Electric Circuits & Electrical Safety (Cambridge (CIE) IGCSE Physics): Exam Questions

A student determines the resistances of some filament lamps.

Fig. 2.1 shows the first circuit she uses. 

(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]

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 $R_3=\frac{V_3}{I_3}$.

$R_3$ = ...................................

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

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

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.

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

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

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

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

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

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

(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]

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 .

(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]