1. Aim: How do we determine the power in a circuit?
HW: Problem Set

2. Ohm’s Law Applied
A 10 volt battery is connected to a circuit, allowing 5 amperes of current flow. Calculate the resistance of the circuit.

3. What do electrons do as they move?
As electrons travel through circuits they do
WORK – as they do this they generate
FRICTION and HEAT
HEAT is just a type of ENERGY!
Power is a RATE of ENERGY usage

4. P=IV Electrical Power CURRENT VOLTAGE POWER LAW
The POWER built up in a circuit depends on:
How many ELECTRONS are flowing through it…
How much ENERGY each electron has…
CURRENT
VOLTAGE
MORE current means more electrons producing
FRICTION  MORE power
MORE voltage means we push electrons
HARDER  MORE power
POWER
LAW
P=IV
Measured in:
WATTS

5. Example #1
How much current is drawn by a 100W light bulb when it is connected to a 120V power supply?
P = IV
100 W = I (120 V)
I = A

6. Example #2
How long does it take for a circuit with a 500 volt source to use 500 joules of energy at a current of 10 amperes?
P = IV
P = (10 A)(500 V)
P = 5000 W
P = W / t
5000 W = (500 J) / t
t = 0.1 s

7. Combine Ohm’s Law and Power Law Recall earlier equation for power…
Power Equations
Combine Ohm’s Law and Power Law
Recall earlier equation for power…
Relate to POWER Rangers… get it? Because they are POWER and morph… like equations… yea.

8. Station Activity
Go to the assigned station and answer the questions involving power.

9. Station #1
What is the power generated in a circuit with a resistance of 2.5 ohms if it is connected to a 5 volt battery?
P = V2 / R
P = (5 V)2 / 2.5 Ω
P = 10 W

10. Station #2
What is the power generated in a circuit with a resistance of 300 ohms if it conducts 2 amps of current?
P = I2R
P = (2 A)2 (300 Ω)
P = 1200 W or 1.2 kW

11. Electric Energy

12. A QUICK REMINDER Power is the rate at which WORK is done per unit TIME
Using the equation above, WORK will equal…

13. MAKING CONNECTIONS

14. Envelope Activity
Answer the following questions inside your envelopes regarding electrical energy.

15. EXAMPLE #1
If an appliance is plugged into a 120V outlet and draws 10 amperes of current for ten minutes, how much energy does the appliance use?

16. EXAMPLE #2
Determine the resistance of an MP3 player that uses 10,000J of energy in 15 minutes when on a 9 volt battery.

17. EXAMPLE #3
How much current goes through a 10 ohm resistor when it uses 54 Joules of energy in one minute?

18. SUMMARY
1. How is electrical energy related to power? 2. Provide 2 examples from previous problems. 3. How do we measure current and potential difference in a circuit?