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, to energy 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 filament lamp is not an ohmic conductor, why a thermistor can run a heating system and an LDR can switch a street light on at dusk, why a diode will pass current one way and refuse it the other, and why a resistor that is doing its job properly always gets warm. Along the way you will work the required practical for this topic, 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.
- There may be an imbalance of charge in an object or area; this is known as static electricity. The charge has no conducting route to travel along. If such a route is provided, the result is a discharge. Students should be aware of some common instances of static electricity, such as lightning, and how they can be explained using the concepts of charge and discharge.
- Electric current is the rate of flow of electric charge. Charge flow, Q, current, I, and time, t, are linked by the equation: I = Q / t
- 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: switch (open) lamp switch (closed) fuse + cell V voltmeter + battery A ammeter diode thermistor resistor variable resistor LDR LED 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: V =I ×R
- 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. Current Potential difference
- 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 thermistor decreases as the temperature increases. Students should be able to describe the applications of thermistors in circuits eg a thermostat.
- The resistance of an LDR decreases as light intensity increases. Students should be able to describe the applications of LDRs in circuits eg switching lights on when it gets dark.
- The resistance of a filament lamp increases as the temperature of the filament increases. Current Potential difference 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. Current Potential difference Required practical: Investigate the V-I characteristics of a filament lamp, a diode and a resistor at constant temperature.
- 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.
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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. Thermistor B. Diode C. Variable resistor D. Fuse Answer: B
- The light intensity falling on a light-dependent resistor increases. What happens to its resistance? A. It increases B. It decreases C. It stays the same D. It becomes zero Answer: B
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