Energy and fields

Track energy through stores and transfers, calculate work, kinetic and potential energy, describe gravitational and electric fields, and use power and efficiency.

GCE A-Level H2 Physics 2027
5 lessons

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Before you begin

Energy is never created or destroyed; it moves between stores. The lessons start with energy stores, transfers and the principle of conservation of energy, then define work done by a force and use it to derive the kinetic energy equation. They introduce gravitational and electric fields and the potential energy linked to them, find elastic potential energy from a force–extension graph, and finish with power and efficiency.

Be comfortable with: the equations of uniformly accelerated motion, because the kinetic energy equation is derived from them.

Learning goals

Track energy stores and transfers, then apply conservation of energy.
Define work and derive and apply the kinetic-energy relationship.
Derive Eₖ = ½mv² from the definition of work done by a force and the uniformly accelerated motion equations.
Represent fields and relate work done by a field to potential-energy change.
Draw field-line representations of uniform and radial gravitational and electric fields.
Use force–extension graphs to determine elastic potential energy.
Apply power, mechanical power and efficiency relationships.

Syllabus statements covered

show an understanding that physical systems can store energy, and that energy can be transferred from one store to another
give examples of different energy stores and energy transfers, and apply the principle of conservation of energy to solve problems
show an understanding that work is a mechanical transfer of energy, and define and use work done by a force as the product of the force and displacement in the direction of the force
derive, from the definition of work done by a force and the equations for uniformly accelerated motion in a straight line, the equation Eₖ = ½mv²
recall and use the equation Eₖ = ½mv² to solve problems
show an understanding of the concept of a field as a region of space in which bodies may experience a force associated with the field
define gravitational field strength at a point as the gravitational force per unit mass on a mass placed at that point, and define electric field strength at a point as the electric force per unit charge on a positive charge placed at that point
represent gravitational fields and electric fields by means of field lines (e.g. for uniform and radial field patterns), and show an understanding of the relationship between equipotential surfaces and field lines
show an understanding that the force on a mass in a gravitational field (or the force on a charge in an electric field) acts along the field lines, and the work done by the field in moving the mass (or charge) is equal to the negative of the change in potential energy
distinguish between gravitational potential energy, electric potential energy and elastic potential energy
recall that the elastic potential energy stored in a deformed material is given by the area under its force–extension graph and use this to solve problems
define power as the rate of energy transfer
show an understanding that mechanical power is the product of a force and velocity in the direction of the force
show an appreciation for the implications of energy losses in practical devices and solve problems using the concept of efficiency of an energy transfer as the ratio of useful energy output to total energy input

Lessons

Work through them in order.

Core lessons

Practise and check

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Topic reference

Quick reference

Idea	Relation	Notes
work done by a force	W = Fs cos θ	θ between force and displacement; area under a force–displacement graph
kinetic energy	Eₖ = (1/2)mv²	total work done on a body = Δ Eₖ
gravitational field strength	g = F/m	N kg⁻¹; 9.81 N kg⁻¹ near the Earth’s surface
electric field strength	E = F/q	N C⁻¹; force on a positive charge
work done by a field	W_field = -Δ Eₚ	positive work, potential energy falls
elastic potential energy	area under force–extension graph	(1/2)Fx = (1/2)kx² when F = kx
power	P = E/t = W/t	1 W = 1 J s⁻¹
mechanical power	P = Fv	force and velocity in the same direction
efficiency	(useful energy output)/(total energy input) × 100%	always less than 100%

Core knowledge to remember

Conservation of energy: energy cannot be created or destroyed, only transferred from one store to another.
Work done by a force is the product of the force and the displacement in the direction of the force. Work is a mechanical transfer of energy.
A field is a region of space in which a body experiences a force associated with the field.
Gravitational field strength at a point is the gravitational force per unit mass on a mass placed at that point. Electric field strength at a point is the electric force per unit charge on a positive charge placed at that point.
Field lines show the direction of the force; equipotentials are at right angles to them.
Power is the rate of energy transfer. Efficiency is the ratio of useful energy output to total energy input.

Common mistakes

Saying friction or an inefficient machine destroys energy, instead of transferring it to internal energy.
Using the whole force in W = Fs cos θ or P = Fv when the force is at an angle to the motion.
Saying positive work by a field increases potential energy.
Using (1/2)Fx for a force–extension graph that is not a straight line through the origin.
Treating power as an amount of energy, or a powerful device as an efficient one.