4ELEC-9

Outcomes

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

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

2.7 use the relationship between energy transferred, current, voltage and time: energy transferred = current × voltage × time E = I × V × t

Starter

Define "Power". Pupils to duscuss in pairs the meaning of power and then feedback to the group. They'll need P=E/t to start with, we'll then link in P=IV and finally E=IVt

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 I against V.

What have students noticed about power of the bulb?

Discuss alternate units (i.e what is a watt?) aiming for joules per second rather than amp volts. See if pupils can combine the two to give an equation with only P,I,V and t

Set up the old skool electricity meter and demo the spinning disk of energy usage (visable inside the meter at the bottom), add more and more load but be aware of the fuse in the extension lead!

Move onto the new(er) meter which will give V,I,P,E,cost etc. readings. Draw a results table on the board with for the various appliances and note down the data you need (you can use I&V to find P, or use E and t to find P, or both and compare, or use I&V to find P then consider typical use time to find typical energy used). Pupils to use the equations to find stuff out and practice using. Maybe throw in some hours for trixiness.

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.

Homework

• Pupils to use their electricity meter to identify the amount of energy used per day.
• Pupils to fill in the worksheet to calcuate various values
• Textbook questions (pg 197 all, 198 7 onwards).

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

Textbook References

Pages 192-193

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 - High voltages.

Notes

Students should be allowed to explore for themselves what happens to the power and brightness of a bulb 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?

Electricity Outline

Electricity