# High School Physics : Circuit Calculations and Concepts

## Example Questions

### Example Question #1 : Understanding Electricity

What is the maximum power consumption of  portable CD player that draw a maximum of  of current?

Explanation:

The power in a circuit is equal to the current times the voltage.

### Example Question #941 : High School Physics

The total amount of charge that passes through a wire’s full cross section at any point per unit of time is referred to as

Wattage

Current

Electric potential

Voltage

Current

Explanation:

Current is defined as the amount of charge that passes through a specific area of a wire in a specific interval of time.  It is measured in Amps which are Coulombs per second.

### Example Question #4 : Electricity

If the resistance in a constant voltage circuit is doubled, the power dissipated by that circuit

Decrease to one fourth its original value

Decrease to one half its original value

Increase by a factor of four

Increase by a factor of two

Decrease to one half its original value

Explanation:

Decrease to one half its original value

The equation for Power is

According to Ohm’s Law

We can rearrange this equation for current as this is what is changing in our circuit.

We can then substitute this back into the power equation.

If we have the original value

and our second value

We can rearrange both for the voltage and set them equal to each other

We know that

So we can substitute this value in

And solve for

Therefore the power would be cut in half.

### Example Question #1 : Understanding Electricity

A current of  flow in a wire.  How many electrons are flowing past any point in the wire per second?

Explanation:

The current is a measure of the amount of charge that passes a given point in a certain amount of time.

To determine how many electrons are passing this point we need to look at the charge of 1 electron and do a conversion.

### Example Question #1 : Electricity

The direction of conventional current is taken to be the direction that __________________ .

negative charges would flow

positive charges would flow

positive charges would flow

Explanation:

We often think of current flowing from the top of the battery to the bottom of the battery.  The top of the battery has a higher electrical potential than the bottom of the battery and is associated as being positive.  Charges interact in such a way where like charges repel and opposite charges attract.  Since we think of the charges traveling away from the positive end of the battery and toward the negative end of the battery, this would model the motion of a positive charge (away from positive and toward negative).  Since it is not protons that move through the circuit, but rather electrons.  It is more accurate to describe the flow of electrons from the negative side of the battery to the positive side.

### Example Question #61 : Electric Circuits

Why might a circuit breaker open if you plug too many electrical devices into a single circuit?

A circuit breaker will not “trip” no matter how many electrical devices you plug into the circuit

The resistance becomes too high

The current becomes too high

The voltage becomes too high

The current becomes too high

Explanation:

When plugin objects into a single circuit, these objects are connected in parallel as each one will receive the 120V from the electrical outlet. However, as additional objects are added, the current is thereby increased. Circuit breakers are designed to trip once the current reaches a maximum load and shuts down the circuit to protect the wires, and the devices that are plugged into the circuit as high current can damage these devices.

### Example Question #2 : Understanding Electricity

A 4.5V battery is connected to a bulb whose resistance is . How many electrons leave the battery per minute?

electrons

electrons

electrons

electrons

electrons

Explanation:

We first need to determine the current coming through the bulb. We can use Ohm’s law to determine this.

Current by definition is the amount of charge per unit time.

We are looking at the number of electrons in one minute which is 60 seconds.

We now know the amount of charge passing through in 60 seconds.  We know that the charge of the electron is .  We can use this to figure out how many electrons are going through.

### Example Question #3 : Understanding Electricity

Nothing happens when birds land on a power line, yet we are warned not to touch a power line with a ladder. What is the difference?

Most birds don’t understand the situation

Birds have extremely high internal resistance compared to humans

Dangerous current comes from the ground only

There is little to no voltage drop between a bird's two feet, but there is a significant voltage drop between the top of a ladder touching a power line and the bottom of the ladder on the ground

There is little to no voltage drop between a bird's two feet, but there is a significant voltage drop between the top of a ladder touching a power line and the bottom of the ladder on the ground

Explanation:

Electricity travels from the point of highest potential to the lowest potential. We measure the difference between these two potentials as the voltage. Since both of the birds' feet are on the wire, both feet have the same potential. However, if the ladder touches the power line and the bottom of the ladder is on the ground, there is a much higher potential difference (as the ground is at 0 potential).  Therefore the electricity will travel down the ladder!

### Example Question #4 : Understanding Electricity

When a light switch is turned on, the light comes on immediately because

The lightbulb may be old with low resistance. It would take longer if the bulb were new and had high resistance

The electrons coming from the power source move through the initially empty wires very fast

The electrons already in the wire are instantly “pushed” by the voltage difference

The electricity bill is paid. The electric company can make it take longer when the bill is unpaid

The electrons already in the wire are instantly “pushed” by the voltage difference

Explanation:

It's kind of like when you turn on the faucet; the water doesn't have to travel from the water tower to your house before it starts flowing. Similarly, the electrons are always in the circuit. Once the switch is turned on, there is a voltage difference present, which is what then pushes these electrons through the circuit (like turning on the faucet).

### Example Question #1 : Kirchoff's Laws

Calculate the current through the 10 ohm resistor.

Explanation:

To begin, let us start with the resistor and the  resistor that are in parallel. In parallel we can add resistors through the equation

This new resistor is now in series with the two  resistors.  In series we can just add these resistors up.

This new resistor is now in parallel with the  resistor. In parallel we can add resistors through the equation

This new resistor is now in series with the  and  resistor.  In series we can just add these resistors up.

Now that we have the equivalent resistance of the circuit.  We can now determine the current flowing out of the battery.

Rearrange to solve for current.

One of the best ways to work through a problem like this is to create a V= IR chart for all the components of the circuit.

We know that the current that is flowing out of the battery is the current that is flowing through both by 5 and 3 Ohm resistors since all of these are in series.  So we can put this information into our chart.

Using Ohm’s Law we can determine the voltage for each of these two resistors.

We can now use Ohm’s law and our voltage to determine the current going through the 14 Ohm resistor.

Next we can analyze the current in the Junction between the 5, 14 and 4 Ohm resistors.  Kirchoff’s laws state that the sum of the current flowing in and out of junction must equal 0.

We have  flowing in from the  resistor and  flowing out to go through the  resistor.

So the current going through the 4 Ohm resistor is 3 amps.  This will be the same for both 4 ohm resistors as both have the same current going into and out of the junctions near them.  We can then use this to determine the voltage drop across each of the resistors.

Now let’s add this to our chart.

We can now use Kirchoff’s loop law to determine the voltage across the 10 Ohm resistor.  Let’s analyze the loop that goes from the 5, to the 4 to the 10 back through the 4 and then through 3 Ohm resistor.

We can now use Ohm’s Law to determine the current through the 10 Ohm resistor.