Electric Circuits Quiz X

A student makes a circuit to switch on a 6.0 V lamp from two different switches X and Y. Fig. 9.1 shows the circuit. Switch X is in position P. State the position of switch Y for the lamp to be lit.

• P
• S
• R
• Q

Answer explanation

1a

(a) For the lamp to light:

Switch Y needs to complete the circuit with no breaks.

Therefore, it must be in position R.

[1 mark]

The current in the lamp is 0.50 A when the potential difference (p.d.) across the lamp is 6.0 V.

Calculate the resistance of the lamp. Include the unit.

• R=12 ohms

• R=2 ohms

• R=6 ohms

• R=4 ohms

Answer explanation

To calculate the resistance of the lamp:

List the known quantities:

• Current, I = 0.50 A

• Potential difference, V = 6.0 V

Recall the Ohm's law equation for resistance:

R = V/I

R = 12 ohms

[Total: 4 marks]

The student connects another 6.0 V lamp in parallel with the first lamp, as shown in Fig. 9.2. Give two advantages of connecting the lamps in parallel.

Answer explanation

Advantages of connecting lamps in parallel:

• 1. Both lamps have the correct p.d. / voltage across them.

• 2. If one lamp fails the other is still lit

[Total: 2 marks]

A student investigates the electrical resistance of some components. Fig. 10.1 shows an incomplete diagram of the circuit used by the student. (i) State the term used for not labelled component. (ii) The student uses the circuit to measure the resistance of component Y. Complete the diagram in Fig. 10.1 by adding electrical symbols to show an ammeter and a voltmeter correctly connected to determine the resistance of component Y.

• Variable Resistor

• Resistor

• fuse

• Diode

Answer explanation

(i) The term used for component Y is a:

Thermistor [1]

(ii)

A diagram that would score 3 marks should look like Fig 10.1 (to the left)

• Ammeter symbol drawn correctly. [1]

• Voltmeter symbol drawn correctly. [1]

• Ammeter positioned in series AND voltmeter position parallel to thermistor. [1]

[Total: 4 marks]

Fig. 10.2 shows two resistors A and B. (i) Resistor A and resistor B are connected in series. State the value of their combined resistance. ............................................ Ω (ii) Resistor A and resistor B are connected in parallel. Compare the combined resistance when in parallel with the resistance of resistor A alone.

Answer explanation

(i) The value of their combined resistance is:

Recall the equation for resistors in series:

• Combined resistance in series = R1 + R2 + R3 + …

Calculate the combined resistance:

• Combined resistance = 5.0 + 7.0 

• combined resistance = 12.0 Ω [1 mark]

(ii) The combined resistance when in parallel with the resistance of A alone is:

• Smaller (than A); [1 mark]

Fig. 10.1 shows a circuit for determining the resistance of a component. a) On Fig. 10.1, label the fixed resistor, by writing the letter R.

b) components in Fig. 10.1 measure electrical quantities. Identify the quantity that each component measures. Write each quantity and the unit of each quantity in the correct place in Table 10.1.

Answer explanation

a) The fixed resistor labelled R on the circuit diagram would look like Fig. 10.1 to the left.

• Fixed resistor labelled R [1 mark]

[Total: 1 mark]

(b) A correctly completed table would look like Table 10.1 to the left.
One mark for each correct answer

[Total: 4 marks]

A student uses the circuit in Fig. 10.1 to determine the resistance of wires made from the same material. State how the resistance of a wire is related to its length and its diameter.

Answer explanation

• Increasing the length increases the resistance OR decreasing the length decreases the resistance; [1 mark]

• Increasing the diameter decreases the resistance OR decreasing the diameter increases the resistance; [1 mark]

[Total: 2 marks]

Fig. 9.1 and Fig. 9.2 each show an electrical circuit. Each circuit has two lamps connected to an electrical supply. State the term used to describe each electrical circuit. Write the term under each circuit. State two disadvantages of the circuit in Fig. 9.2.

Answer explanation

• Fig 9.1 is a parallel circuit; [1 mark]

• Fig 9.2 is a series circuit; [1 mark]

[Total: 2 marks]

Note:

An exam question will often not state the type of circuit, so recognising a parallel and series circuit is extremely important. Current and potential difference (voltage) act differently in both.

Redraw the circuit in Fig. 9.1 with switches that will turn lamps X and Y on and off independently of each other.

Answer explanation

The circuit in Fig. 9.1 can be modified by adding separate switches for lamps X and Y, allowing them to be controlled independently.

Fig. 9.3 shows another circuit. The lamps can be turned on and off using two different switches S and S. Complete the table stating when the lamps are on or off. The first one has been done for you.

Answer explanation

• With switch positions K and W, the lamps are on; [1 mark]

• With switch positions J and W, the lamps are off; [1 mark]

• With switch positions J and Z, the lamps are on; [1 mark]

[Total: 3 marks]