# Student Resources ## What are the factors affecting the brightness of bulbs?

In this article, I will be sharing about the electrical system and the factors that affects the brightness of a light bulb.

Recently, a P5 student asked our teacher for help on a question on the factors affecting the brightness of a light bulb.

These are the questions:

Jess has an electric circuit with two batteries and one bulb, bulb X. She decided to add in another bulb, bulb Y (shown in Figure 1) and her mother gave her another bulb, bulb Z, to form the electric circuit in Figure 2.

a) What happens to the brightness of bulb X after adding bulb Y? Explain why?

b) What happens to the brightness of bulbs X and Y after adding bulb Z? Explain your answer.

First – Let’s understand what an electrical system is.

An electrical system needs a power source to work.

An electrical system consists of many electrical components connected in a loop without a gap to enable an electrical component, such as a light bulb, to light up. In this simple electrical system or circuit, it has wires which connect a bulb to a power source (e.g. battery). The bulb lights up when the electric current flows through the wire and the bulb and back to the power source.

Without a power source, there would not be (electrical) energy for the system to do work (i.e. to light up the bulb in the above electrical circuit). On the other hand, the greater the number of power source, the greater the amount of electric current flowing through the bulb. Therefore, resulting in brighter bulbs when there are
more batteries.

Other than the presence of a power source or the number of batteries, let’s look at what are the other factors affecting the brightness of a light bulb.

Second – The arrangement of bulbs in an electric circuit

An electrical system needs a power source to work.

There are two arrangements in which bulbs can be arranged in, either in series or
in parallel.

When two or more bulbs are arranged in series, the electric current from the power source can flow through the bulbs in a single pathway (see the yellow highlight in Figure 3). Whereas, when two or more bulbs are arranged in parallel, the electric current from the power source can flow through the bulbs in multiple (two or more) pathways (see the blue and red highlight in Figure 4).

Third – The number of bulbs in an electric circuit

The number of bulbs added to an electric circuit will determine how bright the bulbs will light up. For bulbs that are added in series, the additional bulb(s) will share the electric current flowing through the circuit hence the bulbs will become dimmer (Figure 5, right).

For bulbs that are added in parallel, the additional bulb(s) will have the same amount of electric current flowing through and each bulb does not share the electric current. Hence, the brightness of bulb remains the same(Figure 6).

Let’s go back to the questions:

Jess has an electric circuit with two batteries and one bulb, bulb X. She decided to add in another bulb, bulb Y (shown in Figure 1) and her mother gave her another bulb, bulb Z, to form the electric circuit in Figure 2.

a) What happens to the brightness of bulb X after adding bulb Y? Explain why?

Bulb X will become dimmer. As bulbs X and Y are arranged in series hence them electric current is shared among the two bulbs and less electric current will flow through bulb X.

Explanation questions can be answered using the A.B.C. answering technique. First, “Answer with a choice” on whether bulb X will be brighter, dimmer or remain the same after adding bulb Y in series. Even though the options are not stated, students should know the possible changes in the brightness of a bulb.

Second, “Back with evidence”, which is writing the relevant information taken from the diagram. It is observed that bulb Y is added in series to bulb X.

Third, Confirm with concept”, to reason why bulb X is dimmer, as there is less electric current flowing through bulb X. Comparison words such as “dimmer” and “less” are important to show the difference between the two electrical circuits, with and without bulb Y.

b) What happens to the brightness of bulbs X and Y after adding bulb Z? Explain your answer?

The brightness of bulbs X and Y will remain the same. As bulb Z is added, parallel to bulbs X and Y hence the amount of electric current flowing through bulbs X and Y remains the same.

Similar to part (a), this explanation question can be answered using the A.B.C. answering technique. First, Answer with a choice” on whether bulbs X and Y will be brighter, dimmer or remain the same after adding bulb Z in parallel.

Second, Back with evidence”, to state the observed information from the diagram that bulb Z is added in parallel to bulbs X and Y.

Third, Confirm with concept”, to reason why the brightness of bulbs X and Y remain the same, as the amount of electric current flowing through bulbs X and Y is still the same.