Electric Circuits (Cambridge (CIE) O Level 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  across the lamp L₁, as shown on the voltmeter in Fig. 2.2.

 = .................................................. V [1]

(ii) Record the current  in the circuit, as shown in Fig. 2.3.    

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

(iii) Calculate the resistance  of the filament of lamp L₁. Use the equation  . Include the unit.

 = ..................................................... [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 in the circuit and the potential difference  across lamp L₂.

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

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

Calculate the resistance R₂ of the filament of lamp L₂. Use the equation .

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

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  in the circuit and the potential difference  across lamp L₃.            

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

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

Calculate the resistance R₃ of the filament of lamp L₃. Use the equation .

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

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 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

.  

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]

A student investigates the resistance of resistors connected in series.

Fig. 2.1 shows the circuit used.

Fig. 2.1

On Fig. 2.1, draw a voltmeter connected to measure the potential difference across resistor A.

The student closes the switch and records the potential difference across resistor A and current in the circuit. The meter readings are shown in Fig. 2.2.

Fig. 2.2

Record the potential difference and current .

Calculate the resistance of resistor A using the equation .

Give your answer to a suitable number of significant figures for this experiment.

The student then connects the voltmeter to measure the potential difference across both resistors A and B. The new potential difference reading is .

Calculate the combined resistance of the two resistors in series.

The resistance of resistor B is known to be .

A student suggests that should be equal to .

State whether your results support this suggestion. Justify your answer by reference to your values.

(i) The student wants to repeat the experiment with resistor A and resistor B connected in parallel.

Complete the circuit diagram in Fig. 2.3 to show this arrangement.

[1]

Fig. 2.3

(ii) The student notices that the resistors become warm during the experiment.

State one change they could make to the procedure to reduce this heating effect.

[1]

(iii) State one hazard associated with wires overheating in a circuit.

[1]