Electricity and magnetism, Year 10: components, series and parallel circuits

An ohmic conductor

提示The adjective comes from the surname of the scientist the unit of resistance is named after.

为什么A fixed resistor kept at a steady temperature behaves this way. Doubling the potential difference doubles the current, so the resistance stays the same whatever the current. Lamps and diodes do not behave like this.

A straight line through the origin

提示The two quantities are directly proportional; think what that always looks like on a graph.

为什么Because the resistance is constant, equal steps in potential difference give equal steps in current, in either direction. A component with this kind of graph is called linear. Lamps, diodes, thermistors and LDRs give curves, and are called non-linear.

The resistance increases because the filament gets hotter

提示Consider what a glowing wire is doing that a cold one is not.

为什么A larger current heats the filament more, and the resistance of a metal rises with its temperature. Each extra volt therefore produces a smaller extra current than the one before, which bends the graph over. The lamp is a non-linear component.

Diode

提示A light-emitting version of it is used in modern lamps and screens.

为什么In the forward direction it conducts once the potential difference is large enough. In the reverse direction its resistance is very high, so almost no current flows. Its graph of current against potential difference is flat along the axis on the reverse side and climbs steeply on the forward side.

To change the current through the component so a range of readings can be taken

提示One pair of meter values gives one point; a graph needs many.

为什么Altering the variable resistor changes the current in the loop, and with it the potential difference across the test component. Each setting gives one pair of ammeter and voltmeter readings, and the set of pairs is plotted as the graph. This is AQA required practical 16, carried out for a resistor, a filament lamp and a diode.

To take readings with the current flowing the opposite way

提示The finished graph has a negative half as well as a positive half.

为什么Swapping the connections reverses the potential difference across the component, giving negative values of both readings. For a resistor and a lamp the graph looks the same in both halves. For a diode it does not, and this step is what reveals that it conducts one way only.

decreases

提示It behaves the opposite way to the metal filament of a lamp.

为什么This makes a thermistor useful as a temperature sensor: a circuit can detect the change in its resistance and respond. In a thermostat it is used to switch a heater off when a room is warm enough and on again when it cools.

It rises

提示First decide what warming does to the thermistor; then what that does to the flow.

为什么Warming the thermistor lowers its resistance. With the same potential difference from the cell and less resistance in the loop, a larger current flows. A circuit like this turns a change in temperature into a change in current that can be measured.

Thermistor

提示Its name runs 'thermal' and 'resistor' together.

为什么Its resistance falls as it warms. A thermostat circuit uses that change to decide when to switch a heater or a cooling fan on and off, keeping a room, an oven or a fridge near a set temperature. AQA names the thermostat as the application pupils must know.

Its resistance changes with the light level, so the circuit can switch the lamp on when it gets dark

提示Something has to notice that evening has arrived, without anyone telling it.

为什么The resistance of an LDR is low in bright light and high in the dark. The control circuit detects the rise in resistance at dusk and turns the lamp on, then turns it off again at dawn. Switching lights on when it gets dark is the application AQA names.

A light-dependent resistor (LDR)

提示The name simply describes what its resistance relies on, and is usually shortened to three letters.

为什么In the dark its resistance can be very large; in bright light it drops to a small value. That makes it a light sensor for circuits such as automatic night lights and street lamps.

10

提示The current has to get through one and then the other.

为什么For components in series the total resistance is the sum of the separate resistances: R total = R1 + R2 = 4 + 6. AQA's specification states this rule without marking it as either recalled or given on the equation sheet, so it is safest to know it.

It is less than 4 Ω

提示Compare it with the smaller of the two.

为什么Two resistors in parallel always have a total resistance smaller than the smallest individual one. AQA expects pupils to know this rule but says they are not required to calculate the value. Here the true figure is 2.4 Ω, which fits the rule.

It gives the current an extra path, so more current flows for the same potential difference

提示Think of opening a second checkout in a busy shop.

为什么Each branch has the full potential difference of the supply across it, so the first resistor carries the same current as before, and the new branch carries some more on top. More total current for the same potential difference means, by R = V ÷ I, a smaller total resistance.

6; 2; 10

提示Add first, then use the same equation twice: once for the whole loop and once for a single resistor.

为什么Total resistance = 1 + 5. Current = total potential difference ÷ total resistance = 12 ÷ 6, and it is the same through both resistors. Across the 5 Ω resistor V = I × R = 2 × 5; the remaining 2 V is across the 1 Ω resistor, so the two shares add up to the battery's 12 V.

The same current has to pass through every resistor, one after another

提示There is only one route, and now it has one more obstacle on it.

为什么In series there is a single path, so the charge must get through each resistor in turn and the potential difference of the supply is shared between them. Each resistor gets a smaller share, so the current through all of them is smaller. A smaller current for the same supply means a larger total resistance.