Electrical Circuit
Electricity and Magnetism

Power and brightness

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

This demonstration allows you to compare:

  • The brightness of two bulbs commonly used in the home by measuring this in lux at a set distance from each bulb.
  • The relative effectiveness of the bulbs in shifting energy from the original chemical store (assuming fossil fuel power station) to the light pathway. Do this by dividing the lux reading at a set distance from the bulb by the power rating of the bulb to show the brightness per watt.

What to Prepare

  • 2 retort stands
  • 3 bosses and clamps
  • 1 metre ruler
  • 1 digital light meter
  • 2 mains energy meters
  • 1 thick black card big enough to shield the lamp from the light meter
  • 2 electrical lamp housings
  • 1 incandescent bulb (most commonly used 60 watt) attached to one lamp
  • 1 CFL (compact fluorescent lamp, most commonly used 20 watt) attached to other lamp (look for one which has energy saving bulb written on the bulb or on the packaging box)
  • 1 original CFL packaging box

What Happens During this Activity

This demonstration should be set up in a darkened corner of a room away from direct sunlight, with lights off and normal blinds pulled down if it's a sunny day. Black-out blinds could be employed, if available. The lamps should each be plugged in to a mains meter. Bear in mind that throughout this activity the CFL bulb will need to be on for 1 minute before taking a reading because the bulb does not reach maximum brightness immediately.

Comparing bulbs

The following contains sample data. Your own values are likely to differ and should be used instead. Show the class the two bulbs (in their housings) being used and ask which of the students have each kind in their homes.

Teacher: Why do you think that over the last 10 years or so families have been encouraged to switch to these newer CFL bulbs?

Lydia: Well, I think it is because they last longer and because they use less energy.

Teacher: These are good points. You'll notice on the side of the CFL bulb that the words energy saving bulb is printed. What do you think this means?

(This will be printed on either the bulb or the packaging box.)

Alex: This means that they use less energy.

Teacher: What do you mean by use less energy? Doesn't this really depend on how long you leave the bulb on for?

Alex: Yes it does. I mean if you leave both on for the same time, then the CFL bulb will have used less energy.

Teacher: So what quantity are you really talking about?

Lydia: Power. The CFL bulb shifts energy from one store to another at a lower rate.

Teacher: Good. We can see this from the mains meters.

Show the class that the meter reads approximately 60 watt (57 watt, sample data) for the incandescent bulb and much less for the incandescent bulb (16 watt, sample data).

Teacher: Also think about this statement energy saving. Can we really save energy? And what about Alex saying using energy?

Lydia: Energy is always conserved. I think what is really meant is that since the CFL bulb needs energy at a lower rate, then less fuel is burned in a power station. What is really being saved is the fuel!

Teacher: That's much better. Now, if we're to be persuaded to use the CFL bulbs we need to know that they will give a sufficient amount of light. We'll compare their brightness using a light meter.

Brightness and lux

Show the class the meter and demonstrate how placing it near the bulb will give a reading in lux. There's no need to discuss what this unit represents; just accept that it's a useful way to compare the luminous brightness.

Teacher: We can compare the brightness if we compare the reading in lux on the meter. We do have a problem, however.

Alex: Even without the bulb, the light meter will give a reading. We need to take this away from the reading when the meter is put near the bulb.

Place the light meter at the same distance from each bulb (0.5 metre) ensuring that the meter is aligned with the centre of the bulb. Check the distance with the metre rule. Take a background light meter reading with the bulb shielded from the meter using the black card, then take the reading with the black card removed. The reading from the lamp will be the difference between the two.

It's worth writing these readings on the board: Incandescent bulb produces 408 lux at 0.5 metre; CFL bulb produces 192 lux at 0.5 metre .

Teacher: So what do our results show?

Lydia: That the incandescent bulb is better.

Teacher: Better, but how is it better?

Lydia: The incandescent bulb is giving out more light.

Teacher: That's true, so should we therefore use these bulbs instead of CFL bulbs? Do you think that the difference in brightness as read by the light meter would make a difference to how bright our rooms would be at home?

John: Well in my house we have both types of bulbs and all of our rooms are bright. I don't think it makes much of a difference really!

Teacher: Don't forget that the incandescent bulb is also shifting more energy per second than the CFL bulb.

Luminous output and electrical input

Draw the students' attention again to the mains meters.

Teacher: Wouldn't we expect then that the incandescent would give more light since the meter reads a higher power?

Lydia: I suppose so.

Teacher: I wonder if there is a way in which we could compare the brightness, or the amount of light given out by the bulbs, which also takes into account the amount of energy being shifted, or power?

Alex: I don't know!

Teacher: Let me help you. The incandescent bulb shifts energy at a rate of 56 watt and the bulb gave a reading of 408 lux at 0.5 metre away. We could imagine that, for each 1 watt, 7.3 lux of light is measured (408 lux / 56 watt). So the incandescent bulb gives 7.3 lux of light for each watt or joule second-1. Now, what is the calculation for the CFL bulb?

Alex: For the CFL bulb the power read from the meter was 16 watt and 192 lux was recorded at 0.5 metre. So for each 1 watt the light meter recorded 12.0 lux

Focus attention on these values, perhaps by writing them down: the incandescent bulb produces 7.3 lux inverse watt ; the CFL produces 12.0 lux inverse watt.

Teacher: Now which bulb would we consider to be better?

Lydia: It depends what you mean by better! If we mean which is better at switching power to the lighting pathway then the CFL bulb is better.

Teacher: How many times better?

Lydia: 1.6 times better!

This activity might lead to a discussion about whether or not we're convinced that using a CFL bulb is better. They run at a lower power and are better at radiating. This is surely a strong argument in their favour.

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