AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
The electromagnetic force; virtual photons as the exchange particle.
Full specification wording
Particle interactions: The electromagnetic force; virtual photons as the exchange particle.
The electromagnetic force acts between particles that have charge.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 3
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The electromagnetic force acts between particles that have charge. It is responsible for electric attraction and repulsion, and for magnetic forces. In particle physics, the electromagnetic force is transmitted by the exchange of virtual photons. One charged particle emits a virtual photon, which is absorbed by another charged particle, transferring momentum and producing the force. Virtual photons are not directly detectable; they exist only for the brief time of the interaction. The infinite range of the electromagnetic force is consistent with the zero rest mass of the photon.
For example: Two electrons repel each other by exchanging a virtual photon.
What earns the marks
- State that the electromagnetic force acts between charged particles.
- Identify the virtual photon as the exchange particle for the electromagnetic force.
- Explain that a charged particle emits a virtual photon which is absorbed by another charged particle, transferring momentum.
- State that virtual photons are not directly observable and exist only during the interaction.
- Relate the infinite range of the electromagnetic force to the zero rest mass of the photon.
Common mistakes and how to put them right
Thinking the electromagnetic force acts between all particles.
Put it right: It acts only between charged particles.
Believing virtual photons are real photons that can be detected.
Put it right: They are virtual and only mediate the force.
Confusing the exchange particle of the electromagnetic force with that of the weak interaction (W and Z bosons).
Forgetting that the photon has zero rest mass, which allows the electromagnetic force to have infinite range.
Exam tips
- Remember that the exchange particle for the electromagnetic force is the virtual photon.
- When explaining the force, describe the emission and absorption of a virtual photon between two charged particles.
- Link the zero rest mass of the photon to the infinite range of the electromagnetic force.
- Use the term 'virtual photon' rather than just 'photon' when discussing exchange particles.
Try the lesson
The start of the Particle interactions: The electromagnetic force; virtual photons as the exchange particle. lesson, made from checked teaching notes for this specification point.
Have a go first
Which exchange particle is associated with the electromagnetic force?
Listen
The electromagnetic force acts between particles that have charge. It is responsible for electric attraction and repulsion, and for magnetic forces. In particle physics, the electromagnetic force is transmitted by the exchange of virtual photons. One charged particle emits a virtual photon, which is absorbed by another charged particle, transferring momentum and producing the force. Virtual photons are not directly detectable; they exist only for the brief time of the interaction. The infinite range of the electromagnetic force is consistent with the zero rest mass of the photon. For example, two electrons repel each other by exchanging a virtual photon. A strong answer earns marks for these points. State that the electromagnetic force acts between charged particles. Identify the virtual photon as the exchange particle for the electromagnetic force. Explain that a charged particle emits a virtual photon which is absorbed by another charged particle, transferring momentum. State that virtual photons are not directly observable and exist only during the interaction. Exam tip: Remember that the exchange particle for the electromagnetic force is the virtual photon.
See it
Key words
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Electromagnetic force
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Virtual photons
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Range and mass
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See every definition
- Electromagnetic force
- Acts between particles that have charge and is responsible for electric attraction and repulsion, and for magnetic forces.
- Virtual photons
- Not directly detectable and exist only for the brief time of the interaction, transmitting the electromagnetic force between charged particles.
- Range and mass
- The infinite range of the electromagnetic force is consistent with the zero rest mass of the photon.
Label the diagram
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1Box 1
2Box 2
3Box 3
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