# Energy Changes

## Energy Changes Revision

**Energy Changes**

When heating up an object, **thermal energy** is transferred into the object. This causes the object’s **temperature** to increase. For example, when heating up a pan of soup over a flame, the thermal energy of the flame is transferred to the soup. As the thermal energy in the soup increases, the temperature of the soup increases.

We can calculate how much **energy** is required to heat up different objects by a specific amount using the concepts discussed on this page.

**Specific Heat Capacity **

Different materials need different amounts of **energy** to increase their **temperature**. The property that determines this is called the **specific heat capacity**.

The specific heat capacity is represented by a lower-case c and is measured in \text{J/kg} \degree \text{C}.

Materials with a high specific heat capacity need a lot of thermal energy to increase their temperature. Materials with a low specific heat capacity need less thermal energy to increase their temperature.

Some examples of specific heat capacities are shown in the table below.

Material |
Specific Heat Capacity \text{(J/kg}\degree \text{C)} |

Water | 4200 |

Aluminium | 900 |

Ice | 200 |

**Heat Change Calculations**

To calculate the amount of thermal energy needed to increase the temperature of an object by a given amount, we can use the following equation:

\Delta E=mc\Delta\theta

- \Delta E = the change in
**thermal energy**in joules \text{(J)} - m = the
**mass**of the object in kilograms \text{(kg)} - c = the
**specific heat capacity**of the material in joules per kilogram per degree (\text{J/kg} \degree \text{C}) - \Delta \theta = the
**change in temperature**in degrees centigrade or kelvin (\degree \text{C or K)}.

**Required Practical**

**Investigation into Specific Heat Capacity **

**Doing the experiment**

- First, measure the mass of the block using a
**balance**. - Place the block in a container, surrounded by insulating material (such as bubble wrap) to reduce heat losses from the block.
- Place a
**thermometer**into the container and then connect a**power supply**,**heater**and**ammeter**to the block. This is shown in the diagram below. - Record the
**starting temperature**of the block using the thermometer. - Turn the power supply on and start a
**stopwatch**at the same time. - Record the
**current**\text{(I)} from the ammeter and the**potential difference**\text{(V)} from the power source. - After 10 \text{ minutes}, measure the
**final temperature**of the block using the thermometer.

**Calculating the specific heat capacity**

- Calculate the
**change in temperature**using \Delta\theta=\theta_{final}-\theta_{start} - Find the
**power**of the heater using the equation P=VI ( rearranged to V= potential difference, I= current). - The
**energy transferred**from the power source to the heater during the experiment is given by E = Pt (P = the power of the heater, t = the time the heater was switched on =10 \text{ mins or } 600 \text{ s}). - If we assume that
**no heat is lost**to the surroundings (because the block was insulated), then we know that this is the amount of**energy**transferred to the block. Hence E = \Delta E. - By rearranging \Delta E=mc\Delta\theta, we find that c = \dfrac{\Delta E}{m\Delta\theta}.
- Substitute the value of E that you calculated in step 4 and the value of \Delta \theta that you calculated in step 1.

**Example: Heat Capacity Calculation**

A student carries out an investigation on an unknown material called material X. To calculate its specific heat capacity, the student follows the method described in required practical section of this page. The student records their results in the table below.

Calculate the specific heat capacity of material X.

**[4 marks]**

## Energy Changes Example Questions

**Question 1: **Material A has a specific heat capacity that is two times bigger than material B. When materials A and B are placed in an oven, which material’s temperature increases faster?

**[1 mark]**

**Material B**.

A two times bigger heat capacity means that it takes two times as much energy to raise the temperature of the material by a given amount.

**Question 2: **A kettle containing 0.5\text{ kg} water heats the water from 22 \degree C to 50 \degree C? How much thermal energy is transferred to the water?

The specific heat capacity of water is 4200 \text{ J/kg} \degree C.

**[3 marks]**

**Question 3: **Describe an experiment you could use to determine the specific heat capacity of a block of steel.

**[6 marks]**

**Measure the mass** of the block of steel using a balance. Then, **wrap the steel block in bubble wrap** to reduce heat loss and place it in a container. Place a thermometer into the container and then connect a power supply, **heater** and **ammeter** to the container.

**Record the starting temperature** of the steel block using the thermometer then turn the power supply on and start a stopwatch at the same time. **Record the current and potential difference**. After 10 \text{ minutes}, **measure the final temperature** of the block using the thermometer.

**Question 4: **500\text{ J} of energy is transferred to a plastic block of mass 5\text{ kg} in order to raise the temperature of the block from 19 \degree C to 24 \degree C. Calculate the specific heat capacity of the plastic.

**[3 marks]**