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 current to charge and to resistance. You will learn to read and draw the eleven 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 filament lamp is not an ohmic conductor, why a diode will pass current one way and refuse it the other, and why the total opposition of a circuit goes down when you add a second branch to it. Along the way you will work one of the three required practicals for this course, which asks you to measure the current-voltage characteristics of three components and explain the three very different graphs that come out of it.
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. 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:
- 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 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.
- The ‘forward’ resistance is low in a diode and the ‘reverse’ resistance is very high. The current through a diode flows in one direction only. Required practical: Investigate the V-I characteristics of a filament lamp, a diode and a resistor at constant temperature.
- 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.
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- 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
- Which component allows current to flow through it in one direction only? A. Variable resistor B. Diode C. Fuse D. Lamp Answer: B
- Two resistors of different resistance are connected in parallel across a battery. Which statement is correct? A. The current is the same in each resistor B. The combined resistance is greater than that of either resistor C. The current from the supply splits between the branches D. The potential difference across each resistor is half that of the supply Answer: C
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