Electric Circuits & Electrical Safety (CIE IGCSE Physics)

Topic Questions

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Medium

17 marks

A student investigates a wind turbine, which is an electrical generator driven by a propeller blade.

Plan an experiment which will enable him to investigate how the current in a resistor connected across the terminals of the turbine varies with the speed of the air flow through the turbine.

The apparatus available includes:

a model wind turbine as shown in Fig. 4.1

an electric fan to provide the moving air to turn the turbine

a device for measuring air speed.

In your plan, you should:

list any additional apparatus needed
complete the wind turbine circuit diagram on Fig. 4.1
state the key variables to be kept constant
explain briefly how to carry out the experiment, including how the speed of the air flow is to be changed
explain how to use the readings to reach a conclusion.

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2a4 marks

A student determines the resistances of some filament lamps.

Fig. 2.1 shows the first circuit she uses.

p6-circuit

(i)

Record the potential difference

V_{1}

across the lamp L₁, as shown on the voltmeter in Fig. 2.2.

_{$V_{1}$}= .................................................. V [1]

(ii)

Record the current

I_{1}

in the circuit, as shown in Fig. 2.3.

I_{1}

= .................................................. A [1]

(iii)

Calculate the resistance

R_{1}

of the filament of lamp L₁. Use the equation

R_{1} = \frac{V_{1}}{I_{1}}

. Include the unit.

R_{1}

= ..................................................... [2]

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2b1 mark

The student disconnects the voltmeter. She connects lamp L₂ in series with lamp L₁. She connects the voltmeter across lamp L₂.

She measures the current $I_{2}$ in the circuit and the potential difference $V_{2}$ across lamp L₂.

I_{2}

= ..... 0.30 A .....

V_{2}

= ..... 1.7 V .....

Calculate the resistance R₂ of the filament of lamp L₂. Use the equation $R_{2} = \frac{V_{2}}{I_{2}}$ .

R_{2}

= .............................................

The student disconnects the voltmeter. She connects lamp L₃ in series with lamps L₁ and L₂. She connects the voltmeter across lamp L₃.

She measures the current $I_{3}$ in the circuit and the potential difference $V_{3}$ across lamp L₃.

I_{2}

= ..... 0.26 A .....

V_{2}

= ..... 1.2 V .....

Calculate the resistance R₃ of the filament of lamp L₃. Use the equation $R_{3} = \frac{V_{3}}{I_{3}}$ .

$R_{3}$ = ...................................

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2c1 mark

Calculate R₁ + R₂ + R₃. Give your answer to a suitable number of significant figures for this experiment.

R₁ + R₂ + R₃= .....................................................

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2d2 marks

Some students make suggestions about the results of the experiment.

Suggestion A: R₁ + R₂ + R₃ should be equal to 3 × R₁.

Suggestion B: R₁ + R₂ + R₃ should be less than 3 × R₁.

Suggestion C: R₁ + R₂ + R₃ should be greater than 3 × R₁.

State which suggestion A, B or C agrees with your results. Justify your answer by reference to your results.

statement:

justification:

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2e3 marks

Draw a circuit diagram to show the circuit used in part (b) with all three lamps connected in series.

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3a1 mark

A student is investigating a circuit containing different lamps.
She is using the circuit shown in Fig. 3.1.

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On Fig. 3.1, draw a voltmeter connected so that it measures the potential difference (p.d.) across lamp X.

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3b1 mark

The student uses the ammeter to measure the current in the circuit.

Record the current I_S in the circuit, as shown in Fig. 3.2.

I_S= ............................................................

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3c4 marks

(i)

The student uses the voltmeter to measure the p.d. V_X across lamp X and then reconnects the voltmeter to measure the p.d. V_Y across lamp Y.

Record the value of the p.d. V_X across lamp X, shown in Fig. 3.3.

V_X = ...............................................................

Record the value of the p.d. V_Y across lamp Y, shown in Fig. 3.4.

V_Y = ...............................................................
[1]

(ii)

She then measures the p.d. V_S across both lamps in series.

Record the value of the p.d. V_S across both lamps in series, shown in Fig. 3.5.

V_S = ...............................................................[1]

(iii)

A student suggests that V_S should be equal to (V_X + V_Y).
State whether the readings support this suggestion. Justify your statement with reference to the results.

[2]

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3d1 mark

Calculate the resistance R₁ of lamp X. Use the readings from (b) and (c)(i) and the equation

R_{1} = \frac{V_{X}}{I_{S}}

R₁ = ......................................................Ω

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3e4 marks

(i)

The circuit components are to be rearranged so that

lamps X and Y are connected in parallel
the ammeter measures the current in lamp X only
the voltmeter measures the p.d. across the lamps.

Draw a circuit diagram of this arrangement.

[2]

(ii)

The student sets up the circuit as described in (e)(i).

She measures and records the current in lamp X and the p.d. across the lamps.

She then calculates a new resistance R₂ for lamp X in this parallel circuit.

R₂ = 8.3 Ω

The student notices that lamp X is very bright in this parallel circuit, but it was dim in the series circuit in (a).

Suggest how temperature affects the resistance of a lamp.
Justify your suggestion by reference to the value of R₁ from (d) and the value of R₂.

[2]

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