Current, Voltage & Resistance — Print · Grade 12 Science

Current, Voltage & Resistance

Learning goals

• I can define electric current as the rate of flow of charge and calculate it with I = Q/t.
• I can explain voltage as the energy delivered per unit of charge, using V = W/Q.
• I can describe resistance and the wire properties that increase or decrease it.

Readings for this unit

• A lamp transfers energy while charge keeps circulating

In the last lesson charges sat still; now we set them marching. Electric current is simply the rate at which charge flows past a point in a circuit, written I = Q ÷ t — the amount of charge Q, in coulombs, divided by the time t, in seconds. Its unit is the ampere (A), and one amp means one full coulomb of charge (about 6.2 × 10¹⁸ electrons) streaming past every second. By long-standing convention we describe current as flowing from the positive terminal of a battery to the negative one, even though in a metal wire it is actually the negative electrons drifting the opposite way.

Potential difference, or voltage, is energy per charge between two points, in J/C. A steady ideal 9 V source supplies 9 J per coulomb crossing its rise. In an ordinary resistive circuit an electric field supports charge drift, but induction and other devices require their own source model; “voltage always causes current” is not a universal definition. An open circuit can have voltage with almost no steady current, and charge-flow rate also depends on the actual network and operating conditions.

The third member of the trio is resistance, the opposition a material offers to the flow of charge, measured in ohms (Ω). As charge pushes through a material, it collides with the vibrating atoms of the lattice and loses energy, which is why a resistor warms up. Resistance is what converts electrical energy into the heat of a toaster, the glow of a bulb filament, and the warmth of a hair dryer — all deliberately built to resist current so that the energy shows up as heat and light.

For an ideal uniform ohmic wire, R=ρL/A, so a longer wire increases resistance and a larger cross-sectional area reduces it when material and conditions remain fixed. Resistivity depends on material and temperature. Many metals increase in resistivity as they warm, but semiconductors and other materials can behave differently. The long-thin-hot ranking is therefore a conditional metal-wire example, not a universal law for every conductor.

The classic way to feel how these three quantities relate is a water-pipe analogy. Picture water pumped around a loop of pipe: the voltage is like the pressure from the pump that pushes the water, the current is like the amount of water actually flowing past each second, and the resistance is like a narrow, constricted section of pipe that holds the flow back. Raise the pressure (voltage) and more water flows; pinch the pipe tighter (more resistance) and less water flows. The analogy is not perfect, but it captures the essential cause-and-effect: voltage pushes, resistance restrains, and current is the flow that results.

These three ideas — current, voltage, and resistance — are the vocabulary of every electrical device on Earth, and they do not act independently. Push harder (more voltage) and current rises; add more resistance and current falls. That precise relationship is captured by Ohm's law, the centerpiece of the next lesson, which ties all three together into a single equation you can solve. Master what current, voltage, and resistance each *mean* here, and Ohm's law becomes not a formula to memorize but a sentence you already understand.

Model check: Current does not mean charges are consumed; a component transfers energy while charge is conserved. I=Q/t=3 A; conventional current direction is positive-charge flow.

Electric Current

I = Q ÷ t

I is current in amperes (A); Q is charge in coulombs (C); t is time in seconds (s). One amp is one coulomb per second.

Voltage (Potential Difference)

V = W ÷ Q

V is voltage in volts (V); W is the energy in joules (J) given to charge Q in coulombs (C). One volt is one joule per coulomb.

Worked examples

Avoid these mistakes

• Current does not mean charges are consumed; a component transfers energy while charge is conserved.

Words to know

electric current

The rate at which electric charge flows past a point, I = Q/t, measured in amperes (A).

ampere (A)

The SI unit of current; one ampere is one coulomb of charge flowing per second.

voltage (potential difference)

The electrical energy delivered to each unit of charge, V = W/Q, measured in volts (V).

resistance

The opposition a material offers to the flow of current, measured in ohms (Ω).

resistivity

A property of a material itself that sets how strongly it resists current; it, along with a wire's length and thickness, determines the wire's resistance.

Twitching frog legs and the first battery

In the 1780 s, Italian anatomist Luigi Galvani noticed that a dead frog's leg twitched when touched by two different metals, and concluded he had found a mysterious 'animal electricity.' His countryman Alessandro Volta disagreed — he suspected the electricity came from the metals themselves, not the frog. To prove it, in 1800 Volta stacked alternating discs of zinc and copper separated by brine-soaked cloth and produced the first steady source of current in history: the voltaic pile, the ancestor of every battery. Volta was right, and the debate gave us a lasting reward — the unit of voltage, the volt, is named after him, while Galvani's name survives in 'galvanized' metal and the galvanometer.

Electrons crawl, but the signal flies

Flip a switch and a lamp across the room lights instantly, which makes it tempting to think electrons race through the wire near light speed. They don't. The actual drift velocity of electrons in a typical wire is astonishingly slow — often less than a millimeter per second, slower than a snail. What travels fast is the electric field that pushes them: it propagates through the wire at close to the speed of light, nudging electrons everywhere into motion almost at once. Picture a tube already packed with marbles — push one in at your end and a marble instantly pops out the far end, even though no single marble made the whole trip. The 'push' is fast; the marbles themselves barely move.

Superconductors: when resistance drops to exactly zero

Resistance usually rises with temperature, so what happens if you go the other way and make a material extremely cold? In 1911, Dutch physicist Heike Kamerlingh Onnes cooled mercury to about 4 kelvin (−269 °C) and watched its electrical resistance vanish — not just shrink, but drop to exactly zero. A current started in a loop of superconductor can circulate for years without a battery, because nothing dissipates its energy. Today superconductors wind the powerful electromagnets inside MRI scanners and maglev trains, and one of physics' great quests is to find a material that superconducts at everyday temperatures, which would transform how the world moves electricity.

AC vs DC: the War of the Currents

In the 1880 s and 1890 s, Thomas Edison championed direct current (DC), while George Westinghouse and Nikola Tesla backed alternating current (AC) — a rivalry so fierce it became known as the War of the Currents. AC won, and the reason is pure physics. Power lost as heat in a transmission line is P = I²R, so to ship power across long distances with low loss you want very low current and very high voltage. Transformers can step AC voltage up for transmission and back down for safe home use, but they only work on changing (alternating) current — which is exactly why the grid outside your window hums along at AC, while the battery in your phone stores DC.

In the real world

Every device you plug in is rated for a certain voltage and draws a certain current; electricians use these numbers to choose wires thick enough that their resistance won't overheat them.

Exit check

24C passes in 8 s. Find current.