Energy & Fields: stores, work, fields and power
Key idea: Track energy transfers consistently, connect work to kinetic and potential-energy change, then apply power and efficiency.
Before you start: Forces & DynamicsMotion & Forces
By the end, you can
- Describe energy stores and transfers and apply conservation of energy.
- Define work, derive kinetic energy from work and uniformly accelerated motion, and use Eₖ = ½mv².
- Represent gravitational and electric fields and relate field work to potential-energy change.
- Distinguish gravitational, electric and elastic potential energy and use force–extension graph area.
- Apply energy-transfer rate, mechanical power and efficiency, including practical energy losses.
Starting-point self-check
1. Check your starting point
Attempt all three groups without notes and mark the first equation or energy transfer you could not justify. Use the recorded topic diagnostic above when you want scoring and a personalised repair plan.
Fields and potential-energy change 4(f)–(i)
Question 1
A positive test charge is removed from a region that still contains a fixed source charge. Does the electric field at that point disappear? Define electric field strength.
Check the model response
No. The source establishes the field. Electric field strength is force per unit positive charge at the point: E = F/q.
Question 2
How do equipotential surfaces meet field lines, and what is the work done by a field when potential energy decreases by 6.0 J?
Check the model response
Equipotential surfaces are perpendicular to field lines. W_field = −ΔU = +6.0 J.
repair
2. Repair the six common breaks
Use only the repair matching an error, then repeat the corresponding diagnostic model.
Fields and potential-energy change 4(f)–(i)
Check this idea
Misconception: A field exists only when a test body is present.
Repair: A source establishes the field. A test mass or positive test charge only defines its strength and direction.
Check this idea
Misconception: Positive work by a field increases potential energy.
Repair: Work done by a field is W_field = −ΔU, so positive field work corresponds to decreasing potential energy.
worked example
3. Follow three worked models
Track the system, transfer, direction and graph area before substituting values.
Fields and potential-energy change 4(f)–(i)
Model 1
A positive charge moves along a uniform electric field and its potential energy changes by −4.8 × 10⁻¹⁶ J. Find field work and explain the force direction.
Check the model response
A positive charge experiences force along the field lines. W_field = −ΔU = +4.8 × 10⁻¹⁶ J, consistent with displacement along the force.
guided practice
4. Guided practice
Use each hint only to select the governing relationship.
Fields and potential-energy change 4(f)–(i)
Question 1
A 2.0 kg mass experiences 19.6 N downward. Calculate g and state the direction of the gravitational field.
Hint: Use the defining ratio.
Check the model response
g = F/m = 19.6/2.0 = 9.8 N kg⁻¹ downward, along the gravitational field lines.
independent practice
5. Independent practice
Solve without repair notes and state every energy store, transfer and sign convention used.
Fields and potential-energy change 4(f)–(i)
Question 1
Compare uniform and radial fields, define both gravitational and electric field strength, and state the relationship between field work and potential-energy change.
Check the model response
Uniform fields have parallel equally spaced lines; radial lines converge on or diverge from a centre. g = F/m and E = F/q for a positive test charge. Equipotentials are perpendicular to field lines and W_field = −ΔU.
Practice exit check
6. Practice assessment
Use this as extra closed-book practice, then complete the separate recorded assessment in your plan.
Fields and potential-energy change 4(f)–(i)
Question 1
A +2.0 nC charge experiences 1.0 × 10⁻⁴ N away from a positive source. Find E and the work done by the field when its potential energy falls by 1.5 × 10⁻¹³ J.
Check the model response
E = F/q = 5.0 × 10⁴ N C⁻¹ away from the source. W_field = −ΔU = +1.5 × 10⁻¹³ J.
Re-test practice
7. Delayed re-test practice
Return after at least three days and solve these fresh contexts without reopening earlier responses. The recorded plan enforces the delay and uses a separate re-test family for selected-response skill-group evidence.
Fields and potential-energy change 4(f)–(i)
Question 1
A +4.0 μC bead is in a uniform field of 2.5 × 10³ N C⁻¹. Find its force. If the field does 8.0 × 10⁻⁴ J of work, find ΔU and orient an equipotential.
Check the model response
F = qE = 1.0 × 10⁻² N along the field. ΔU = −8.0 × 10⁻⁴ J. Equipotential surfaces are perpendicular to field lines.