3-ELE-06

Outcomes

• Pupils can use the equation P=IV to find out how that power of a light bulb changes with V change.

Specification References

2.6 recall and use the relationship, power = current x voltage, P=IV and apply the relationship to the selection of appropriate fuses

Starter

• Demo Joule meter
• Can use the power results table flipchart to record the results (available in Science Shared)
• Use the various modes on the meter to record the energy used, time taken, power and voltage
• Use these values to calculate the power and find the fuse needed for the appliance

Main Body of Lesson

• Power output of a bulb

Intro to practical: Use an ammeter and voltmeter to measure I and V of a bulb while changing V. Will need to show a clear circuit diagram to help them set up. (Could use variable supply power supply with set voltages 2V to 12V instead of a voltmeter. No need then for voltmeter or variable resistor.)

Record values of V and I and note the brightness of the bulb each time. Leave a column to calculate Power = IV each time.

Plot V against I.

What have students noticed about power of the bulb?

Plenary

Conclusion : Power of the bulb is only as written on the side when the voltage across it is that stated on the bulb.

Extension: Could talk about what the area under the V against I graph represents: Power.

Additional Information

Resources Required

12V bulbs 2V to 12V variable dc power supply or 12V dc supply and variable resistor. Ammeter Voltmeter (if not using pre-set variable power supply) graph paper

Skills Addressed

Observation and recording of results. Graph plotting.

Safety/Hazards

Bulbs can get warm. Do not supply power supply greater than 12V or bulbs will blow.

Notes

Students should be allowed to explore for themselves what happens to the power and brightness of a buld as the voltage changes. Can link conclusion back to ideas of:

• higher voltage = more joules per coulomb converted by bulb to light and heat.
• higher voltage leads to greater current, so power increases.

Extension: Would the power go on increasing as voltage increases? Why?