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Performance (effort/work) is important, but the SPEED at which it is performed is also important.
POWER = WORK / TIME
We measure this in J/s, or Watt (W)
1J per second provides 1 Watt of power
Complete the following table:
Aspect | Measurement |
Step height | |
Mass | |
Weight | |
Work performed: F x s | |
Time |
QUESTIONS
1. What was your power?
2. Explain the following:
a) Kilowatt
b) Kilowatt-hour
3. Find out what the power rating of the average car (your educator's car, for instance) is.
4. Would you be prepared to push your educator's car all the way home? Why not?
5. Why would it be easier for the car engine to do this, even though you might have enough time and help from your friends?
6. Calculate the cost of operating a 3 000 W heater for a month (July) for 4 hours per day at a tariff of R0,35 per kWh.
Assessment of the CALCULATION of POWER CAPACITY
Were you able to apply knowledge and do the calculations?
[LO 2.3; LO 2.4]
Find out what 1 kWh of electricity costs in your municipal area.
Suppose you use the following appliances during the day:
Electrical appliance | Power (kW) |
Computer | |
Television | |
Lamp | |
Kettle | |
Stove | |
Hairdryer |
Now complete the following cost assessment:
APPLIANCE | POWER (kW) as above | TIME Usage(hours) | Energy Usage (kW/h) | Cost @ R1,50 per hour | |
Computer | |||||
Television | |||||
Lamp | |||||
Kettle | |||||
Stove | |||||
Hairdryer |
QUESTIONS
1. What is your deduction?
2. Are you aware of the need to use energy more economically? Give your own opinion of this.
3. Calculate the following:
a) A mountaineer whose mass is 75 kg climbs a 320 m rock face in 20 minutes. Calculate his power.
b) A large cupboard weighing 150 kg has to be carried up a stairway approximately 5 m high during a move.
Assessment of RESEARCH on appliances
Were you able to obtain all the required information and do the calculations?
Could you categorise information?
[LO 1.1; LO 1.2; LO 2.2; LO 2.4]
LO 1: Scientific investigations:
The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, technological and environmental contexts.
This is evident when the learner:
LO 2: Constructing Science Knowledge:
The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.
This is evident when the learner:
2.4 applies knowledge.
Activity
Questions:
Power = Labour (work) Time
Labour = Power (F) × s
Weight = Mass × 10 = Kg
= F × s Time =
2. a) 1000 Watt, in other words, 1 000 × Joule/second
b) The amount of kW that is delivered or consumed in one hour
3. Larger cars approximately 100 kW; smaller cars approximately 70 kW
4. –
5. Engine has greater power / capacity for doing work
6. First calculate total consumption of energy (kWh):
= kW × h (hour)= 3 kW × 124 h= 372 Kw
Then calculate the costs:
= 372 × R0,35= R130,20
Activity
The power per hour of appliances (more of less):
Questions:
1. The greater the power, the higher the cost of using it
2. Debate
3. a) W = F x s= (75 x 10) x 320= 750 x 320= 240 000 J
$\begin{array}{}\text{Power}=\frac{\text{240}\text{000}J}{(\text{20}\times \text{60})}\\ =\frac{\text{240}\text{000}}{1\text{200}}\\ =\text{200}W(\text{or}\mathrm{0,2}\text{lW})\end{array}$
b) W = F x s = (150 x 10) x 5= 7 500 J
One worker's performance = 12,5 2= 6,25 W
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