Electricity and Magnetism

The One Thing

Electric current is the organized flow of charge through a complete conducting circuit, and its behavior is determined by potential difference and resistance. Current also produces magnetic fields, linking electricity and magnetism through electromagnetism.

Reactions, Processes and Experiments

Key Terms

• Electric charge: A property of matter responsible for electrical effects; charge may be positive or negative and is measured in coulombs.
• Electric current: The rate of flow of electric charge through a conductor. It is measured in amperes.
• Conventional current: The assumed direction of current from the positive terminal to the negative terminal of a cell through the external circuit.
• Electric circuit: A complete conducting path through which electric current can flow.
• Cell: A source of electrical energy that produces a potential difference using chemical reactions.
• Battery: A combination of two or more cells connected together.
• Potential difference: The work done per unit charge to move charge between two points; it is measured in volts.
• Resistance: The opposition offered by a conductor to the flow of current; it is measured in ohms.
• Ohm’s law: For a conductor at constant physical conditions, the current is directly proportional to the potential difference across it.
• Conductor: A material, such as copper or aluminium, that allows electric current to pass through it easily.
• Insulator: A material, such as rubber or plastic, that strongly resists the flow of electric current.
• Ammeter: An instrument used to measure current; it is connected in series and has very low resistance.
• Voltmeter: An instrument used to measure potential difference; it is connected in parallel and has very high resistance.
• Series circuit: A circuit in which components are connected along one path, so the same current flows through every component.
• Parallel circuit: A circuit in which components are connected across separate branches, so each branch has the same potential difference.
• Magnet: A material or object that produces a magnetic field and attracts magnetic materials such as iron, nickel, and cobalt.
• Magnetic poles: The two regions of a magnet, called north and south poles, where its magnetic effect is strongest.
• Magnetic field: The region around a magnet or current-carrying conductor in which magnetic force can be detected.
• Magnetic field lines: Imaginary lines used to represent the direction and strength of a magnetic field; they emerge from the north pole and enter the south pole outside a magnet.
• Electromagnet: A temporary magnet produced when electric current flows through a coil, usually wound around a soft iron core.
• Magnetic effect of current: The production of a magnetic field around a conductor when electric current passes through it.
• Right-hand thumb rule: If the right-hand thumb points in the direction of current, the curled fingers show the direction of the magnetic field around a straight conductor.
• Electromagnetic induction: The production of an induced current when the magnetic field linked with a conductor changes.

Easily Confused

• Cell and battery: A cell is a single source of electrical energy; a battery is a combination of two or more cells.
• Ammeter and voltmeter: An ammeter measures current and is connected in series; a voltmeter measures potential difference and is connected in parallel.
• Series and parallel circuits: A series circuit has one path and the same current through every component; a parallel circuit has separate branches with the same potential difference across each branch.
• Conventional current and electron flow: Conventional current is assumed to flow from the positive terminal to the negative terminal through the external circuit; the summary defines conventional current rather than electron-flow direction.
• Conductor and insulator: A conductor, such as copper or aluminium, allows current to pass easily; an insulator, such as rubber or plastic, strongly resists current.
• Magnet and electromagnet: A magnet produces a magnetic field as a permanent or continuing property; an electromagnet is a temporary magnet produced by current in a coil.
• Magnetic poles and magnetic field lines: Poles are the north and south regions where magnetic effects are strongest; field lines are an imaginary representation of magnetic-field direction and strength.
• Magnetic effect of current and electromagnetic induction: The magnetic effect of current is the production of a magnetic field by current; electromagnetic induction is the production of current by a changing linked magnetic field.
• Like and unlike magnetic poles: Like poles repel, whereas unlike poles attract.
• Soft iron and permanent magnetic materials: Soft iron is used in electromagnets because it becomes strongly magnetised when current flows and loses most of its magnetism when current stops.

What Gets Asked

• Definitions and units: Questions may ask for definitions of charge, current, potential difference, resistance, conductors, insulators, cells, batteries, or magnetic fields. Marks are lost by omitting the required units: coulomb (C), ampere (A), volt (V), or ohm (Ω).
• Numerical calculations: Questions may require use of $I=Q/t$, $V=W/Q$, $V=IR$, or $R=V/I$. Marks are lost by substituting the wrong physical quantity or giving an answer without the appropriate unit.
• Circuit arrangements: Questions may ask students to distinguish series and parallel circuits or calculate equivalent resistance using $R=R1+R2+R3+...$ and $1/R=1/R1+1/R2+1/R3+...$. Marks are lost by reversing the series and parallel rules.
• Instrument connections: Questions may require the correct placement of an ammeter or voltmeter. Marks are lost by connecting the ammeter in parallel or the voltmeter in series.
• Magnetic effects and field representation: Questions may ask for the result of Oersted’s 1820 experiment, the field around a straight current-carrying conductor, the right-hand thumb rule, or the properties of magnetic field lines. Marks are lost by stating that field lines intersect or by giving the wrong field direction.
• Electromagnets and safety: Questions may ask how to strengthen an electromagnet, why soft iron is used, or how a fuse protects a circuit. Marks are lost by failing to state that electromagnets can be switched on, switched off, or varied, or by omitting that a fuse melts and breaks the circuit when excessive current flows.