. At an example price of 12 cents per kWh, how much does this cost, in cents?
cents
Electrical Energy, Power and Cost
18
A device uses 1 kW of power for 1 hour. Name the unit of energy that equals exactly this amount.
Efficiency and Energy Transformations in Devices
19
A device takes in 200 J of electrical energy and gives out 50 J of useful energy. What is its percent efficiency?
%
Efficiency and Energy Transformations in Devices
20
Why does an LED bulb cost less to run than an incandescent bulb that gives the same amount of light?
Efficiency and Energy Transformations in Devices
21
A heat pump delivers 3 J of heat to a house for every 1 J of electrical energy it uses. Which explanation is correct?
Generating Electricity Sustainably
22
Which source produces the largest share of Ontario's electricity?
Generating Electricity Sustainably
23
A family replaces 5 incandescent bulbs (60 W each) with LED bulbs (9 W each). Each bulb is on 4.0 h a day for 30 days. At an example price of 10 cents per kWh, how much money do they save in those 30 days, in cents?
cents
Generating Electricity Sustainably
24
Which best describes a smart grid?

Endless drill

Fresh numbers every time, for the calculation questions in this unit.

Endless drillnew numbers every time

Generating a problem…

Principles & Applications of Electricity: Test Review

Key facts

Where marks get lost

Practice test

Endless drill

Keep going