Question

Chargers for cell phones get warm while they are being used. Some of the energy that they are using is being used to power the cell phone and the rest is wasted as heat. If a cell phone battery needs \(235 \mathrm{~J}\) of energy and \(345 \mathrm{~J}\) are wasted as heat, how many joules are required to charge the cell phone?

Short answer

580 J are required to charge the cell phone.

Step-by-step solution

Understand the Problem

The problem gives us two different energy quantities: the energy required to power the cell phone and the energy wasted as heat. To find the total energy needed to charge the cell phone, we need to sum these quantities.

Define the Variables

Let's define the variables: Let E_cell be the energy needed to charge the cell phone battery, which is 235 J, and E_heat to be the energy wasted as heat, which is 345 J.

Set Up the Equation

To find the total energy required, E_total, we add the energy for the battery and the energy wasted: E_total = E_cell + E_heat.

Calculate the Total Energy Required

Substitute the given values into the equation: E_total = 235 J + 345 J = 580 J.

State the Final Answer

The total amount of energy required to charge the cell phone is 580 J.

Key concepts

Energy Transfer

Energy transfer is a key principle in physics that refers to the movement of energy from one place to another or from one form to another. For example, when charging a cell phone, electrical energy from the charger is transferred to the cell phone battery. This process is not 100% efficient, resulting in some energy being transformed into heat energy, which is felt as warmth in the charger.

During energy transfer, the total amount of energy remains constant as per the law of conservation of energy; energy cannot be created or destroyed, only transformed. So, in our context, the electric energy converts partially into the chemical energy stored in the battery and partially into heat energy, which is an unintended byproduct.

Joules

In physics, energy is measured in units called joules (symbol: J). A joule is defined as the amount of work done when a force of one newton displaces an object by one meter in the direction of the force. It's a small unit, so activities like charging a phone involve hundreds or thousands of joules.

In the cell phone charging example, we speak of 235 J of energy needed for the battery and 345 J dissipated as heat. The use of joules provides a standardized way to quantify and compare different forms of energy transfer.

Heat Energy

Heat energy, also known as thermal energy, relates to the internal motion of particles within an object. The faster these particles move, the warmer the object feels. It's a form of kinetic energy because it results from movement at the microscopic level.

In the context of our cell phone charging, 345 J of electrical energy is transferred as heat instead of being used to charge the cell phone. It's essential to understand that this isn't 'lost' energy but energy that has changed form and is no longer useful for the charging process.

Electric Energy

Electric energy refers to the power derived from the flow of electric charge through a conductor, like the electricity that runs through a charging cable to your cell phone. We use electric energy for countless tasks every day, each time measuring the consumption in joules or kilowatt-hours for larger scales.

When this energy is provided to a cell phone charger, part of it is used efficiently to charge the battery (235 J in our case), while the rest may be converted to other forms like heat energy due to resistance and inefficiencies in the electronic components.

Thermodynamics

Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. The concepts within thermodynamics help us understand how energy is converted and transferred, including the reasons behind energy loss as heat during processes.

For our cell phone example, thermodynamics principles tell us why all the electrical energy isn't converted into chemical energy in the battery. Due to the second law of thermodynamics, no process is perfect, and some energy will always be dissipated, often in the form of heat, as is seen with the 345 J wasted while charging the phone.