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OxfordAQA · International GCSE · Physics Тема 7 4:06 Английское озвучивание · Английский + китайские субтитры (встроенные)
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An atom is about 10 to the minus 10 metres across, with a tiny nucleus at its centre and electrons around it.
原子直径约为 10 的负 10 次方米，中心是一个很小的原子核，电子在它周围。
The nucleus is about a hundred-thousandth of the atom's width, yet it holds nearly all the mass.
原子核的宽度只有原子的十万分之一左右，却集中了几乎全部质量。
Unstable nuclei give out energy as radiation.
不稳定的原子核会以辐射的形式释放能量。
Protons and neutrons each have a relative mass of 1.
质子和中子的相对质量都是 1。
A proton has a charge of plus 1, a neutron none.
质子带 +1 电荷，中子不带电。
An electron has a charge of minus 1 and almost no mass.
电子带 −1 电荷，质量几乎为零。
A neutral atom has equal numbers of protons and electrons; lose or gain electrons and it becomes an ion.
中性原子的质子数等于电子数；失去或得到电子就成为离子。
Fire alpha particles at thin gold foil.
用 α 粒子轰击很薄的金箔。
Most alpha particles go straight through, so the atom is mostly empty space.
大多数 α 粒子直接穿过，说明原子大部分是空的。
A few are deflected, and a few bounce straight back: the mass and positive charge are concentrated in a tiny nucleus.
少数发生偏转，极少数被直接弹回：质量和正电荷集中在一个很小的原子核里。
Read fluorine-19.
来读氟-19。
The top number is the mass number: protons plus neutrons, 19.
上面的数是质量数：质子数加中子数，为 19。
The bottom number is the proton number, 9.
下面的数是质子数，为 9。
So it has 19 minus 9: 10 neutrons.
所以中子数是 19 减 9，即 10。
Isotopes of an element share the proton number but differ in neutrons.
同一元素的同位素质子数相同，中子数不同。
From a source, alpha is stopped by a sheet of paper.
从放射源出发，α 射线被一张纸挡住。
Beta passes paper but is stopped by a few millimetres of aluminium.
β 射线能穿过纸，但被几毫米厚的铝挡住。
Only gamma gets through, and thick lead reduces it without stopping it completely.
只有 γ 射线能穿过，厚铅板能减弱它，但不能完全挡住。
Alpha is a helium nucleus, two protons and two neutrons: the most ionising, and it travels only a few centimetres in air.
α 粒子是氦原子核，由两个质子和两个中子组成：电离能力最强，在空气中只能前进几厘米。
Beta is a fast electron made in the nucleus.
β 粒子是原子核中产生的高速电子。
Gamma is an electromagnetic wave: weakly ionising, it travels metres.
γ 射线是电磁波：电离能力弱，能传播数米。
Background radiation surrounds us all the time.
我们周围时刻都有本底辐射。
Polonium-210 emits an alpha particle.
钋-210 放出一个 α 粒子。
The top numbers must balance: 210 equals A plus 4, so A is 206.
上面的数必须守恒：210 等于 A 加 4，所以 A 为 206。
The bottom numbers must balance too: 84 equals Z plus 2, so Z is 82.
下面的数也要守恒：84 等于 Z 加 2，所以 Z 为 82。
The product is lead-206.
生成物是铅-206。
The half-life is the time for half the nuclei to decay.
半衰期是一半原子核发生衰变所需的时间。
A count rate falls from 1200 to 600, to 300, and to 150 in 15 hours.
计数率从 1200 降到 600、再到 300，15 小时后降到 150。
That is three half-lives, so the half-life is 5.0 hours.
这是三个半衰期，所以半衰期为 5.0 小时。
Each halving takes the same time.
每次减半所用的时间都相同。
Contamination means radioactive atoms are on you or in you: the hazard lasts until they are removed.
沾污是指放射性原子附着在你身上或进入体内：危害一直持续到它们被清除。
Irradiation is exposure from a source outside; it ends when the source is taken away, and does not make you radioactive.
照射是来自外部放射源的辐射；放射源移走后照射就停止，也不会使你带有放射性。
A U-235 nucleus absorbs a neutron and splits into two smaller nuclei, with 2 or 3 neutrons and far more energy than any chemical reaction.
铀-235 原子核吸收一个中子后分裂成两个较小的原子核，放出 2 到 3 个中子，能量远超任何化学反应。
Those neutrons can split more nuclei: a chain reaction.
这些中子又能使更多原子核分裂：形成链式反应。
In a reactor, control rods absorb neutrons, so that on average one neutron goes on to cause the next fission.
在反应堆中，控制棒吸收中子，使平均每次裂变只有一个中子去引发下一次裂变。
Fusion joins two light nuclei into a heavier one, turning some mass into energy.
聚变把两个轻原子核结合成一个较重的原子核，把部分质量转化为能量。
It powers the stars.
恒星就靠它发光。
But positive nuclei repel each other, so they must collide very fast: fusion needs extreme temperature and pressure.
但带正电的原子核相互排斥，必须以极高速度相撞：所以聚变需要极高的温度和压强。
Four common mistakes. The mass number does not change in beta decay; only alpha decay changes it.
四个常见错误：β 衰变时质量数不变，只有 α 衰变才改变质量数。
After two half-lives a quarter is left, not nothing.
两个半衰期后还剩四分之一，而不是全部消失。
Inside the body alpha does the most damage, because it is the most ionising.
在体内，α 射线危害最大，因为它电离能力最强。
And an irradiated object does not become radioactive.
被照射的物体不会变得有放射性。
Check yourself.
自我检测。
Fluorine-19 has 9 protons and 10 neutrons.
氟-19 有 9 个质子和 10 个中子。
Paper stops alpha.
纸能挡住 α 射线。
Radium-226 decays by alpha to radon-222 and a helium nucleus.
镭-226 经 α 衰变生成氡-222 和一个氦原子核。
Control rods absorb neutrons to keep the chain reaction steady.
控制棒吸收中子，使链式反应保持稳定。