Current, potential difference and resistance

Current

Current is the rate of flow of charge: I = ΔQ / Δt. Charge is in coulombs (C), current in amperes (A): 1 A = 1 C/s. In metals the charge carriers are free electrons, each with charge -1.60 x 10^-19 C; conventional current flows from + to -.

Potential difference

The potential difference (p.d.) between two points is the energy transferred per unit charge: V = W / Q. 1 volt = 1 joule per coulomb.

Resistance and Ohm's law

Resistance is defined by R = V / I, measured in ohms (Ω). This definition always applies. Ohm's law is a special case: for some conductors (such as a metal wire) at constant temperature, current is directly proportional to p.d., so R is constant. Components like filament lamps do not obey Ohm's law, but R = V / I still gives their resistance at any instant.

Worked example

A current of 0.40 A flows for 2.0 minutes. How much charge flows, and how many electrons is that?

1. Q = IΔt = 0.40 x 120 = 48 C
2. Number of electrons = 48 / 1.60 x 10^-19

Answer: 48 C, which is 3.0 x 10²⁰ electrons

Check yourself

1. A current of 3.0 A flows for 20 s. How much charge flows, in C?
   Show answer

   60 C. Q = IΔt = 3.0 x 20 = 60 C.

2. 90 C of charge passes a point in 1.5 minutes. Find the current in A.
   Show answer

   1 A. I = 90 / 90 = 1.0 A.

3. How much energy does a 1.5 V cell transfer to 20 C of charge, in J?
   Show answer

   30 J. W = VQ = 1.5 x 20 = 30 J.

4. A p.d. of 6.0 V drives a current of 0.25 A through a resistor. Find its resistance in Ω.
   Show answer

   24 Ω. R = V / I = 6.0 / 0.25 = 24 Ω.

5. How many electrons pass a point each second in a current of 1.0 A? Give your answer in units of 10¹⁸, to 3 s.f.
   Show answer

   6.25 x 10¹⁸. 1.0 C/s divided by 1.60 x 10^-19 C per electron = 6.25 x 10¹⁸ electrons per second.

6. Under what condition does a metal wire obey Ohm's law?
   • The current is very large
   • The p.d. is alternating
   • It is connected in parallel
   • Its temperature stays constant
   Show answer

   Its temperature stays constant. As a wire heats up its resistance rises, so I is no longer proportional to V.

7. What is 1 volt equal to?
   • 1 coulomb per second
   • 1 joule per coulomb
   • 1 ampere per ohm
   • 1 joule per second
   Show answer

   1 joule per coulomb. V = W / Q.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)
• Pearson Edexcel IAL Physics Unit 2 (WPH12) question paper, January 2024: list of data (e, m_e, h, c, eV)

Last updated 7 November 2026. Spotted a mistake? Tell us.