SPH3U · Unit 5 test review

Electricity and Magnetism: Test Review

Everything for the Unit 5 test on one page: the key facts from all 5 lessons, the mistakes that cost the most marks, and a practice test that points you back to the lessons you need.

Key facts

Static Charge, Current and Electron Flow

Like charges repel, opposite charges attract. Charge is measured in coulombs.
Only electrons move in a solid: gaining them makes an object negative, losing them makes it positive.
Charging happens by friction, contact or induction.
$I = \frac{q}{Δ t}$ — current in amperes, where $1 A = 1 C/s$ .
Conventional current runs positive to negative and is used in every equation and diagram.
Electron flow runs negative to positive and is what physically happens in a metal.
Electron drift velocity is under a millimetre per second; the field, not the electrons, travels near light speed.
Ammeters go in series and have near-zero resistance. Voltmeters go in parallel and have near-infinite resistance.

Voltage, Resistance, Ohm's Law and Electrical Power

A volt is a joule per coulomb: $V$ is the energy given to each unit of charge.
$R = \frac{V}{I}$ , rearranged as $V = I R$ or $I = \frac{V}{R}$ .
An ohmic component has constant resistance and gives a straight line through the origin on a $V$ – $I$ graph.
Resistance rises with length and with temperature, and falls with cross-sectional area.
$P = V I$ — volts times amperes gives watts, because $\frac{J}{C} \times \frac{C}{s} = \frac{J}{s}$ .
$P = I^{2} R$ means power loss grows with the square of the current.
$1 kWh = 3.6 \times 1 0^{6} J$ , and it is a unit of energy, not power.
$E (kWh) = \frac{P (W) \times t (h)}{1000}$ , then multiply by the rate for the cost.

Series and Parallel Circuits

Series: $R_{T} = R_{1} + R_{2} + \dots$ — the total exceeds any individual resistor.
Parallel: $\frac{1}{R _{T}} = \frac{1}{R _{1}} + \frac{1}{R _{2}} + \dots$ — the total is less than the smallest resistor.
Remember the final reciprocal in a parallel calculation.
Series: current is the same everywhere; voltages add to the supply.
Parallel: voltage is the same across every branch; currents add to the total.
In series the largest resistor takes the largest voltage; in parallel the smallest resistor takes the largest current.
Two equal resistors in parallel give half of one. For two unequal ones, $R_{T} = \frac{R _{1} R _{2}}{R _{1} + R _{2}}$ .
Houses are wired in parallel so every outlet gets full voltage and devices work independently.

Magnetic Fields, Motors and Transformers

Field lines run from north to south outside a magnet, never cross, and are closest where the field is strongest.
Isolated poles do not exist — cutting a magnet gives two smaller magnets.
Right-hand rules use conventional current: thumb along the current for a straight wire, thumb out of the north pole for a coil.
$F_{M} \propto I$ , $F_{M} \propto n$ , $F_{M} \propto κ$ — an electromagnet is stronger with more current, more turns and a better core.
An electromagnet can be switched off and varied; a permanent magnet cannot.
A current-carrying wire in a magnetic field feels a force perpendicular to both — the motor principle.
Induction requires change: a stationary magnet in a stationary coil induces nothing.
$\frac{N _{1}}{N _{2}} = \frac{V _{1}}{V _{2}} = \frac{I _{2}}{I _{1}}$ — the current ratio is inverted.
Transformers only work on AC, and the grid transmits at high voltage to keep $I^{2} R$ losses low.

Household Circuits, Safety and Static in Practice

A house receives two live conductors and a neutral: $120 V$ from either live to neutral, $240 V$ across both.
Live (black or red) carries current out, neutral (white) carries it back, and ground (bare or green) carries none until a fault occurs.
Branch circuits are wired in parallel so every device gets the full voltage and switches independently; the branch currents add.
A fuse or breaker is a deliberate weak point wired in series, protecting the wiring. A fuse is consumed; a breaker resets.
An overload is too much load on one branch and trips slowly; a short circuit is live touching neutral or ground and trips instantly.
A GFCI compares live and neutral currents and opens on about a $5 mA$ imbalance — it protects people, while a breaker protects wiring.
Current, not voltage, causes harm. $I = \frac{V}{R}$ with dry skin near $100000 Ω$ and wet skin near $1000 Ω$ .
Roughly $100 mA$ through the body causes ventricular fibrillation, while $10$ to $20 mA$ already makes letting go impossible.

Where marks get lost

Saying a positive object 'gained protons'. Protons cannot move in a solid — the object lost electrons. (Static Charge, Current and Electron Flow)
Leaving time in minutes or hours in $I = \frac{q}{Δ t}$ . Convert to seconds first. (Static Charge, Current and Electron Flow)
Drawing current from negative to positive on a circuit diagram. Conventional current is always positive to negative. (Static Charge, Current and Electron Flow)
Claiming electrons travel near the speed of light in a wire. Their drift velocity is under a millimetre per second. (Static Charge, Current and Electron Flow)
Connecting an ammeter in parallel, which short-circuits the supply, or a voltmeter in series, which stops the circuit. (Static Charge, Current and Electron Flow)
Confusing static electricity with current electricity. Static is charge at rest; current is charge in motion. (Static Charge, Current and Electron Flow)
Treating the kilowatt-hour as a unit of power. It is energy — a kilowatt sustained for an hour. (Voltage, Resistance, Ohm's Law and Electrical Power)
Forgetting to divide watts by 1000 before multiplying by hours to get kilowatt-hours. (Voltage, Resistance, Ohm's Law and Electrical Power)
Using seconds with kilowatt-hours, or hours with joules. Match the units to the quantity you want. (Voltage, Resistance, Ohm's Law and Electrical Power)
Rearranging Ohm's law wrongly. Check with units: volts divided by amperes gives ohms. (Voltage, Resistance, Ohm's Law and Electrical Power)
Assuming every component is ohmic. A filament lamp's resistance rises sharply as it heats. (Voltage, Resistance, Ohm's Law and Electrical Power)
Thinking a higher-resistance appliance draws more power at fixed voltage. At constant $V$ , $P = \frac{V ^{2}}{R}$ — higher resistance means less power. (Voltage, Resistance, Ohm's Law and Electrical Power)
Ignoring that $P = I^{2} R$ is a square law, so doubling the current quadruples the heating. (Voltage, Resistance, Ohm's Law and Electrical Power)
Forgetting the final reciprocal in a parallel calculation, leaving the answer as $\frac{1}{R _{T}}$ . (Series and Parallel Circuits)
Adding resistances in parallel as though they were in series, which always gives a total that is far too large. (Series and Parallel Circuits)
Expecting the current to divide equally between unequal parallel branches. The smaller resistance takes more. (Series and Parallel Circuits)
Assuming the voltage divides equally between unequal series resistors. It divides in proportion to resistance. (Series and Parallel Circuits)
Believing the current is 'used up' as it passes through a series circuit. It is identical at every point. (Series and Parallel Circuits)
Applying series voltage rules to a parallel circuit, or parallel current rules to a series one. (Series and Parallel Circuits)
Never checking the answer. Series voltages must sum to the supply, and parallel currents must sum to the total. (Series and Parallel Circuits)
Inverting the transformer current ratio. $\frac{N _{1}}{N _{2}} = \frac{I _{2}}{I _{1}}$ , with the subscripts reversed. (Magnetic Fields, Motors and Transformers)
Mixing up primary and secondary. Subscript 1 is always the input side. (Magnetic Fields, Motors and Transformers)
Using the left hand, or electron flow, with the right-hand rules. State and stick to conventional current. (Magnetic Fields, Motors and Transformers)
Expecting a transformer to work on DC. Induction needs a changing field, which steady DC does not provide. (Magnetic Fields, Motors and Transformers)
Thinking a step-up transformer creates power. Voltage rises and current falls in exact proportion. (Magnetic Fields, Motors and Transformers)
Calculating transmission loss with $P = V I$ using the line voltage. Line loss is $I^{2} R$ , using the line's own resistance. (Magnetic Fields, Motors and Transformers)
Claiming Earth's geographic north pole is a magnetic north pole. It attracts compass norths, so it is magnetically south. (Magnetic Fields, Motors and Transformers)
Saying voltage is what kills. Current through the body does the damage; the voltage matters only because it drives that current through your resistance. (Household Circuits, Safety and Static in Practice)
Thinking the ground wire carries current in normal use. It carries none at all — it is an idle spare path that only conducts during a fault. (Household Circuits, Safety and Static in Practice)
Believing a circuit breaker protects people. It protects wiring from tens of amperes, and a lethal current is a few hundredths of an ampere. That is the GFCI's job. (Household Circuits, Safety and Static in Practice)
Fitting a higher-rated fuse to stop a breaker or fuse from operating. It removes the protection and leaves the wiring in the wall as the next weakest point. (Household Circuits, Safety and Static in Practice)
Confusing an overload with a short circuit. An overload is too much legitimate load and trips slowly; a short is live touching neutral or ground and trips instantly. (Household Circuits, Safety and Static in Practice)
Adding household appliances as though they were in series. Every outlet is in parallel, so each device gets the full $120 V$ and the currents add. (Household Circuits, Safety and Static in Practice)
Claiming a lightning rod attracts lightning to a building. It provides a low-resistance path to earth, and its point also bleeds charge away continuously. (Household Circuits, Safety and Static in Practice)

Practice test

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

Static Charge, Current and Electron Flow
1
A charge of $45 C$ flows past a point in $9.0 s$ . What is the current, in amperes?
A
Static Charge, Current and Electron Flow
2
A glass rod becomes positively charged after being rubbed with silk. What happened?
Static Charge, Current and Electron Flow
3
Why does a lamp light instantly when you flip a switch, even though electrons drift very slowly?
Static Charge, Current and Electron Flow
4
What happens if a voltmeter is mistakenly connected in series with a component?
Static Charge, Current and Electron Flow
5
Order these materials from best conductor to best insulator.
1. 1Salt water
2. 2Glass
3. 3Dry wood
4. 4Copper
Voltage, Resistance, Ohm's Law and Electrical Power
6
A $9.0 V$ battery drives $0.30 A$ through a resistor. What is its resistance, in ohms?
ohms
Voltage, Resistance, Ohm's Law and Electrical Power
7
A device draws $2.5 A$ from a $120 V$ supply. What power does it use, in watts?
W
Voltage, Resistance, Ohm's Law and Electrical Power
8
A $2000 W$ heater runs for $3.0 hours$ . How much energy does it use, in kilowatt-hours?
kWh
Voltage, Resistance, Ohm's Law and Electrical Power
9
A graph of voltage against current for a component is a straight line through the origin. What does this show?
Voltage, Resistance, Ohm's Law and Electrical Power
10
Order these appliances by the energy each uses in one hour, largest first.
1. 1Electric kettle ( $1500 W$ )
2. 2LED bulb ( $10 W$ )
3. 3Refrigerator running ( $150 W$ )
4. 4Clothes dryer ( $3000 W$ )
Series and Parallel Circuits
11
Resistors of $8.0 Ω$ , $12 Ω$ and $20 Ω$ are connected in series. What is the total resistance, in ohms?
ohms
Series and Parallel Circuits
12
A $4.0 Ω$ and a $6.0 Ω$ resistor are in parallel. What is the total resistance, in ohms?
ohms
Series and Parallel Circuits
13
A $6.0 V$ battery is connected to two resistors in series, $4.0 Ω$ and $8.0 Ω$ . What current flows, in amperes?
A
Series and Parallel Circuits
14
In a parallel resistance calculation, what step is most often forgotten?
Series and Parallel Circuits
15
Order the steps for analysing a circuit.
1. 1Find the total current from the supply voltage
2. 2Check that series voltages sum to the supply, or parallel currents to the total
3. 3Work back to find the voltage or current at each component
4. 4Find the total resistance using the appropriate combination rule
Magnetic Fields, Motors and Transformers
16
A transformer has 100 primary turns and 400 secondary turns. With $60 V$ on the primary, what is the secondary voltage, in volts?
V
Magnetic Fields, Motors and Transformers
17
Why will a transformer not work with direct current?
Magnetic Fields, Motors and Transformers
18
A transmission line of resistance $3.0 Ω$ carries $40 A$ . How much power is lost as heat, in watts?
W
Magnetic Fields, Motors and Transformers
19
What does a split-ring commutator do in a DC motor?
Magnetic Fields, Motors and Transformers
20
Order the stages electricity passes through from the generator to a household outlet.
1. 1Step-up transformer to $230$ – $500 kV$
2. 2Pole transformer steps down to $120/240 V$
3. 3Substation steps down to $27.6 kV$
4. 4Generator at about $20 kV$
Household Circuits, Safety and Static in Practice
21
A $1500 W$ kettle runs on a $120 V$ outlet. What current does it draw, in amperes?
A
Household Circuits, Safety and Static in Practice
22
A $15 A$ breaker trips the instant an appliance is plugged in, with a loud bang. What is the most likely cause?
Household Circuits, Safety and Static in Practice
23
Why are heavy appliances such as ranges and dryers supplied at $240 V$ rather than $120 V$ ?
Household Circuits, Safety and Static in Practice
24
Why is a carpet shock of $20000 V$ merely startling while a $120 V$ outlet can be lethal?
Household Circuits, Safety and Static in Practice
25
A technician handles a memory module without a wrist strap and it fails afterwards. What most likely happened?

Endless drill

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

Endless drillnew numbers every time

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