SITEMAP   School Physics Notes: Electricity 2.5 Solving lamp circuit current questions

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Electrical circuits: 2.5 More complex bulb circuits: Deducing the current flow at various points in the circuit

Doc Brown's Physics exam study revision notes

There are various sections to work through,

after 1 they can be read and studied in any order.

ANSWERS to ALL the QUESTIONS at the end of the page

2.5 More complex bulb circuits and deducing the current flow at various points in the circuit

Q1.

Circuit 18 shows how three lamps and four ammeters are wired together. The lamps are NOT identical but all four switches are closed.

Q2.

Circuit 18 shows how three lamps and four ammeters are wired together. The lamps are NOT identical but all four switches are closed.

Q3.

Circuit 20 shows a circuit arrangement for five switches (s) and six bulbs (b).

(a) Which bulbs light up if switch s5 is closed?

(b) Which bulbs light up if switches s1, s3 and s5 are closed?

(c) Which bulbs light up if switches s1, s2 and s5 are closed?

(d) Which bulbs light up if switches s1, s2, s4 and s5 are closed?

(e) Which bulbs light up if switches s1, s3, s4 and s5 are closed?

Q4.

Circuit 21 shows the arrangement of six identical bulbs (X) and four ammeters (a1 to a4). To solve the problem you need to realise that each bulb has an identical resistance. The more bulbs there are in series, the greater the resistance to current flow.

Q5.

The Circuit 23 shows a two way switch s1 and a simple switch s2, connected to a power supply and five lamps (b1 to b5

(a) Which lamps will be ON when only switch s1a is closed?

(b) Which lamps will be ON when only switch s1b is closed?

(c) Which lamps will be ON when switches s1a and s2 are closed?

(d) Which lamps will be ON when switches s1b and s2 are closed?

Q6.

In the circuit 20 shown above, all the bulbs light up when all the switches are closed.

(a) What happens to the other bulbs if bulb b2 'blows'?

(b) What happens to the other bulbs if bulb b3 'blows'?

(c) What happens to the other bulbs if bulb b4 'blows'?

(d) What happens to the other bulbs if bulb b5 'blows?

(e) What happens to the other bulbs if bulb b6 'blows'?

Keywords, phrases and learning objectives for more complex electrical circuits

Be able to deduce which bulbs light up and what relative current might flow in more complex bulb (lamp circuits)

e.g. be able to deduce the current flowing at various points in the circuit

Practice questions problem solving with both series circuits and parallel circuits

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Q1.

Circuit 18 shows how three lamps and four ammeters are wired together. The lamps are NOT identical but all four switches are closed. If ammeter a4 reads 8A, a1 reads 3A and a2 reads 2A, what will be the current reading on ammeter a3?

Answer: Readings a4 = a1 + a2 + a3, therefore a3 = a4 - a1 - a2 = 8 -3 -2 = 3A

Q2.

Circuit 18 shows how three lamps and four ammeters are wired together. The lamps are NOT identical but all four switches are closed. If ammeter a1 reads 2A, a2 reads 1A and a3 reads 4A, what will be the current reading on ammeter a4?

Answer: Readings a4 = a1 + a2 + a3 = 2 + 1 + 4  = 7A

Q3.

Circuit 20 shows a circuit arrangement for five switches (s) and six bulbs (b).

(a) Which bulbs light up if switch s5 is closed?  Answer b1 only

(b) Which bulbs light up if switches s1, s3 and s5 are closed?  Answer: b1, b2, b3 and b6

(c) Which bulbs light up if switches s1, s2 and s5 are closed?  Answer: b1, b2, b4 and b5

(d) Which bulbs light up if switches s1, s2, s4 and s5 are closed?  Answer: b1, b2, b4, b5 and b6

(e) Which bulbs light up if switches s1, s3, s4 and s5 are closed?    Answer: b1, b2, b3, b5 and b6

Q4.

Circuit 21 shows the arrangement of six identical bulbs (X) and four ammeters (a1 to a4). To solve the problem you need to realise that each bulb has an identical resistance. The more bulbs there are in series, the greater the resistance to current flow.

Answer: The ratio of the resistances is 1 : 2 : 3

The ratio of the current will be 3 : 2 : 1

a2 will be 1/2 of a1 due to double the resistance, so a2 will read 1.5A

(a3 will be 1/3 of a1 due to triple resistance)

Answer: The ratio of the resistances is 1 : 2 : 3

The ratio of the current will be 3 : 2 : 1

a3 will read 1/3 of a1 = 2A (triple the resistance, 1/3 of the current)

a4 = a1 + a2 + a3 = 6 + 3 + 2 =  11A

Answer: The ratio of the resistances is 1 : 2 : 3

The ratio of the current will be 3 : 2 : 1

a2 will be 1/2 a1, so a1 reading = 9.0 A

Q5.

The Circuit 23 shows a two way switch s1 and a simple switch s2, connected to a power supply and five lamps (b1 to b5

(a) Which lamps will be ON when only switch s1a is closed?  Answer: b2 and b3

(b) Which lamps will be ON when only switch s1b is closed?  Answer: b1 and b4

(c) Which lamps will be ON when switches s1a and s2 are closed?  Answer: b1, b2, b3 and b5

(d) Which lamps will be ON when switches s1b and s2 are closed?  Answer: b1, b2, b4 and b5

Q6.

In the circuit 20 shown above, all the bulbs light up when all the switches are closed.

(a) What happens to the other bulbs if bulb b2 'blows'?

Answer: b1 still lit, b3, b4, b5 and b6 go out

(b) What happens to the other bulbs if bulb b3 'blows'?

Answer: b6 goes out, b1, b2, b4 and b5 stay lit

(c) What happens to the other bulbs if bulb b4 'blows'?

Answer: b5 goes out, b1, b2, b3 and b6 stay lit

(d) What happens to the other bulbs if bulb b5 'blows?

Answer: b4 goes out, b1, b2, b3 and b6 stay lit

(e) What happens to the other bulbs if bulb b6 'blows'?

Answer: b3 goes out, b1, b2, b4 and b5 stay lit

INDEX on notes on drawing and interpreting simple electrical circuits

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