Electricity, Magnetism & Circuits — Chapter quiz · Physics & Environmental Science, Grade 12

Relate charge, fields, voltage, circuits, induction, stored energy, and electrical power.

Problem 1

Find the force between two 2 μC charges placed 0.2 m apart. (k = 9 × 10⁹ N·m²/C²)
Answer and explanation

0.9 N

F = kq₁q₂/r² = (9 × 10⁹)(2 × 10⁻⁶)(2 × 10⁻⁶) ÷ (0.2)² = 0.036 ÷ 0.04 = 0.9 N.

Problem 2

Two positive charges are placed near each other. They will:
  1. cancel each other out
  2. push apart (repel)
  3. pull together (attract)
  4. form a magnet
Answer and explanation

push apart (repel)

Like charges repel, so two positive charges push apart.

Problem 3

A charge of 3 μC (3 × 10⁻⁶ C) feels a force of 0.15 N at a point. Find the electric field strength there.
Answer and explanation

50000 N/C

E = F ÷ q = 0.15 ÷ (3 × 10⁻⁶) = 50,000 N/C.

Problem 4

A charge of 30 C flows past a point in 6 seconds. Find the current.
Answer and explanation

5 A

I = Q ÷ t = 30 ÷ 6 = 5 A.

Problem 5

Electrical resistance is measured in:
  1. volts
  2. watts
  3. ohms
  4. amperes
Answer and explanation

ohms

Resistance is measured in ohms (Ω); amps measure current, volts measure voltage, and watts measure power.

Problem 6

A battery gives 40 J of energy to 5 C of charge. Find the voltage.
Answer and explanation

8 V

V = W ÷ Q = 40 ÷ 5 = 8 V.

Problem 7

A 10 V battery is connected across a 2 Ω resistor. Find the current.
Answer and explanation

5 A

I = V ÷ R = 10 ÷ 2 = 5 A.

Problem 8

Three resistors of 5 Ω, 10 Ω, and 15 Ω are wired in series. Find the total resistance.
Answer and explanation

30 Ω

In series, resistances add: 5 + 10 + 15 = 30 Ω.

Problem 9

Two 8 Ω resistors are connected in parallel. Find the equivalent resistance.
Answer and explanation

4 Ω

1/R = 1/8 + 1/8 = 1/4, so R = 4 Ω. (Two equal resistors in parallel give half of one.)

Problem 10

An electric current flowing through a wire always produces a:
  1. sound wave
  2. chemical reaction
  3. gravitational field
  4. magnetic field
Answer and explanation

magnetic field

Oersted discovered that any electric current creates a magnetic field around it — the basis of electromagnets.

Problem 11

A generator in a power plant converts:
  1. mechanical energy into electrical energy
  2. heat energy into sound energy
  3. electrical energy into mechanical energy
  4. chemical energy into light energy
Answer and explanation

mechanical energy into electrical energy

A generator uses electromagnetic induction to turn the motion of a spinning coil or magnet into electrical energy.

Problem 12

A device operates at 120 V and draws 5 A. Find its electrical power.
Answer and explanation

600 W

P = IV = 5 × 120 = 600 W.

Problem 13

Explain the following claim and supply a valid example or counterexample: Field exists as a model independent of the particular small test charge.
Answer and explanation

F=qE=.002N along the field; a negative charge would reverse direction. Field exists as a model independent of the particular small test charge.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 14

Independent chapter check: A+2 μC test charge in a uniform500 N/C field points east. Find its force. What force acts on a−3 μC test at the same location?
Answer and explanation

F=qE gives+0.0010 N east for+2 μC and−0.0015 N, i.e.west, for−3 μC. The field is unchanged in the ideal nonperturbing-test model.

F=qE gives+0.0010 N east for+2 μC and−0.0015 N, i.e.west, for−3 μC.

The field is unchanged in the ideal nonperturbing-test model.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 15

Explain the following claim and supply a valid example or counterexample: Current does not mean charges are consumed; a component transfers energy while charge is conserved.
Answer and explanation

I=Q/t=3 A; conventional current direction is positive-charge flow. Current does not mean charges are consumed; a component transfers energy while charge is conserved.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 16

Independent chapter check: An ohmic12 Ω resistor has9 V across it for40 s. Find current, power and transferred energy.
Answer and explanation

I=9/12=0.75 A; P=VI=6.75 W; E=Pt=270 J. The stated constant resistance and voltage must persist over the interval.

I=9/12=0.75 A; P=VI=6.75 W; E=Pt=270 J.

The stated constant resistance and voltage must persist over the interval.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 17

Explain the following claim and supply a valid example or counterexample: A series circuit has equal current; a parallel circuit has equal branch voltage.
Answer and explanation

1/R=1/6+1/6=1/3, soR=3ohm; lower than either branch. A series circuit has equal current; a parallel circuit has equal branch voltage.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 18

Independent chapter check: Two resistors4 Ω and12 Ω are in parallel across an ideal12 V source. Find each current, total current, equivalent resistance and total power.
Answer and explanation

Currents are3 A and1 A; total4 A. Req=12/4=3 Ω and P=12×4=48 W. Branch powers36 W and12 W sum consistently.

Currents are3 A and1 A; total4 A.

Req=12/4=3 Ω and P=12×4=48 W.

Branch powers36 W and12 W sum consistently.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 19

Explain the following claim and supply a valid example or counterexample: A static magnet beside a stationary coil does not continuously generate emf.
Answer and explanation

A changing magnetic flux, through changing field, area, or orientation; a moving magnet can supply that change. A static magnet beside a stationary coil does not continuously generate emf.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 20

Independent chapter check: A charge+2 μC moves at3.0×10⁴ m/s perpendicular to a0.20 T field. Find magnetic force magnitude. What happens if velocity becomes parallel to the field?
Answer and explanation

F=|q|vB=0.012 N. Parallel motion gives sin0=0 and zero magnetic-force magnitude in this model; electric forces would need a separate account.

F=|q|vB=0.012 N.

Parallel motion gives sin0=0 and zero magnetic-force magnitude in this model; electric forces would need a separate account.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 21

C=.001F,V=10 V. Find stored energy.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

0.05 J

.5×.001×100=.050.

Problem 22

What happens to stored energy if voltage doubles at fixed C?
Answer and explanation

Energy increases by four because it depends on V².

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 23

Independent chapter check: A50 μF capacitor is at8 V. Find Q and U; compare U at16 V in the same ideal model.
Answer and explanation

Q=400 μC; U=½×50×10⁻⁶×64=0.0016 J=1.6 mJ. At16 V, U=6.4 mJ, four times as much. Actual ratings must be separately respected.

Q=400 μC; U=½×50×10⁻⁶×64=0.0016 J=1.6 mJ.

At16 V, U=6.4 mJ, four times as much.

Actual ratings must be separately respected.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 24

An ohmic 10ohm resistor has 2 A. Find power.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

40 W

I²R=4×10=40.

Problem 25

Which voltage should be used for a parallel branch power calculation?
Answer and explanation

The branch potential difference, equal to supply voltage in the ideal parallel model.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 26

Independent chapter check: A15 W device is replaced by a9 W device providing the same service for2500 h/year. Find annual energy saved in kWh and joules, and name one uncounted life-cycle term.
Answer and explanation

Saving=(0.015−0.009)×2500=15 kWh=5.40×10⁷ J/year. Production, transport, replacement lifespan or disposal is outside this operational-only result.

Saving=(0.015−0.009)×2500=15 kWh=5.40×10⁷ J/year.

Production, transport, replacement lifespan or disposal is outside this operational-only result.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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