Kirchhoff's laws

Diodes are used in a variety of electrical circuits.

Sketch the current-potential difference graph for a diode.

A package containing a light emitting diode (LED) is marked

“20 mA 3.2 V to operate normally”.

A student uses a 5 V power supply of negligible internal resistance to operate the LED.

Calculate the resistance of a protective resistor required to operate the LED normally from this power supply.

*3 A student connects three identical 3 V bulbs to a 6 V battery of negligible internal resistance. The circuit includes two switches, S₁ and S₂, as shown.

The student closes S₁ and records the brightness of each bulb.

With S₁ still closed, the student closes S₂.

Explain how the brightness of bulb 1 compares with the brightness of bulb 2 before and after S₂ is closed.

A student is planning to collect data to produce a current-potential difference graph for a filament lamp. Her teacher suggests two circuits that she could use.

Circuit 1 uses a potential divider and circuit 2 uses a variable resistor to vary the potential difference across the filament lamp.

*(a) Discuss the suitability of each circuit to collect the data.

The student sets up the following circuit with the filament lamp. The battery has negligible internal resistance.

The reading on the ammeter is 17.5 mA. Calculate the value of the potential difference (p.d.) across the filament lamp.

p.d. across filament lamp =_........

When a voltmeter with a resistance of 1.5 kΩ is connected as shown, the p.d. across the filament lamp decreases.

Explain why the p.d. across the filament lamp decreases.

Questions 4 and 5 refer to the information below.

Two resistors are connected in parallel and the current in one of them is 2.0 A, as shown.

4 Which of the following is the current I in ampere?

A 3.0

B 4.0

C 5.0

D 6.0

Show that the shunt would need to have a resistance of about 0.01 Ω to adapt this ammeter to read up to a maximum current of 2.0 A.

A shunt of this resistance was usually made from Manganin wire.

Calculate the length of Manganin wire of radius 0.95 mm required to make this shunt.

Analogue ammeters were used before digital meters became widely available. The analogue ammeter shown will measure a maximum current of 1.0 mA and has a resistance of 18 Ω.

(Source: © David J. Green/Alamy Stock Photo)

The analogue ammeter can be adapted to measure a larger current by adding a resistor, known as a shunt, in parallel with the ammeter. The arrangement is shown below. The analogue ammeter is represented by the 18 Ω resistor.

The maximum current through the 18 Ω resistor remains as 1.0 mA.

Show that the shunt would need to have a resistance of about 0.01 Ω to adapt this ammeter to read up to a maximum current of 2.0 A.

A shunt of this resistance was usually made from Manganin wire.

Calculate the length of Manganin wire of radius 0.95 mm required to make this shunt.

Part of an electric circuit is shown.

What is the current shown by the ammeter?

A student wanted to plot a graph of current against potential difference for a light emitting diode (LED). He used the circuit shown.

Add an ammeter and a voltmeter to the circuit diagram that would enable the data to be collected.

The graph of current against potential difference obtained by the student is shown.

The student wrote the following conclusion.

“The graph shows that in general the LED is not an ohmic conductor. However, for potential differences greater than +2 V, Ohm’s law is obeyed since the graph is linear in this region.”

Criticise the student’s conclusion.

The student used the LED with a 5.0 V power supply as shown in the circuit.

To be lit to normal brightness the current through the LED must be 18 mA.

Calculate the resistance of the resistor needed in the circuit.