According to the standard model, the proton consists of three quarks (uud). The three quarks are bound together by the strong force. u quarks have a 2/3 charge and d quarks have a -1/3 charge, so the proton carries an equivalent positive charge [1]. The electromagnetic force is realised by exchanging virtual photons and the strong force is realised by exchanging virtual gluons. If we move a proton by electromagnetic force, it is equivalent to moving three quarks bound together by the strong force, and the strong force needs to change at the same time. So how is the electromagnetic force transferred to the strong force, or how is it converted to the strong force?
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References
[1] Schmitz, W. 2019. Particles, Fields and Forces: Springer.