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Botswana General Certificate of Secondary EducationQueued — topic map only

Physics — BGCSE

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A uniform metre rule is pivoted at its 50 cm mark. A 2.0 N weight hangs at the 20 cm mark. A 3.0 N weight is hung on the other side to balance the rule. Calculate the position of the 3.0 N weight.

3 marks · Calculate · Forces, moments and motion

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Answer: At the 70 cm mark

  1. Moment of the 2.0 N weight: 2.0 N × 30 cm = 60 N cm
  2. Balance: 3.0 N × d = 60 N cm, so d = 20 cm from the pivot
  3. Position: 50 + 20 = 70 cm

Never take moments about the end of the rule — take them about the pivot. The distance that matters is always the distance from the pivot.

A car travels 150 m in 12 s. What is its average speed?

1 mark · Calculate · Forces, moments and motion

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Answer: 12.5 m/s

  1. Speed = distance ÷ time
  2. 150 ÷ 12 = 12.5
  3. Unit: m/s

Estimate before you divide: 150 m in 12 s must be a little over 10 m/s, which rules out three of the four options immediately.

A 2.0 kW kettle is used for 6 minutes each day for 30 days. Electricity costs P1.85 per kWh. Calculate the total cost of running the kettle for the 30 days.

4 marks · Calculate · Work, energy, power and cost of electricity

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Answer: P11.10

  1. Time per day in hours: 6 ÷ 60 = 0.1 h
  2. Energy per day: 2.0 kW × 0.1 h = 0.2 kWh
  3. Energy over 30 days: 6 kWh
  4. Cost: 6 × 1.85 = P11.10

Kilowatt-hours need hours, not seconds. Convert the time before you multiply, and keep the unit attached to the number all the way through.

A 12 V battery is connected across a 6.0 Ω resistor. Calculate the current in the circuit and the power dissipated by the resistor.

3 marks · Calculate · Circuits, resistance and electrical power

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Answer: 2.0 A and 24 W

  1. I = V ÷ R = 12 ÷ 6.0 = 2.0 A
  2. P = VI = 12 × 2.0
  3. P = 24 W

Two answers means two calculations and two units. An unlabelled "2.0, 24" answer is at the mercy of the marker, and there is an easier way to live.

Two 6.0 Ω resistors are connected in parallel. What is their combined resistance?

1 mark · Calculate · Circuits, resistance and electrical power

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Answer: 3.0 Ω

  1. 1/R = 1/6.0 + 1/6.0 = 2/6.0
  2. 1/R = 0.333…, so R = 3.0 Ω
  3. Sanity check: less than either single resistor

Two equal resistors in parallel always give half the value. Knowing that lets you check every answer in one second.

A student has two lamps and one battery. Describe what happens when the lamps are connected in series, then compare that with connecting them in parallel, referring to current, p.d. and brightness in each case.

4 marks · Compare · Circuits, resistance and electrical power

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Answer: Series: one path, same current through both lamps, p.d. shared, both dimmer. Parallel: two paths, same p.d. across each lamp, larger total current from the battery, each lamp at normal brightness.

  1. Series: a single path, so the current through both lamps is the same (1 mark)
  2. Series: the battery p.d. is shared between the lamps, so each lamp is dimmer (1 mark)
  3. Parallel: each lamp is connected across the battery, so each has the full p.d. and normal brightness (1 mark)
  4. Parallel: the total current from the battery is the sum of the branch currents, so it is larger and the battery drains faster (1 mark)

Write comparisons as pairs — one sentence with "whereas" in the middle scores two marks that two separate paragraphs do not.