Electricity

A student wants to measure the current through a lamp and the potential difference across it. Which arrangement is correct?

1. A

   Both meters in series with the lamp

2. B

   Both meters in parallel with the lamp

3. C

   Ammeter in series; voltmeter in parallel

4. D

   Ammeter in parallel; voltmeter in series

1. a

   Select and explain Select the correct arrangement and explain the purpose of each connection.

   3
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   Correct choiceC

   1. 1

      Selects option C: the ammeter is in series and the voltmeter is in parallel.

   2. 2

      Explains that the ammeter is in series so the same charge flow passes through it and the lamp.

   3. 3

      Explains that the voltmeter is in parallel to compare energy per charge across the lamp.

A class records current at different potential differences for a fixed resistor and a filament lamp.

1. a

   Calculate Calculate the resistance of the fixed resistor at 4.0 V.

   2
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   1. 1

      Uses R = V ÷ I with V = 4.0 V and I = 0.40 A.

   2. 2

      Obtains R = 10 Ω.

2. b

   Analyse Use the data to decide which component is ohmic over this range.

   3
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   1. 1

      Identifies the fixed resistor as ohmic.

   2. 2

      Uses evidence that current is directly proportional to potential difference, with V ÷ I constant at 10 Ω.

   3. 3

      Explains that the lamp is non-ohmic because its current does not rise in direct proportion to voltage, so its resistance changes.

3. c

   Explain Explain the physical reason for the changing resistance of the filament lamp.

   2
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   1. 1

      States that the filament becomes hotter as current or power increases.

   2. 2

      Explains that increased lattice vibration makes electron motion more difficult, increasing resistance.

A light-dependent resistor (LDR) will control a night light. A team has an LDR, a multimeter, a lamp with a dimmer, a metre rule, and a phone light-meter app. They want to determine how LDR resistance depends on illuminance.

1. a

   Formulate Write a focused research question and a testable hypothesis.

   2
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   1. 1

      Writes a question linking measured illuminance to LDR resistance with other conditions controlled.

   2. 2

      Predicts that resistance decreases as illuminance increases, with a defensible physical or component-based reason.

2. b

   Design Design a method that would produce a useful graph and valid evidence.

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   1. 1

      Collects paired illuminance and resistance readings across at least five well-spaced light levels.

   2. 2

      Uses repeats and a mean at each light level.

   3. 3

      Controls relevant variables such as sensor angle, lamp spectrum, ambient light, and LDR temperature.

   4. 4

      Plots resistance against illuminance with units and selects scales that reveal the relationship.

3. c

   Evaluate Explain why lamp distance alone is not a sufficient measure of the light reaching the LDR.

   2
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   1. 1

      Explains that reflected light, ambient light, source geometry, and lamp output affect illuminance as well as distance.

   2. 2

      Concludes that illuminance should be measured at the LDR rather than inferred from distance alone.