Testing understanding: adding lamps in series

What the Activity is for

The diagnostic questions can be used to check the pupils' understanding of key ideas introduced in this episode.

What to Prepare

• printed copies of these four questions

Support sheet

What Happens During this Activity

The questions might be used for homework or as the basis for discussion in class.

The What happens to the current? question probes the effect on the electric current of adding a bulb to a circuit. When the extra bulb is added:

• The current in the circuit gets less, but does not fall to zero.
• The battery cannot push as big a current through two bulbs.

We find that many pupils incorrectly select the fifth option to explain what happens (the current is shared between the two bulbs, so each gets half). While it is acceptable to say that the energy is shared, it does not make sense to say that the current is shared.

This answer suggests that these pupils may not have separated in their minds the two distinct ideas of current and energy.

The Both ammeters question probes the effect on the electric current of adding resistance to a circuit. When the large resistance is placed in the circuit:

• The reading on ammeter A1 gets smaller.
• The reading on ammeter A2 gets smaller.
• This is because increasing the resistance makes the current smaller everywhere in the circuit.

The Two bulbs question probes the effect on bulb brightness of adding a second bulb to a circuit. When the extra bulb is added:

• Both bulbs are lit with the same brightness as each other.
• Energy is dissipated equally in the two bulbs as the charge passes around the circuit and the electric current is reduced everywhere in the circuit.

The Bright and dim question probes the pupils' understanding of what happens in a circuit with two non-identical bulbs when the current is reversed.

• The bulbs are the same as before. Bulb 1 is bright. Bulb 2 is dim.
• After turning the battery around, the current is the same (but reversed in direction) and energy is shared between the two bulbs as in the original circuit.

Some pupils may think that when the current is reversed, the brightness of the bulbs also changes over. Some pupils will be interested to talk through why bulb 1 is brighter (it must have a bigger resistance, so the rate at which energy is shifted is bigger in bulb 1).