Currents
Study H1 Physics current and drift velocity, relate e.m.f. and potential difference to energy per charge, and calculate electrical power.
Offline pages
Saved on this device only. Connect at least once every 30 days to confirm Premium. Sign-out or a change of account removes saved copies.
Before you begin
Every circuit calculation rests on three quantities: how much charge flows each second, how much energy each coulomb carries or gives up, and how fast energy is transferred. The lessons start with current as the rate of flow of charge and derive I = nAvq to link it to the slow drift of electrons. They then define potential difference and e.m.f. as energy per unit charge, and finish with electrical power.
Be comfortable with: work and energy, because a p.d. is work done per unit charge.
Learning goals
- Relate current to charge flow, number density and drift velocity.
- Apply potential difference, e.m.f. and electrical power relationships.
Syllabus statements covered
- show an understanding that electric current is the rate of flow of charge and solve problems using I = Q/t
- derive and use the equation I = nAvq for a current-carrying conductor, where n is the number density of charge carriers and v is the drift velocity
- recall and solve problems using the equation for potential difference in terms of electrical work done per unit charge, V = W/Q
- distinguish between electromotive force (e.m.f.) and potential difference (p.d.) using energy considerations
- recall and solve problems using the equations for electrical power P = VI, P = I²R and P = V²/R
Lessons
Work through them in order.
- Current, charge flow and drift velocityCurrent as the rate of flow of charge, and how I = nAvq links it to the drift velocity of the charge carriers.
- Potential difference and electromotive forcePotential difference and e.m.f. as energy per unit charge, and how they differ.
- Electrical powerElectrical power as the rate of energy transfer, using P = VI, I²R and V²/R.
Practise and check
Or choose
Topic reference
Quick reference
| Idea | Relationship | Note |
|---|---|---|
| current | I = Q/t | 1 A = 1 C s⁻¹ |
| current and drift | I = nAvq | n carriers per m³; A in m² |
| potential difference | V = W/Q | electrical energy to other forms, per unit charge |
| e.m.f. | ε = W/Q | other forms to electrical energy, per unit charge |
| power | P = VI | V across and I through the same component |
| power in a resistor | P = I²R = V²/R | V, I and R under the same conditions |
| energy | W = Pt = VIt | t in seconds |
Core knowledge to remember
- Electric current is the rate of flow of charge.
- For a current-carrying conductor, I = nAvq, where n is the number density of charge carriers and v is the drift velocity.
- The potential difference between two points is the electrical work done per unit charge, V = W/Q. One volt is one joule per coulomb.
- 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, in a component.
- Electrical power is P = VI = I²R = V²/R.
Common mistakes
- Converting 1 mm² to 10⁻³ m² instead of 10⁻⁶ m².
- Thinking the drift velocity is the speed at which a circuit responds when it is switched on.
- Saying current is used up in a component. The charge flows on; energy is transferred.
- Calling e.m.f. a force, or defining e.m.f. and p.d. in the same words.
- Using a resistance measured under different conditions, such as a cold filament, in P = V²/R.