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F. Electricity and Magnetism · Electricity and Magnetism

Electricity and Magnetism: Unit Review

A1.1 A1.4 A1.5 A1.6 A1.8 A1.9 A1.10 A1.11 A1.12 A1.13 F1.1 F1.2 F2.1 F2.2 F2.3 F2.4 F2.5 F2.6 F2.7 F2.8 F3.1 F3.2 F3.3 F3.4 F3.5 F3.6 F3.7 F3.8 F3.9About 40 minutes
Learning goals
  • I can analyse series, parallel and mixed DC circuits with Ohm's and Kirchhoff's laws.
  • I can describe magnetic fields and use the right-hand (and left-hand) rules.
  • I can explain motors, generators and transformers using the motor principle, Faraday's law and Lenz's law, and solve transformer and transmission problems.
  • I can describe electrical safety precautions and analyse the impact of electromagnetic technologies and electricity production.

Words to know

Kirchhoff's laws
Current in = current out at a junction; potential rises = drops around a loop.
Oersted's principle
A current produces a magnetic field around the conductor.
motor principle
A current-carrying conductor in an external field feels a force perpendicular to both.
Faraday's law
A changing magnetic field induces a potential difference in a conductor.
Lenz's law
The induced current opposes the change that produced it.
transformer equation
Vₛ ÷ Vₚ = Nₛ ÷ Nₚ; ideally VₚIₚ = VₛIₛ.

Let's review!

In this unit you analysed DC circuits with Ohm's law and Kirchhoff's laws, including mixed series-parallel circuits, and designed and tested your own. You mapped magnetic fields, used Oersted's principle and the right-hand rules, and saw how the motor principle drives motors and speakers. You learned how Faraday's law and Lenz's law make generators and transformers work, why the grid uses high-voltage AC, how to stay safe around electricity, and how to judge the social, economic and environmental impact of electromagnetic technologies and electricity production.

This review has a few questions from every lesson. Try each one, then press Show answer to check.

How to use this review

Do two or three sections at a time. Draw a circuit diagram or field sketch for every question, and use your hand for every hand rule. Write units at every step and convert mA, kV, MW and minutes before calculating. For each question you miss, write down why (wrong law? wrong hand? ratio upside down? unit conversion?) and go back to that lesson's worked examples. Finish with the 10-question quiz.

CircuitsV = IR, P = VI = I²RKCL: in = outKVL: rises = dropsFieldsfield lines N → S outsideRHR #1: thumb I, fingers BRHR #2: fingers I, thumb NMotor principleRHR #3: thumb I,fingers B, palm Fleft hand = electron flowInductionchanging field → currentLenz: opposes the changeVₛ/Vₚ = Nₛ/Nₚ, P_loss = I²R
Your electricity and magnetism toolbox: circuit laws, field rules, the motor principle and induction.
The three right-hand rules (conventional current)RHR #1straight wirethumb = currentfingers curl = fieldRHR #2coil / solenoidfingers curl = currentthumb = N poleRHR #3motor rulethumb = current, fingers = fieldpalm = forceUsing electron flow? Do exactly the same with your LEFT hand.
The three right-hand rules in one place. With electron flow, use the left hand in the same way.
Just for fun

If you did this unit properly, your right hand has probably made more strange shapes in the last few weeks than in all your previous years of school combined. Physics students are the only people who get caught waving at nobody during a test.

Charge, Current and Potential Difference: Conventional Current and Electron Flow

1

A current of 0.40 A flows for 5.0 min. How much charge passes, and how many electrons is that?

2

A 12 V battery gives 30 J to the charge passing through it. How much charge passed?

3

Which way do electrons move in the external circuit? Which way is conventional current?

Ohm's Law and Kirchhoff's Laws: Series and Parallel Circuits

1

Find R_T for 15 Ω and 30 Ω (a) in series, (b) in parallel.

2

4.5 A enters a junction; two branches leave, one carrying 2.0 A. Find the other.

3

A lamp's resistance rises as it heats. Is it ohmic? What does its V–I graph look like?

Analysing Mixed Series-Parallel Circuits

1

A 12 V battery is connected to R₁ = 3.0 Ω in series with a parallel pair of 6.0 Ω and 6.0 Ω. Find I_T and the current in each 6.0 Ω resistor.

2

In the last question, find the potential difference across R₁ and across the pair. Check with KVL.

3

Find the power of R₁ in that circuit.

Designing, Building and Testing Mixed DC Circuits

1

How are an ammeter and a voltmeter connected, and why?

2

An LED needs 2.0 V at 15 mA from a 6.0 V supply. Find the series resistor needed.

3

Predicted 1.20 A, measured 1.14 A. Find the percent error.

Magnets and Magnetic Fields

1

State two rules for drawing magnetic field lines.

2

Use domain theory to explain why dropping a magnet can weaken it.

3

What is between two N poles facing each other?

Oersted's Principle, the Right-Hand Rules and Electromagnets

1

Conventional current flows into the page. Which way does the field circle?

2

Looking at one end of a solenoid, the current is clockwise. Is that end N or S?

3

Give two ways to strengthen an electromagnet.

The Motor Principle and the DC Motor

1

The field points right and conventional current flows up the page. Find the force.

2

What does the split-ring commutator do in a DC motor?

3

How does a loudspeaker use the motor principle?

Build, Test and Refine an Electromagnetic Device

1

Why is the insulation removed from only half of one lead in a paper-clip motor?

2

Name the independent, dependent and one controlled variable for testing how the number of turns affects motor speed.

3

A coil makes 25 turns in 2.5 s. Find its speed in rpm.

Electromagnetic Induction: Faraday's Law and Lenz's Law

1

State Lenz's law.

2

A magnet's S pole is pulled away from a coil. What pole forms at the near end of the coil?

3

Why does a magnet resting inside a coil induce no current?

Generators, Alternating Current and Direct Current

1

What is the job of slip rings in an AC generator?

2

Find the period of 60 Hz AC and the peak voltage of a 120 V (effective) outlet.

3

Why is a generator harder to turn when more current is drawn?

Transformers: Stepping Voltage Up and Down

1

Nₚ = 600, Nₛ = 30, Vₚ = 120 V. Find Vₛ. Step-up or step-down?

2

An ideal transformer gives 6.0 V at 3.0 A from 120 V. Find the primary current.

3

A transformer takes in 400 W and gives out 380 W. Find its efficiency.

Transmitting Electrical Energy: Why the Grid Uses High-Voltage AC

1

Power of 40 MW is sent at 200 kV through lines of 5.0 Ω. Find the current and the power lost.

2

If the voltage were 400 kV instead, what would the loss be?

3

Why is AC used for transmission?

Electrical Safety: Circuits, Homes and High-Voltage Lines

1

What does a GFCI protect against, and how?

2

Will a 1800 W heater and a 600 W lamp trip a 15 A, 120 V breaker?

3

What should you do before digging in your yard in Ontario?

Electromagnetic Technologies and Society

1

How does an induction loop detect a car?

2

Give one social and one economic impact of MRI.

3

What privacy question could be asked about red-light cameras?

Producing Electrical Energy: Efficiency and Environmental Impact

1

A station takes in 2400 MW and produces 840 MW. Find its efficiency.

2

Give two environmental impacts of coal-fired generation.

3

Suggest one way to make electricity production more sustainable.

Check your understanding

Quiz

1.

A 6.0 Ω and a 12 Ω resistor are in parallel across 12 V. What is the total current?

2.

Kirchhoff's voltage law is based on conservation of…

3.

A 2.0 Ω resistor is in series with two 4.0 Ω resistors in parallel. What is R_T?

4.

Outside a bar magnet, field lines point…

5.

Conventional current flows out of the page. The magnetic field around the wire is…

6.

A wire carries conventional current to the right in a field pointing into the page. The force on it is…

7.

An N pole is pushed into a coil. The near end of the coil becomes…

8.

A transformer has Nₚ = 50 and Nₛ = 500. With 12 V AC in, the output is…

9.

Doubling the transmission voltage for the same power changes the line loss to…

10.

A station takes in 3000 MW and outputs 1050 MW. Its efficiency is…

For parents and teachers

How to use the results: for each kind of mistake, revisit the lesson shown.

  • Charge, current, energy or unit errors (Q = IΔt, V = E ÷ Q): Charge, Current and Potential Difference.
  • Series and parallel rules, Kirchhoff's laws: Ohm's Law and Kirchhoff's Laws.
  • Getting lost in mixed circuits: Analysing Mixed Series-Parallel Circuits (redraw after each step; use an R–I–V–P table).
  • Meter placement, resistor choice, percent error: Designing, Building and Testing Mixed DC Circuits.
  • Field-line drawings, domains, Earth's field: Magnets and Magnetic Fields.
  • Wrong field direction around wires or coils: Oersted's Principle, the Right-Hand Rules and Electromagnets.
  • Wrong force direction, commutator questions: The Motor Principle and the DC Motor.
  • Fair-test and design questions: Build, Test and Refine an Electromagnetic Device.
  • Induced pole or current direction: Electromagnetic Induction: Faraday's Law and Lenz's Law.
  • AC/DC, slip rings, frequency, peak voltage: Generators, Alternating Current and Direct Current.
  • Turns ratios upside down, current ratios: Transformers.
  • Using V²/R for line losses: Transmitting Electrical Energy.
  • Breakers versus GFCIs, outdoor safety: Electrical Safety.
  • One-sided impact analyses: Electromagnetic Technologies and Society and Producing Electrical Energy.

In Grade 12 Physics (SPH4U), these ideas extend to electric and magnetic fields, forces on moving charges (F = qvB) and electromagnetic waves.