Electric potential & current
The idea
Voltage and current are the two bookkeeping quantities of electricity, and keeping them distinct is half the battle. Electric potential difference (voltage) measures energy per charge: a 9.0 V battery gives every coulomb of charge 9.0 joules of energy to spend on its trip around the circuit. Current measures flow rate: I = q/t, in amperes, where 1 A means one coulomb passing a point each second. Voltage is the push per charge; current is how much charge actually moves.
A water analogy organizes the picture: voltage is like the pressure difference a pump maintains, current is like the liters per second flowing through the pipe. Combining the two definitions gives the energy delivered: E = qV, and since power is energy per time, P = IV. These two relations let you track exactly how fast a battery drains and how much energy a device consumes.
The misconception to kill early is that charge or current is used up by a bulb. The same current that enters a bulb leaves it — charge is conserved. What the charge spends is its ENERGY: it arrives at high potential, hands energy to the filament as light and heat, and continues on at lower potential. Current is the courier; voltage marks how much each courier delivers.
Worked example
A flashlight bulb draws a steady 0.50 A from a 9.0 V battery for 2.0 minutes. How much charge passes through the bulb, how much energy does the battery deliver, and at what power?
- Convert the time to seconds so the units cooperate: 2.0 min = 120 s.
- Use the definition of current to count the charge: q = It = 0.50 × 120 = 60 C passing through the bulb.
- Each coulomb carries 9.0 J from the battery, so the energy delivered is E = qV = 60 × 9.0 = 540 J.
- Compute the power as the delivery rate: P = IV = 0.50 × 9.0 = 4.5 W.
- Sanity-check for consistency: power times time should reproduce the energy, and 4.5 W × 120 s = 540 J — the two routes agree, and 4.5 W is sensible for a small bulb.
Answer. In two minutes, 60 C of charge flows through the bulb, carrying 540 J of energy at a steady power of 4.5 W.
Check your understanding
- How would you explain the difference between voltage and current to a friend using a water-pipe or delivery-truck analogy?
- Why does the current entering a bulb equal the current leaving it, and what exactly does the bulb take from the charge?
- What does it mean physically to say a battery is rated at 9.0 volts, in terms of joules and coulombs?
- If two bulbs draw the same current but one sits across a larger voltage, which consumes energy faster and why?
Build the foundations first
Electric potential & current builds on these concepts. If any feel shaky, start there.