--- title: "How does current flow?" subtitle: "Year 9 Physics · Wave and particle models · Lesson 4" --- # How does current flow? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A copper wire and a plastic ruler both contain millions of electrons. Only one of them can light a bulb. What is different? :::teacher-note **In one sentence:** Students explain electric current as a directed flow of free electrons driven by voltage, and distinguish conductors from insulators using free and bound electrons (AC9S9U04, node 3). **Students should be able to say:** I can explain what an electric current is. I can say why copper conducts and plastic doesn't. I can say what the battery does. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **A battery is joined to a bulb by copper wire and the bulb lights. What is flowing round the circuit?** ☐ Electrons that were already in the wire, pushed round by the battery ☐ Electrons that come out of the battery and are used up in the bulb ☐ Nothing flows: the bulb makes the electricity ☐ Protons, pushed round by the battery My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Electrons that were already in the wire. A metal is full of free electrons; the battery pushes them round the loop. They are not used up: as many go back into the battery as come out. What the bulb uses is the energy they carry. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: the particle model of current :::keep-together ### Current **What is an electric current in a metal wire?** ☐ A flow of free electrons ☐ A flow of atoms ☐ A flow of protons ::: :::teacher-note **Answer:** A flow of free electrons The atoms are fixed; the free electrons between them drift along when pushed. ::: :::keep-together ### Conductor and insulator **Why does copper conduct while plastic doesn't?** ☐ Copper has free electrons that can move; in plastic they are tied to their atoms ☐ Copper has more protons ☐ Plastic has no electrons ::: :::teacher-note **Answer:** Copper has free electrons that can move; in plastic they are tied to their atoms Conductors have many free electrons; in insulators almost all the electrons are held tightly by their atoms. ::: :::keep-together ### The battery **What does the battery do?** ☐ Stores electrons and releases them until it is empty ☐ Makes the electrons ☐ Pushes the free electrons round the circuit, giving them energy ::: :::teacher-note **Answer:** Pushes the free electrons round the circuit, giving them energy The battery's voltage is the push. A bigger voltage pushes harder, so the electrons drift faster and more energy is delivered each second. ::: :::keep-together ### Used up? **Are the electrons used up in the bulb?** ☐ Only some of them ☐ Yes: that is where the light comes from ☐ No: the same number go in as come out; the bulb transfers the energy they carry as light and heat ::: :::teacher-note **Answer:** No: the same number go in as come out; the bulb transfers the energy they carry as light and heat Current is the same at every point in the loop; the energy, not the electrons, is what the bulb gets. ::: :::keep-together ### Which way **On the board, electrons drift round one way. By convention the current is drawn the other way. Why?** ☐ The convention was chosen before electrons were discovered, and has stuck ☐ The board has a mistake ☐ Current is the flow of protons ::: :::teacher-note **Answer:** The convention was chosen before electrons were discovered, and has stuck Conventional current is drawn from + to −, a choice made before electrons were known. Electrons actually flow the other way. ::: :::typical ## Elaborate: how bright? :::keep-together ### What changes **What happens to the electrons and the bulb when the voltage doubles?** ☐ The electrons get bigger ☐ The electrons drift faster, the current is bigger and the bulb is brighter ☐ Nothing changes: the bulb has a fixed brightness ::: :::teacher-note **Answer:** The electrons drift faster, the current is bigger and the bulb is brighter More volts means a stronger push. The electrons drift faster, so more charge passes each second and more energy reaches the bulb. ::: ::: :::wider ## Elaborate: in between :::keep-together **Silicon has only a few free electrons, far fewer than copper and far more than plastic. It is used in computer chips. Why is that useful?** ☐ It is a perfect insulator ☐ It is cheap ☐ Its conduction can be controlled (by adding other atoms, or a voltage), so it can act as a switch ::: :::teacher-note **Answer:** Its conduction can be controlled (by adding other atoms, or a voltage), so it can act as a switch A material that sits between conductors and insulators can be switched on and off, which is how transistors in chips work. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **conductor** has **free electrons**; an **insulator** has almost none: its electrons are held tightly by their atoms. - A **current** is a flow of free electrons. The **battery** pushes them round; more volts means a stronger push. - The electrons are **not used up**; the bulb transfers the energy they carry. - **Conventional current** is drawn the opposite way to the electrons, by a historical choice. ## Exit ticket :::keep-together **A student says 'the bulb uses up the electrons so the wire after it is empty'. What would you say?** {{lines-3}} ::: :::teacher-note **Answer:** The same number of electrons leave the bulb as enter it. They carry on round the loop. The bulb transfers some of the energy they carry as light and heat. :::