MYP Integrated Sciences Work, Power and Efficiency Notes

How Does Work Link Forces To Energy Transfer

1. In physics, work is not the same as effort or tiredness.
2. Work describes how energy is transferred when a force causes an object to move.
3. Work is only done when a force causes movement in the direction of that force.
4. If there is no movement, no work is done, even if a force is applied.

Calculating Work Done

1. The amount of work done depends on how large the force is.
2. It also depends on how far the object moves.
3. The distance considered is only the distance moved in the direction of the force.

$$W = Fd$$

where:

1. $W$ is work (joules, J)
2. $F$ is force (newtons, N)
3. $d$ is distance moved (metres, m)

When Work Is Done

1. Work is done when a force causes movement in the same direction as the force.
2. Work is also done when movement is opposite to the force, such as lifting against gravity.
3. In these cases, energy is transferred into or out of the object.

When Work Is NOT Done

1. No work is done if an object does not move.
2. No work is done if the motion is at right angles to the force.
3. Even strong forces may transfer no energy if they do not cause movement.

Positive And Negative Work

1. Positive work: the force adds energy to the object (speed increases).
2. Negative work: the force removes energy from the object (speed decreases), for example friction or braking.

Do machines reduce the amount of work needed?

1. Machines make tasks easier by changing how forces are applied.
2. They allow a smaller force to be used over a longer distance.
3. Machines do not reduce the total work done.
4. They change how the work is carried out.

Power Tells You How Fast Energy Is Transferred

1. Sometimes the key question is not only "How much energy?" but also "How quickly?"
2. Power describes how quickly energy is transferred.
3. Two systems may transfer the same energy but at different speeds.
4. The system that transfers energy faster has greater power.

Calculating power

The defining equation is

$$P = \frac{E}{t}$$

where:

1. $P$ is power (watts, W)
2. $E$ is energy transferred (joules, J)
3. $t$ is time taken (seconds, s)

Connecting Work And Power In Mechanical Situations

If the energy transfer is mechanical work, then

$$P = \frac{W}{t}$$

A useful rearrangement for constant speed motion is:

$$P = Fv$$

because if $W = Fd$ and $d = vt$, then $P = \frac{Fd}{t} = Fv$.

Why Is Not all Input Energy Useful?

1. Real processes never convert all input energy into the desired output.
2. Some energy is always transferred into other forms, very often thermal energy due to friction or electrical resistance.

Calculating efficiency

Using energy:

$$\text{efficiency} = \frac{\text{useful output energy}}{\text{total input energy}}\times 100\%$$

Using power:

$$\text{efficiency} = \frac{\text{useful output power}}{\text{total input power}}\times 100\%$$

Why Wasted Energy Is Often Thermal

1. In mechanical systems, friction converts kinetic energy into microscopic motion of particles, increasing temperature.
2. In electrical systems, resistance causes heating in wires and components.
3. This "wasted" energy is still conserved, but it is usually less useful because it spreads into the surroundings.