1. Electricity
7-3 Circuits

2. Electrical Circuits
Circuits rely on generators at power plants to produce a voltage difference across the outlet, causing the charge to move when the circuit is complete

3. Example Hair dryer plugged into wall outlet
Generator at power plant produces voltage difference
Dryer and circuit in house contain conducting wires to carry current
Hair dryer turns electrical energy into thermal energy and mechanical energy
Unplug hair dryer opens circuit and breaks path of current

4. The Current in a Simple Circuit
A simple electric circuit contains a source of voltage difference, such as a battery, a device, such as a lightbulb, that has resistance, and conductors that connect the device to the battery terminals.
When the wires are connected to the battery terminals, current flows in the closed path.

5. Circuit Components
A - battery C - light bulb
B - switch D - resistor

6. Types of Circuits
Series circuit – the current has only one loop to flow through.
Series circuits are used in flashlights and some holiday lights.

7. Series Circuit Current travels in a single path
Current is the same throughout circuit
lights are equal brightness
Each device receives a fraction of the total voltage
- gets dimmer as lights are added

8. Series Circuit
Open circuit – if any part of a series circuit is disconnected, no current flows through the circuit
Example: holiday lights

9. Series Circuit

10. Holiday Lights – Series Circuit

11. Parallel Circuits
Parallel circuit – contains two or more branches for current to move through.
The current can flow through both or either of the branches.
Example: household circuits

12. Parallel Circuit

13. Parallel Circuits Advantage
When one branch of the circuit is opened, such as when you turn a light off, the current continues to flow through the other branches.
Click box to play movie.

14. Household Circuits
Household circuits mainly use parallel circuits connected in a logical network.
Electrical energy enters your home at the circuit breaker or fuse box and branches out to wall sockets, major appliances, and lights.

16. Guards against overheating electric wires:
Electric fuse – contains a small piece of metal that melts if the current becomes too high, opening the circuit and stopping the flow of current.
To enable current to flow again in the circuit, you must replace the blown fuse with a new one.
Too many appliances in use at the same time is the most likely cause for the overheating of the circuit.

17. Household Fuse Box

18. Guards against overheating electric wires:
Circuit breaker – contains a small piece of metal that bends when it gets hot, opening the circuit and stopping the flow of current.
Circuit breakers usually can be
reset by pushing the switch to
its “on” position.

19. Circuit Breaker Box

20. Electric Power
Electrical energy is easily converted to mechanical, thermal, or light energy.
Electrical power – the rate at which electrical energy is converted to another form of energy.

21. P = I × V Electrical Power P = power (W) I = current (A)
rate at which electrical energy is converted to another form of energy
P = power (W)
I = current (A)
V = potential (voltage) difference (V)
P = I × V

22. Electrical Power Electrical power is expressed in watts (W)
The unit for power is the watt (W). Because the watt is a small unit of power, electric power is often expressed in kilowatts (kW).
One kilowatt equals 1,000 watts.
Power =
current x voltage difference

23. Electrical Power P (watts) = I (amperes) x V (volts)
To calculate the amount of energy an appliance uses:
The unit of electrical energy is the kilowatt-hour, which equals 1000 watts of power used for one hour

24. Electrical Power
Energy = power x time
E (kWh) = P (kW) x t (h)

25. Electrical Power V P I I = 0.01 A P = I · V V = 9 V P = (0.01 A) (9 V)
A calculator has a 0.01-A current flowing through it. It operates with a potential difference of 9 V. How much power does it use?
GIVEN:
I = 0.01 A
V = 9 V
P = ?
WORK:
P = I · V
P = (0.01 A) (9 V)
P = 0.09 W P I V

26. Electrical Energy
Electric companies charge by the amount of electrical energy used, rather than by the electric power used.
Electrical energy usually is measured in units of kilowatt hours (kWh) and can be calculated from this equation:

27. E = P × t Electrical Energy E: energy (kWh) P: power (kW) t: time (h)
energy use of an appliance depends on power required and time used
E: energy (kWh)
P: power (kW)
t: time (h)
E = P × t

28. The Cost of Using Electrical Energy
The cost of using some household appliances is given in this table, where the cost per kWh is assumed to be $0.09/kWh.

29. Electrical Energy t E P P = 700 W = 0.7 kW E = P · t
A refrigerator is a major user of electrical power. If it uses 700 W and runs 10 hours each day, how much energy (in kWh) is used in one day?
GIVEN:
P = 700 W = 0.7 kW
t = 10 h
E = ?
WORK:
E = P · t
E = (0.7 kW) (10 h)
E = 7 kWh E P t

30. Question 1 Answer What is an open circuit?
Section Check
Question 1
What is an open circuit?
Answer
An open circuit is a series circuit in which one part is disconnected. This prevents current from flowing through the circuit.

31. Section Check 3
Question 2
The rate at which electrical energy is converted to another form of energy is called __________.
A. electrical fuse
B. electrical switching
C. electric power
D. thermal energy

32. Section Check 3
Question 3
Which of the following equations is used to calculate electric power?
A. P = IV
B. P = I/V
C. P = V/I
D. P = AV

33. Section Check 3
Answer
The answer is A. Electric power is equal to the current, I, multiplied by the voltage difference, V.

34. Why do homes have a fuse box or circuit breaker?
It is there to make sure the electrical wires in our home do not get too hot and start fires. If the wires get too hot, they melt the piece of metal in the fuse or bend the piece of metal in the circuit.

35. How does it work?
This opens the circuit and stops the flow of current. If we have a fuse box, we have to replace the melted fuse with a new one. If we have a circuit breaker, we only need to unplug some appliances and flip the switch in the breaker box.