Understand that mechanical or electrical work done is equal to the energy transferred

1.7.2 Work – Energy Transfer

Work is the amount of energy that a force transfers to an object when it moves that object. Think of it as the “push” that gives a ball a kick‑start. ⚡

Mechanical Work

When a force acts in the direction of motion, the work done is:

\$W = F \, d \, \cos\theta\$

• \$F\$ = force applied (N)
• \$d\$ = displacement of the point of application (m)
• \$\theta\$ = angle between force and displacement

📐 Example: A student pushes a 10 kg box 5 m across a floor with a force of 30 N at 0° (straight ahead).

\$W = 30 \times 5 \times \cos0° = 150\,\text{J}\$.

Electrical Work

When an electric current flows through a potential difference, the work done (energy transferred) is:

\$W = V \, Q\$

or, using current and time:

\$W = I \, V \, t\$

• \$V\$ = voltage (V)
• \$Q\$ = charge (C)
• \$I\$ = current (A)
• \$t\$ = time (s)

⚡ Example: A 12 V battery supplies 2 A for 10 s.

\$W = 2 \times 12 \times 10 = 240\,\text{J}\$.

Work = Energy Transferred

Both mechanical and electrical work represent the same physical concept: the transfer of energy. When work is done on an object, its energy changes by the same amount.

📚 Key Point: The units of work and energy are identical – joules (J). 1 J = 1 N m.

Units & Conversion Table

Exam Tips for 1.7.2 Work

1. Always check the angle \$\theta\$ – if the force is not parallel to displacement, use \$\cos\theta\$.
2. Remember that work can be negative if the force opposes motion (e.g., friction).
3. For electrical work, choose the correct formula: \$W = VQ\$ or \$W = IVt\$ depending on the given data.
4. Convert all quantities to SI units before calculating.
5. Show all steps and keep units in the final answer (J).
6. Use the word “work” to indicate energy transfer, not just the action of applying a force.