High School Physics : Understanding Electricity

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #51 : Electric Circuits

What is the maximum power consumption of \displaystyle 3.0V portable CD player that draw a maximum of \displaystyle 240mA of current?

Possible Answers:

\displaystyle 720W

\displaystyle 0.17W

\displaystyle 80W

\displaystyle 0.72W

Correct answer:

\displaystyle 0.72W

Explanation:

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

 

\displaystyle P = IV

\displaystyle P = (240x10^{-3}A)(3v)

\displaystyle P = 0.72W

Example Question #81 : Electricity And Magnetism

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

 

Possible Answers:

Current

Voltage

Electric potential

Wattage

Correct answer:

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 #53 : Electric Circuits

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

Possible Answers:

Decrease to one fourth its original value

Increase by a factor of two

Decrease to one half its original value

Increase by a factor of four

Correct answer:

Decrease to one half its original value

Explanation:

Decrease to one half its original value

 

The equation for Power is 

 

\displaystyle P = IV

 

According to Ohm’s Law

 

\displaystyle V = IR

 

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

 

\displaystyle I = \frac{V}{R}

 

We can then substitute this back into the power equation.

 

\displaystyle P = (\frac{V}{R})V

 

\displaystyle P = \frac{V^2}{R}

 

If we have the original value

\displaystyle P_1 = \frac{V^2}{R_1}  and our second value \displaystyle P_2 = \frac{V^2}{R_2}

 

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

 

\displaystyle V^2 = P_1R_1\displaystyle V^2 = P_2R_2

\displaystyle P_1R_1 = P_2R_2

 

 

We know that

 

So we can substitute this value in

 

\displaystyle R_2 = 2R_1P_1R_1 =P_22R_1

 

And solve for \displaystyle P_2

 

\displaystyle \frac{P_1}{2} = P_2

 

Therefore the power would be cut in half.

 

Example Question #82 : Electricity And Magnetism

A current of \displaystyle 1.60A flow in a wire.  How many electrons are flowing past any point in the wire per second? 

Possible Answers:

\displaystyle 3.2x10^{12}

\displaystyle 1.6

\displaystyle 3.2x10^{19}

\displaystyle 1.6x10^{12}

\displaystyle 1x10^{19}

Correct answer:

\displaystyle 1x10^{19}

Explanation:

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

 

\displaystyle I = \frac{Q}{t}

\displaystyle 1.60A = \frac{Q}{1s}

\displaystyle Q =(1.60A)(1s)

\displaystyle Q = 1.6C

 

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

 

\displaystyle 1.6C * \frac{1 electron}{1.6x10^{-19}} = 1x10^{19} electrons

Example Question #83 : Electricity And Magnetism

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

Possible Answers:

positive charges would flow

negative charges would flow

Correct answer:

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 #1 : Understanding Electricity

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

Possible Answers:

The current becomes too high

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

The voltage becomes too high

The resistance becomes too high

Correct answer:

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 #1 : Understanding Electricity

A 4.5V battery is connected to a bulb whose resistance is \displaystyle 1.3\Omega. How many electrons leave the battery per minute?

 

Possible Answers:

\displaystyle 4.51x10^{21} electrons

\displaystyle 1.29x10^{21} electrons

\displaystyle 2.46x10^{21} electrons

\displaystyle 3.78x10^{21} electrons

Correct answer:

\displaystyle 1.29x10^{21} electrons

Explanation:

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

\displaystyle V=IR

\displaystyle 4.5V=I(1.3\Omega )

\displaystyle I=3.46A

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

\displaystyle I=Q/t

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

\displaystyle 3.46A=\frac{Q}{60s}

\displaystyle Q=207.7C

We now know the amount of charge passing through in 60 seconds.  We know that the charge of the electron is \displaystyle 1.6x10^{-19}C.  We can use this to figure out how many electrons are going through.

\displaystyle 207.7C*\frac{of electrons}{1.6x10^{-19}C}=1.29x10^{21} electrons

 

Example Question #91 : Electricity And Magnetism

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?

Possible Answers:

Birds have extremely high internal resistance compared to humans

Most birds don’t understand the situation

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

Dangerous current comes from the ground only

Correct answer:

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 #92 : Electricity And Magnetism

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

Possible Answers:

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

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

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

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

Correct answer:

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).

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