Electricity and magnetism, Year 8: more practice：全部卡片

250

提示1 A is 1000 mA.

为什么Multiply amps by 1000 to get milliamps: 0.25 × 1000 = 250 mA.

The current stops, so the magnetic field stops

提示What flows through the coil to make it act as a magnet?

为什么A coil's magnetic field exists only while current flows. An iron core stays magnetised only weakly and briefly, so the clips drop.

Its north and south poles swap

提示Reverse the current and the ends reverse too.

为什么The direction of the magnetic field depends on the direction of the current. Reversing the current reverses the poles.

A voltmeter

提示The letter is the first of the quantity it measures.

为什么A voltmeter measures potential difference. It is drawn as a circle containing V.

A resistor

提示It resists the flow of current.

为什么A resistor is a rectangle on the wire. A variable resistor is the same rectangle with an arrow through it.

So anyone can read and build the same circuit

提示The aim is agreement between different people.

为什么Symbols are international, so a diagram can be understood without words and without being redrawn to look like real parts.

No: the current is the same after the bulb as before

提示Compare ammeter readings either side of it.

为什么The charge flows round the whole loop and is not used up. It is the energy it carries that is transferred in the bulb.

The volt (V)

提示It is named after a scientist.

为什么Potential difference is measured in volts, with a voltmeter connected across the component.

3

提示Series cells add together.

为什么The voltages of cells in series add up: 1.5 V + 1.5 V = 3 V.

The same

提示Each branch gets the full push of the battery.

为什么In a parallel circuit each branch gets the full potential difference of the battery, so each identical bulb is as bright as one on its own.