Content

• Steps
• Method 1 of 5: Calculating Work (J)
• Method 2 of 5: Calculate Energy (J) from a Given Power (W)
• Method 3 of 5: Calculating Kinetic Energy (J)
• Method 4 of 5: Calculating the Amount of Heat (J)
• Method 5 of 5: Calculating Electrical Energy (J)
• Tips
• Warnings
• What do you need

The joule (J) is one of the most important units in the International System of Units (SI). Joules measure work, energy and heat. To represent the final result in joules, use SI units.If other units of measurement are given in the task, convert them to units of measurement from the International System of Units.

Steps

Method 1 of 5: Calculating Work (J)

1. 1 The concept of work in physics. If you move the box, then you get the job done. If you lift the box, then you have done the job. For the work to be done, two conditions must be met:
   • You are applying constant force.
   • Under the action of the applied force, the body moves in the direction of the action of the force.
2. 2 Calculate the work. To do this, multiply the force and the distance (by which the body moved). In SI, force is measured in newtons and distance in meters. If you use these units, the resulting work will be measured in joules.
   • When solving problems, determine the direction of the applied force. When lifting the box, the force is directed from the bottom up, but if you take the box in your hands and walk a certain distance, then you will not do the work - you are applying force so that the box does not fall, but this force does not move the box.
3. 3 Find your body weight. It is needed to calculate the force that needs to be applied to move the body. Consider an example: calculate the work done by an athlete when lifting (from floor to chest) a barbell weighing 10 kg.
   • If the problem contains non-standard units of measurement, convert them to SI units.
4. 4 Calculate the strength. Force = mass x acceleration. In our example, we take into account the acceleration of gravity, which is equal to 9.8 m / s. The force that needs to be applied to move the bar up is 10 (kg) x 9.8 (m / s) = 98 kg ∙ m / s = 98 N.
   • If the body moves in a horizontal plane, ignore the acceleration due to gravity. Perhaps the task will require calculating the force required to overcome friction. If acceleration is given in the problem, simply multiply it by the given body mass.
5. 5 Measure the distance traveled. In our example, let's say that the bar is lifted to a height of 1.5 m. (If non-standard units of measurement are given in the problem, convert them to SI units.)
6. 6 Multiply the force by the distance. In order to lift a barbell weighing 10 kg to a height of 1.5 m, the athlete will perform work equal to 98 x 1.5 = 147 J.
7. 7 Calculate the work when the force is directed at an angle. The previous example was quite simple: the directions of force and movement of the body coincided. But in some cases, the force is directed at an angle to the direction of travel. Consider an example: calculate the work done by a child pulling a sled 25 m with a rope that is 30 degrees off the horizontal. In this case work = force x cosine (θ) x distance. The angle θ is the angle between the direction of the force and the direction of movement.
8. 8 Find the total applied force. In our example, let's say that the child applies a force equal to 10 N.
   • If the problem says that the force is directed upwards, or to the right / left, or its direction coincides with the direction of movement of the body, then to calculate the work, simply multiply the force and the distance.
9. 9 Calculate the corresponding force. In our example, only a fraction of the total force pulls the sled forward. Since the rope is directed upward (at an angle to the horizontal), another part of the total force is trying to lift the sled. Therefore, calculate the force, the direction of which coincides with the direction of movement.
   • In our example, the angle θ (between the ground and the rope) is 30º.
   • cosθ = cos30º = (√3) / 2 = 0.866. Find this value using a calculator; set the angle unit in the calculator to degrees.
   • Multiply the total force by cosθ. In our example: 10 x 0.866 = 8.66 N - this is a force whose direction coincides with the direction of movement.
10. 10 Multiply the corresponding force by the distance to calculate the work. In our example: 8.66 (H) x 20 (m) = 173.2 J.

Method 2 of 5: Calculate Energy (J) from a Given Power (W)

1. 1 Power and energy. Power is measured in watts (W) and describes the rate of change, conversion, transmission or consumption of energy, which is measured in joules (J).To calculate the energy (J) for a given power (W), you need to know the length of time.
2. 2 To calculate energy (J), multiply power (W) by time (s). A device with a power of 1 W consumes 1 J of energy for every 1 s. For example, let's calculate the energy consumed by a 60 W bulb for 120 seconds: 60 (W) x 120 (s) = 7200 J
   • This formula is correct for any power measured in watts, but is most often used in tasks involving electricity.

Method 3 of 5: Calculating Kinetic Energy (J)

1. 1 Kinetic energy is the energy of motion. It can be expressed in joules (J).
   • Kinetic energy is equivalent to the work done to accelerate a stationary body to a certain speed. Having reached a certain speed, the kinetic energy of the body remains constant until it is converted into heat (from friction), gravitational potential energy (when moving against gravity), or other types of energy.
2. 2 Find your body weight. For example, calculate the kinetic energy of a bicycle and a cyclist. The cyclist weighs 50 kg and the bike weighs 20 kg, which means the total body weight is 70 kg (consider the bike and the cyclist as a single body, since they will move in the same direction and at the same speed).
3. 3 Calculate the speed. If speed is given in the problem, go to the next step; otherwise, calculate it using one of the methods below. Note that the direction of speed is negligible here; moreover, suppose the cyclist is driving in a straight line.
   • If the cyclist was riding at a constant speed (no acceleration), measure the distance traveled (m) and divide it by the time (s) taken to cover this distance. This will give you average speed.
   • If the cyclist was accelerating, and the value of acceleration and the direction of movement did not change, then the speed at a given time t is calculated by the formula: acceleration x t + initial speed. Time is measured in seconds, speed in m / s, acceleration in m / s.
4. 4 Plug in the values ​​into the formula. Kinetic energy = (1/2) mv, where m is mass, v is velocity. For example, if a cyclist's speed is 15 m / s, then his kinetic energy K = (1/2) (70 kg) (15 m / s) = (1/2) (70 kg) (15 m / s) (15 m / s) = 7875 kg ∙ m / s = 7875 N ∙ m = 7875 J
   • The formula for calculating the kinetic energy is derived from the definition of work (W = FΔs) and the kinematic equation (v = v₀ + 2aΔs, where Δs is the distance traveled).

Method 4 of 5: Calculating the Amount of Heat (J)

1. 1 Find the mass of the heated body. To do this, use a balance or spring scale. If the body is a liquid, first weigh the empty container (into which you will pour the liquid) to find its mass. After weighing the liquid, subtract the mass of the empty container from this value to find the mass of the liquid. For example, consider water weighing 500 g.
   • For the result to be measured in joules, the mass must be measured in grams.
2. 2 Find the specific heat of the body. It can be found in a chemistry, physics textbook, or on the internet. The specific heat capacity of water is 4.19 J / g.
   • Specific heat varies slightly with temperature and pressure. For example, in some sources the specific heat capacity of water is 4.18 J / g (since different sources choose different values ​​of the "reference temperature").
   • Temperature can be measured in degrees Kelvin or Celsius (since the difference between the two temperatures will be the same), but not in degrees Fahrenheit.
3. 3 Find your starting body temperature. If the body is liquid, use a thermometer.
4. 4 Heat the body and find its final temperature. This way you can find the amount of heat transferred to the body when it is heated.
   • If you want to find the total energy converted to heat, consider the initial body temperature to be absolute zero (0 Kelvin or -273.15 Celsius). This usually does not apply.
5. 5 Subtract the starting body temperature from the ending temperature to find the change in body temperature. For example, water is heated from 15 degrees Celsius to 35 degrees Celsius, that is, the change in water temperature is 20 degrees Celsius.
6. 6 Multiply the body weight, its specific heat, and the change in body temperature. Formula: H = mcΔT, where ΔT is the change in temperature. In our example: 500 x 4.19 x 20 = 41.900 J
   • Heat is sometimes measured in calories or kilocalories. Calories are the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius; kilocalories is the amount of heat required to raise the temperature of 1 kg of water by 1 degree Celsius. In the example above, it would take 10,000 calories or 10 kcal to raise the temperature of 500 grams of water by 20 degrees Celsius.

Method 5 of 5: Calculating Electrical Energy (J)

1. 1 This describes a method for calculating the flow of energy in an electrical circuit. A practical example is given on the basis of which one can solve physical problems. To begin with, let's calculate the power according to the formula P = I x R, where I is the current strength (A), R is the resistance (Ohm). You will find the power (W) with which you can calculate the energy (J) (see second chapter).
2. 2 Take a resistor. The resistance value (Ohm) of the resistor is indicated by a number or color-coded marking. You can also determine the resistance of the resistor by connecting it to an ohmmeter or multimeter. For example, let's take a 10 ohm resistor.
3. 3 Connect the resistor to the current source. To do this, use crocodile clips or an experimental stand with an electrical circuit.
4. 4 Pass a current through the circuit for a certain time. For example, do this for 10 seconds.
5. 5 Determine the amperage. To do this, use an ammeter or multimeter. For example, the current is 100 mA = 0.1 A.
6. 6 Calculate the power (W) using the formula P = I x R. In our example: P = 0.1 x 10 = 0.01 x 10 = 0.1 W = 100 mW
7. 7 Multiply power and time to find energy (J). In our example: 0.1 (W) x 10 (s) = 1 J.
   • Since 1 joule is a small value, and the power of electrical appliances is indicated in watts, milliwatts and kilowatts, in the housing and communal sector, energy is usually measured in kilowatt-hours. If 1 W = 1 J / s, then 1 J = 1 W ∙ s; if 1 kW = 1 kJ / s, then 1 kJ = 1 kW ∙ s. Since 1 h = 3600 s, then 1 kW ∙ h = 3600 kW ∙ s = 3600 kJ = 3600000 J.

Tips

• In SI, energy and work are also measured in ergs. 1 erg = 1 dyne (unit of force measurement) x 1 cm. 1 J = 10,000,000 erg.

Warnings

• The joule and newton meter are units of measure for work. Joules measure the energy and work done when a body moves in a straight line. If the body rotates, the unit of measurement is newton-meter.

What do you need

Work and kinetic energy:

• Stopwatch or timer
• scales
• Cosine calculator

Electric Energy:

• Resistor
• Wires or experimental stand
• Multimeter (or ohmmeter and ammeter)
• Crocodile clips

Quantity of heat:

• Heated body
• Heat source (e.g. burner)
• Thermometer
• Handbook for determining the specific heat of a heated body