OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) distinction between e.m.f. and p.d. in terms of energy transfer
Full specification wording
E.m.f. and p.d: (c) distinction between e.m.f. and p.d. in terms of energy transfer
The distinction between e.m.f. and p.d. lies in the direction of energy transfer.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 5
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The distinction between e.m.f. and p.d. lies in the direction of energy transfer. E.m.f. is the energy transferred from other forms, such as chemical energy in a cell, to electrical energy per unit charge as charge passes through the source. P.d. is the energy transferred from electrical energy to other forms, such as thermal energy in a resistor, per unit charge as charge passes through a component. Both are measured in volts, so the unit alone cannot distinguish them; the definition must state which way the energy is transferred. In a circuit, the e.m.f. of the source supplies energy that is then transferred away by the p.d.s across the components.
What earns the marks
- E.m.f. is the energy transferred from other forms to electrical energy per unit charge in a source.
- P.d. is the energy transferred from electrical energy to other forms per unit charge across a component.
- In a circuit, the total energy supplied per unit charge by the source is transferred away by the components.
- The e.m.f. is associated with a source, whereas p.d. is measured between two points across a component.
Common mistakes and how to put them right
Saying e.m.f. and p.d. differ because they have different units.
Put it right: Both are measured in volts, so the difference is the direction of energy transfer.
Describing p.d. as energy supplied to the circuit.
Put it right: p.d. is energy transferred from electrical energy to other forms, whereas e.m.f. is energy transferred to electrical energy.
Using the terms interchangeably in circuit calculations.
Put it right: Keep e.m.f. for the source and p.d. for components to make energy transfers clear.
Exam tips
- In comparison questions, state the direction of energy transfer for each quantity rather than only giving the unit.
- Use a concrete example, such as a cell charging a resistor, to show e.m.f. supplying energy and p.d. transferring it away.
- Check that your answer mentions both quantities and the per-unit-charge basis of each.
Try the lesson
The start of the E.m.f. and p.d: (c) distinction between e.m.f. and p.d. in terms of energy transfer lesson, made from checked teaching notes for this specification point.
Listen
The distinction between e.m.f. and p.d. lies in the direction of energy transfer. E.m.f. is the energy transferred from other forms, such as chemical energy in a cell, to electrical energy per unit charge as charge passes through the source. P.d. is the energy transferred from electrical energy to other forms, such as thermal energy in a resistor, per unit charge as charge passes through a component. Both are measured in volts, so the unit alone cannot distinguish them; the definition must state which way the energy is transferred. In a circuit, the e.m.f. of the source supplies energy that is then transferred away by the p.d.s across the components. A strong answer earns marks for these points. E.m.f. is the energy transferred from other forms to electrical energy per unit charge in a source. P.d. is the energy transferred from electrical energy to other forms per unit charge across a component. In a circuit, the total energy supplied per unit charge by the source is transferred away by the components. The e.m.f. is associated with a source, whereas p.d. is measured between two points across a component. Exam tip: In comparison questions, state the direction of energy transfer for each quantity rather than only giving the unit.
See it
Key words
Say what each word means before you turn the card.
E.m.f.
Try answering aloud or in your head before turning the card.
P.d.
Try answering aloud or in your head before turning the card.
See every definition
- E.m.f.
- The energy transferred from other forms, such as chemical energy in a cell, to electrical energy per unit charge as charge passes through the source.
- P.d.
- The energy transferred from electrical energy to other forms, such as thermal energy in a resistor, per unit charge as charge passes through a component.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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