WJEC AS Physics Unit 1.4 - Energy Concepts

Work

• also called Work Done

• Represented by W

• Work done by a force is the product of the magnitude of the force and the distance moved in the direction of the force.

What Work Done is always equal to

the energy transferred

Unit for Work Done

Joules (J)

What must be done to overcome resistive forces?

Work

Name 2 resistive forces

Air resistance and friction

Equation for Work Done

W=Fx

Where W=Work, F=Force and x=distance

The calculation of the work done for constant forces, when the force is not along the line of motion

W.D. = Fxcos θ

Principle of Conservation of Energy

Energy cannot be created or destroyed, only transferred from one form to another. Energy is a scalar.

Gravitational Potential Energy

This is energy possessed by an object by virtue of its position. Unit; J

Equation for Gravitational Potential Energy

E =  mg Δh

Where E = energy, m = mass, g = acceleration due to gravity and h = height.

Elastic Potential Energy

This is the energy possessed by an object when it has been deformed due to forces acting on it. Unit; J

First equation for Elastic Potential Energy

E = ½ Fx

Where E = energy, F = force, x = distance.=

Second Equation for Elastic Potential Energy

E = ½ kx2 Where E = energy, x = distance and k = spring constant.

Kinetic Energy

This is energy possessed by an object by virtue of its motion. Unit; J

Equation for Kinetic Energy

E_k = ½ mv² 

Where E= Kinetic Energy, m = mass and v = velocity.

Energy

The energy of a body or system is the amount of work it can do. Unit: J

Work-Energy relationship

The work done on a body is equal to the change in the kinetic energy of the body.

Equation for the Work-Energy relationship

Fx = ½ mv² - ½ mu² 

Where F = force, x = extension, m = mass, v = final velocity and u = initial velocity.

Explain the Work-Energy relationship

If a force is pushing an object horizontally it does work on the object equal to “F x”, and in the absence of friction etc, this work translates to a gain in kinetic energy.

SUVAT equation that the Work-Energy relationship is derived from

v² = u²+2ax

Proof of the Work-Energy relationship

v² = u²+2ax

v² - u² - 2ax

½  v² - ½ u² = ax

½  v² - ½ u² = (f/m)x
½ mv² - ½ mu² = (f)x

Power

This is the work done per second, or energy transferred per second. Unit: W [= Js-1 ]

Unit of Power

Watts (W) or Joules per second (Js-1)

1 Watt is equal to

1 Js-1

1st equation for Power

P = W/t

Where P = power,  W = work done, t = time,

2nd equation for Power

P = E/t 

Where P = power, E = energy and t = time.

3rd equation for Power

P = Fv

Where P = power, F = force and v = velocity

Efficiency

The percentage of useful energy transferred out of the total input energy.

Why is energy lost to surroundings when being transferred?

Because of dissipative forces, such as friction, air resistance and fluid friction.

Dissipative forces

• Dissipative forces, for example friction and drag, cause energy to be transferred from a system and reduce the overall efficiency of the system

List 3 dissipative forces

Friction, air resistance and fluid riction

Equation for Efficiency

Efficiency % = (useful energy transfer / total input energy). * 100