A metal wire is full of electrons that are normally drifting around at random, going nowhere. Connect a battery and a field appears inside the wire that nudges them all to drift the same way, that is a current. But there are two flavours: the steady one-way flow from a battery (DC) and the back-and-forth flow from the mains (AC). Both deliver energy; they just do it differently.
A copper wire is full of free electrons even when no battery is connected, yet no current flows.
Before reading: what do you think the battery changes about the electrons to make a current flow? Write your prediction.
A voltage sets up a field that drives the free electrons
A metal is already full of free electrons; the battery's job is to give them a reason to all move the same way.
A metallic conductor contains free electrons, delocalised electrons that can move through the metal lattice. With no source connected, these electrons move randomly in all directions, so there is no net flow and no current. When the conductor is connected to an external voltage (a battery or supply):
A metal contains free electrons that normally move randomly with no net flow. Connecting an external voltage sets up an electric field along the conductor that exerts a force on the electrons, giving them a net drift, this flow of charge is the electric current.
Pause, copy the highlighted explanation into your book before moving on.
Did you get this? True or false: connecting an external voltage sets up an electric field in the conductor that gives the free electrons a net drift.
One-way flow versus back-and-forth flow
We just saw how a voltage drives electrons one way. That raises a question: what if the voltage keeps reversing? This card answers it → that produces alternating current, which we compare with direct current.
DC and AC differ only in whether the flow keeps to one direction or repeatedly switches back and forth.
The two types of current are distinguished by how their direction behaves over time:
Direct current (DC) flows steadily in one direction (flat current–time graph; batteries). Alternating current (AC) periodically reverses direction (sinusoidal current–time graph; the mains). Both transfer energy as charge moves.
Add the highlighted comparison to your notes before the check below.
Quick check: Which best describes alternating current (AC)?
Pattern B, Compare and contrast
In your book: (1) sketch current–time graphs for both DC and AC on the same set of axes; (2) make a two-column table listing one similarity and two differences between DC and AC; (3) explain in one sentence why AC can still deliver energy even though the electrons end up back where they started.
Match the pairs: Match each feature of a metallic conductor to its role in carrying current.
Dot point: Explain why electrons flow through a metallic conductor when connected to an external voltage
More voltage means more current when resistance is fixed
We just learned the DC/AC distinction. That raises a question: what sets how much current flows? This card answers it → the applied voltage, when the resistance is held constant.
The bigger the voltage you apply, the harder the field pushes the electrons, so more charge drifts each second, a larger current.
For a conductor of constant resistance, the voltage applied directly controls the current. A larger voltage produces a stronger field in the wire, pushes the electrons harder, and so drives a larger current. In fact, for a constant resistance the current is directly proportional to the voltage:
$$I \propto V \quad\text{(constant resistance)}$$
So doubling the voltage doubles the current, halving the voltage halves it. This direct proportionality is the experimental seed of Ohm's law, $V = IR$, which you will meet in L14, the constant of proportionality turns out to be the resistance $R$.
A conductor of constant resistance carries $0.40\ \text{A}$ when $6.0\ \text{V}$ is applied across it. Find the current when the applied voltage is raised to $15\ \text{V}$, and state how the current–time graph would differ if that $15\ \text{V}$ were supplied as AC rather than DC.
For a conductor of constant resistance, a larger applied voltage produces a stronger internal field and drives a larger current: $I \propto V$. Doubling the voltage doubles the current. This proportionality leads directly to Ohm's law $V = IR$.
Pause, copy the highlighted relationship into your book before the check below.
Last check: Decide if the statement is true or false.
For a conductor of constant resistance, doubling the applied voltage doubles the current through it.
Odd one out: three of these are sources of DIRECT current. Which one is not?
Dot point: Compare direct current (DC) and alternating current (AC)
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 5(4 marks) 1. Explain why electrons flow through a metallic conductor when it is connected to an external voltage, and compare direct current (DC) with alternating current (AC).
1 mark: a metal has free electrons that move randomly with no net flow · 1 mark: the voltage sets up a field that gives the electrons a net drift · 1 mark: DC flows steadily in one direction · 1 mark: AC periodically reverses direction
ExplainBand 5(3 marks) 2. A student says that a lamp does not light instantly when the switch is closed, because the electrons must first travel all the way from the switch to the lamp. Explain why the lamp in fact lights almost immediately, referring to the electric field in the conductor and to the drift velocity of the electrons.
1 mark: the electric field is established along the whole conductor almost instantly · 1 mark: free electrons are already present throughout the wire and all begin to drift at once · 1 mark: drift velocity itself is very slow, so it is the field, not the journey of any one electron, that explains the speed
Dot point: Explain why electrons flow through a metallic conductor when connected to an external voltage
AnalyseBand 5(3 marks) 3. A small motor designed to run on a DC supply is instead connected to an AC supply of the same peak voltage. Describe two ways in which the current it receives differs from the current it was designed for, and identify a typical source of each kind of supply.
1 mark: the AC current reverses direction periodically whereas the DC current flows one way only · 1 mark: the AC current varies continuously and passes through zero twice each cycle, whereas the DC current is essentially constant · 1 mark: names sources, e.g. a battery or solar cell for DC and the mains outlet for AC
Dot point: Compare direct current (DC) and alternating current (AC)
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): A metallic conductor contains free (delocalised) electrons that normally move randomly in all directions, so there is no net flow of charge and no current. When the conductor is connected to an external voltage, the voltage sets up an electric field along the wire that exerts a force on each free electron, giving them all a slow net drift in one direction, this net flow of charge is the current. Direct current (DC) flows steadily in one direction only, as produced by a battery, and appears as a flat current–time graph. Alternating current (AC) periodically reverses direction, as supplied by the mains, and appears as a sinusoidal current–time graph.
Q2 (3 marks): The student's reasoning would be right if the current depended on particular electrons travelling the length of the circuit, but it does not. Closing the switch establishes an electric field along the whole length of the conductor essentially at once, at close to the speed of light. Free electrons are already distributed throughout the wire, including inside the lamp filament itself, so every one of them feels a force and begins to drift at nearly the same instant. The lamp therefore carries a current immediately. The drift velocity of any individual electron is in fact extremely slow, typically a fraction of a millimetre per second, so if the student were right the lamp would take hours to light. What travels quickly is the field and the resulting change in electron motion, not the electrons themselves.
Q3 (3 marks): The first difference is direction. The motor was designed for a current that flows steadily in one direction, but an alternating supply reverses the direction of charge flow periodically, so the electrons oscillate back and forth rather than drifting consistently one way. The second difference is magnitude over time. A DC supply delivers a current of essentially constant size, whereas an AC current varies continuously, typically as a sine wave, rising to a peak, falling back through zero, reversing, and passing through zero twice in every cycle. So the motor receives a current that is neither steady in size nor constant in direction. A battery or a solar cell is a typical DC source; the mains outlet supplies AC.
You can now explain why electrons flow in a conductor, compare DC and AC, and relate voltage to current at constant resistance. Next lesson examines what opposes the flow, resistance, and the four factors that affect it.
↩ Back to focus-area hubAt the start you predicted what the battery changes about the electrons. Without a source the free electrons move randomly with no net flow; the battery sets up an electric field in the wire that gives them all a net drift, which is the current. Whether that flow is steady (DC) or reversing (AC), it still delivers energy.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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