Which Circuit Runs a Battery Down Quickest?

Imagine three circuits.

1. One bulb.
2. Two bulbs in series.
3. Two bulbs in parallel.

The batteries are the same in each circuit. In what order do the batteries run down? (If it matters to you, assume that the batteries all have negligible internal resistance and that the resistance of the bulbs doesn't change with current).

This is the correct order

Circuit 3 (parallel) runs down after, say 1 hr
Circuit 1 (single) runs down after 2 hours
Circuit 2 (series) runs down after 4 hours

The times assume (incorrectly) that the batteries provide a constant voltage before suddenly dropping to zero.

Remember that each of the parallel bulbs are as bright as the single bulb. So the two bulbs in parallel demand energy from the battery twice as quickly as one alone. That’s why the parallel circuit runs down first.

It might be tempting to believe that the series circuit runs down at the same time as the single bulb. This would assume that each series bulb was half as bright as one alone. But in fact each of the series bulbs is a quarter as bright. Brightness depends on how quickly energy is transferred, in other words, power.

Power depends on both current (how quickly charges are arriving) and voltage (how much energy each charge transfers). In the series circuit the current round the circuit is halved and the voltage is shared between two bulbs, so energy is transferred to each bulb only a quarter as quickly as if it were by itself.

In this simulation you can drag a power meter to different parts of a series and parallel circuit to see how the different brightnesses can be explained.

The Difference Between Irradiation and Contamination

It’s important to distinguish between ’irradiation’ and ’contamination’.

Contamination involves radioactive material, like dust, moving from one place to another. If radioactive dust settles on your skin then we say your skin has been ’contaminated’. The radioactive nuclei in the dust give off radiation (like alpha or beta particles) all the time. When these hit your skin we say the skin has been ’irradiated’.

Alpha, beta and even gamma radiation have only a limited range in air, perhaps up to a few tens of metres. But radioactive dust can be blown for hundreds or even thousands of miles.

This animation explains the difference between contamination and irradiation.

Why Nuclear Radiation Isn’t Infectious

A key misconception that many students have is that nuclear radiation has the same properties as a disease, in that it can 'infect' anything it touches.

But nuclear radiation doesn't cause other atoms to become radioactive. This is why any risks from irradiating food are to do with chemistry and biology rather than with radioactivity.

Here are some activities about irradiating food.

What Makes a Good Electric Circuit Analogy?

There are two key ideas that you need to grasp when trying to understand electric circuits.

There's something that moves and there's something that batteries run out of, and they're different.

The things that move are charges. They are already there everywhere in the circuit and they all start moving very slowly everywhere at the same time as soon as the circuit is connected. Charges aren’t lost, they just go round and round.

The stuff that batteries run out of is energy. Energy is transferred from the battery to the components in the circuit almost instantly. Though the mechanism for this transfer is very complex, it can help to imagine that each charge carries with it some energy, which it transports from the battery to the components.

This model of individual charges carrying energy, though not strictly correct, makes it easier to understand qualitatively the ideas of current, voltage and power.

Our interactive circuit animation models charges as black blobs and energy as a red glow.