Good Afternoon! 04/21/23

•Starter: On holiday lights, why is it that on some strings, when one bulb goes out, they all go out?

•Today we will be learning about the law that governs circuits.

•Also we will be playing with electric Circuits.

Fuses

• 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.

7.37.3Electrical EnergyElectrical Energy

Circuit Breaker • A circuit breaker contains a piece of metal

that bends when the current in it is so large that it gets hot.

• The bending causes a switch to flip and open the circuit, stopping the flow of current.

7.37.3Electrical EnergyElectrical Energy

• Circuit breakers usually can be reset by pushing the switch to its "on" position.

Electric Power

• The reason that electricity is so useful is that electrical energy is converted easily to other types of energy.

• The rate at which electrical energy is converted to another form of energy is the electric power.

7.37.3Electrical EnergyElectrical Energy

Calculating Electric Power

• Electric power can be calculated from the following equation.

• 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.

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:

7.37.3Electrical EnergyElectrical Energy

The Cost of Using Electrical Energy

• The cost of using the appliance can be computed by multiplying the electrical energy used by the amount the power company charges for each kWh.

7.37.3Electrical EnergyElectrical Energy

• For example, if a 100-W lightbulb is left on for 5 h, the amount of electrical energy used is

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.

7.37.3Section CheckSection Check

Question

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.

7.37.3Section CheckSection Check

Question The rate at which electrical energy is converted to another form of energy is called __________.

A. electrical fuseB. electrical switchingC. electric powerD. thermal energy

7.37.3Section CheckSection Check

Answer

The answer is C. Electric power is the rate of energy conversion and is measured in watts.

7.37.3Section CheckSection Check

Question

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

7.37.3Section CheckSection Check

Answer

The answer is A. Electric power is equal to the current, I, multiplied by the voltage difference, V.

Please match the object on the right with the correct symbol

on the left. 

Light Bulb

Battery

Will the light bulb in the circuit below light up?

•No•What could

you do to make the light bulb light up?

•Flip the switch

Which of the following circuits will have the

brightest bulb?

• 3, there are 2 batteries hooked up in series.

1 2

3 3

Which one will not light up?

•2, both wires are connected to the same side of the bulb.

1 2

3 3

Which one will have the dimmest bulb?

•1, there is only one battery.

1 2

3 3

Ohm’s Law

• Voltage (V) # of Batteries

• Current (I) Brightness of bulb(s)

• Resistance (R) # of Lightbulbs

Today we will be exploring relationships between Voltage (V), Current (I) and Resistance

(R)

First let’s explore the relationship between current and voltage

•We will keep the resistance (# of light bulbs) the same and increase the voltage (# of batteries) and then watch what happens to the current (brightness).

First, we will look at a simple circuit with one

battery, and one light bulb.

1.5 V+

-

Now if you add a second battery to one light bulb

what will happen?•Make sure that

you wire the batteries by going from negative of one to the positive of the other.

1.5 V

1.5 V

+

+

Brighter bulbs, Good now what does that mean?

•What is the relationship between Voltage and current?

•When voltage goes up the current goes up, therefore they are proportional.

Now let’s explore the relationship between Current

and Resistance.•So lets keep the voltage constant

(Two batteries) and change the resistance (add light bulbs), and see what the current does.

Now we will

•add a second light bulb

1.5 V

1.5 V

What happened?

• The bulbs got dimmer (current decreased)

• V was the same, R was increased, and the current went down. This means that they are…

• Inversely Proportional.

Putting it all together: so what do we know?

• V & I are proportional (more batteries brighter bulbs)

• I & R are inversely proportional (more light bulbs, dimmer bulbs)

• So V= I•? & ? = I•R, see the connection between these equations?

• V = I•R this governs any path that electrons can flow.

Now finally, we will answer the starter

• It has to do with series and parallel circuits.

•When you buy holiday lights, they will say either series or parallel, one is more expensive but it doesn’t go out when one bulb is broken.

Series (if you have three batteries in you kit, you will need all of them)

Please construct the circuit that you see. • In this circuit

there is only one path so the resistances add up and lower the current.

• What happens when you unscrew one of the light bulbs?

1.5 V

1.5 V

Parallel Please construct the circuit that you see.

• In this circuit V = I•R is applies for every possible circuit. So the resistance of each light bulb doesn’t effect the others.

1.5 V

1.5 V

They all have the same voltage, and this is why they are brighter. Now unscrew one of the bulbs and see

what happens.

So which type of circuit do you need to buy if you don’t

want to be frustrated by light bulbs this holiday

season?•Parallel !