Electrical Circuits
Visão Geral
Press a light switch and the room is lit before your finger has left the plastic. Nothing you can see has crossed the ceiling. No pipe has filled, no tank has emptied, nothing has arrived from the fuse box. And yet a wire that looked completely inert a moment ago is now carrying something so real that it will melt a fuse, spin a motor, boil a kettle or stop a heart. Learning what that something is, and learning to predict exactly how much of it goes where, is the single most useful piece of physics in the whole course, because almost every object you own runs on it.
This lesson builds the whole picture from one idea: charge that is free to move. You will meet the two quantities that describe it, current and potential difference, and the equations that tie them to charge and to resistance. You will learn to read and draw the fourteen circuit symbols the specification names, and to say what happens at every point of a circuit when components are joined one after another or side by side. You will find out why a lamp, a thermistor and a light-dependent resistor refuse to keep one fixed value of resistance, how that refusal is exactly what makes the last two useful as sensors in a thermostat or a street light, and how to get a resistance out of a component whose graph is a curve. Extension Tier candidates go further in three places, each of them marked in the lesson where it arrives.
Objetivos
- Electrical charges can move easily through some substances; for example metals have many charges (electrons) that are free to move.
- Electric current is the rate of flow of electric charge. Charge flow, Q, current, I, and time, t, are linked by the equation:
- The voltage of a source is the energy supplied by a source in driving charges round a complete circuit and is measured in volts.
- Potential difference across a component measures the energy transfer by charges and is measured in volts.
- The relationship between potential difference, V, energy transferred, E, and charge, Q, is: V = E / Q Teachers can use either of the terms potential difference or voltage. Questions will be set using the term potential difference. Students will gain credit for the correct use of either term.
- Circuit diagrams use standard symbols. Students will be required to interpret and draw circuit diagrams. Students should know the following standard symbols: Students should understand the use of thermistors in circuits, for example thermostats. Students should understand the use of light-dependent resistors (LDRs) in circuits, for example switching lights on when it gets dark.
- Components resist the flow of charge through them. The greater the resistance the smaller the current for a given potential difference across the component. The resistance of a component can be found by measuring the current through and potential difference across, the component. The relationship between potential difference, V, current, I, and resistance, R, is:
- The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.
- The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.
- The resistance of a filament lamp increases as the temperature of the filament increases. Students should be able to explain change in resistance in terms of ions and electrons.
- An LED emits light when a current flows through it in the forward direction. Students should be aware that the use of LEDs for lighting is increasing, as they use a much smaller current than other forms of lighting.
- The combined voltage of several sources in series is their sum.
- There are two ways of joining electrical components: in series and in parallel. Some circuits include both series and parallel parts.
- For components connected in series: the combined resistance is the sum of the resistance of each component; the current is the same in each component; the total potential difference of the power supply is shared between the components.
- For components connected in parallel: the combined resistance is less than that of either component; the current from the supply splits in the branches; the potential difference across each component is the same.
- When an electrical charge flows through a resistor, the resistor gets hot because of collisions between moving charges and stationary atoms in the wire. Students should understand that a lot of energy is wasted in filament bulbs by heating. Less energy is wasted in power saving lamps such as Compact Fluorescent Lamps (CFLs). They should understand that there is a choice when buying new appliances in how efficiently they transfer energy.
Mapa mental
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Nota de Aula
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Avaliação da Lição
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Use esta secção de avaliação como uma oportunidade para reforçar a tua compreensão do tema e identificar quaisquer áreas onde possas precisar de estudo adicional. Não te deixes desencorajar pelos desafios que encontrares; em vez disso, vê-os como oportunidades de crescimento e melhoria.
- Which statement defines electric current? A. The energy transferred by each unit of charge B. The rate of flow of electric charge C. The opposition of a component to the flow of charge D. The total charge stored in a component Answer: B
- A steady current of 2.0 A flows through a lamp for 30 seconds. How much charge passes through the lamp? A. 0.067 C B. 15 C C. 32 C D. 60 C Answer: D
- A 4 ohm resistor and an 8 ohm resistor are connected in series with a 6 V battery. What is the current in the circuit? A. 0.5 A B. 1.5 A C. 2.0 A D. 72 A Answer: A
- Two resistors are connected in parallel across a supply. Which statement is correct? A. The current is the same in both resistors B. The potential difference across each resistor is the same C. The combined resistance is the sum of the two resistances D. The combined resistance is larger than either resistance Answer: B
- Why is the use of LEDs for lighting increasing? A. They use a much smaller current than other forms of lighting B. They have a much larger resistance than a filament lamp C. They can only be connected in parallel D. They cost less to buy than a filament lamp Answer: A
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