1. 1
   A washing machine has a small LED indicator light connected in series with a resistor and a low-voltage supply, to show the user when a wash cycle is running.

   (a)

   What is a light-emitting diode (LED) designed to do that a normal diode does not?

   [1 mark]

   • AEmit light when it conducts current in the forward direction
   • BConduct current equally well in both directions
   • CStore electrical charge for later release
   • DConvert alternating current directly into mechanical motion

   (b)

   The engineer connects the LED the wrong way round by mistake. What would happen to the LED in this case?

   [1 mark]

   • AThe LED would not light up, since it is reverse biased and blocks the current
   • BThe LED would light up exactly as normal
   • CThe LED would light up more brightly than normal
   • DThe resistor would stop working entirely

   (c)

   Explain why a fixed resistor is always connected in series with an LED in a circuit like this, rather than connecting the LED directly across the supply on its own.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An audio engineer at a small recording studio uses a variable potential divider, formed from a single variable resistor with a sliding contact connected across a supply, as a volume control on a mixing desk.

   (a)

   An audio engineer uses a variable potential divider, formed from a single variable resistor connected across a supply, as a volume control on a small mixing desk. As the engineer moves the control to increase the output p.d. taken from part of the variable resistor, what is happening inside the potential divider?

   [1 mark]

   • AThe total resistance of the whole variable resistor changes to zero
   • BThe position of the sliding contact changes the proportion of the total resistance on each side of the contact, changing the output p.d.
   • CThe supply e.m.f. itself increases as the control is turned
   • DThe current in the circuit becomes an alternating current instead of a direct current

   (b)

   The variable resistor in the volume control is split by the sliding contact into two parts, of resistance $R_1$ and $R_2$, with potential differences $V_1$ and $V_2$ across each part respectively. Which equation correctly relates these four quantities for this potential divider?

   [1 mark]

   • A$\frac{R_1}{R_2} = \frac{V_1}{V_2}$
   • B$R_1 R_2 = V_1 V_2$
   • C$R_1 + R_2 = V_1 + V_2$
   • D$\frac{R_1}{V_1} = \frac{R_2}{V_1}$

   (c)

   If $R_1 = 200 \, \Omega$ and $R_2 = 800 \, \Omega$ for a particular position of the sliding contact, and the p.d. across $R_1$ is measured as 3.0 V, calculate the p.d. across $R_2$.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician in an electronics laboratory passes a constant current through several different electrical conductors of different resistances, one at a time, and measures the potential difference across each to investigate the relationship between resistance and potential difference.

   (a)

   A technician passes a constant current through several different electrical conductors of different resistances, one at a time, and measures the potential difference across each. State how the potential difference across a conductor changes as its resistance increases, for a constant current, and explain this relationship using the equation for resistance.

   [3 marks]

   (b)

   The technician repeats the test on a filament lamp instead of a fixed resistor, keeping the supply current constant using a variable resistor to compensate. Explain why the relationship between p.d. and resistance for the filament lamp is more complicated to interpret than for a fixed resistor, referring to how the filament lamp's own resistance changes with current.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A theatre's lighting engineer uses a large variable potential divider, known as a dimmer, to smoothly control the brightness of the stage lamps during a performance. The engineer also fits small LED indicator lamps, each with its own series resistor, next to the controls in the dark backstage area.

   (a)

   Explain, using the action of a variable potential divider and the equation $\frac{R_1}{R_2} = \frac{V_1}{V_2}$, how moving the dimmer's control smoothly changes the p.d. supplied to the stage lamps, and why this method gives smooth control of brightness rather than the lamps simply switching fully on or off.

   [6 marks]

   (b)

   The theatre's lighting engineer also fits small LED indicator lamps, each with its own series resistor, next to the main controls in the dark backstage area, so that technicians can see which controls are active without shining a bright light that would distract the audience. Explain why LEDs, together with their series resistors, are well suited to this backstage indicator role, referring to the action of an LED, the purpose of the series resistor, and the relationship between p.d. and resistance for a constant current.

   [6 marks]

   Total for question 4: 12 marks

End of questions