# 物理代写|电动力学代写electromagnetism代考|PHYC20014

## 物理代写|电动力学代写electromagnetism代考|Combinations of Capacitors

In engineering, we often have to make something new out of existing components. This is because manufacturers like to produce standard components to keep costs down. When designing a circuit we often need a particular value of capacitor that is not available from any source. We could pick a value close to the one we require, and then physically altề the area of the capacitone to gêt the capaciciance ręuiréd hy filling the top down. This requires the use of a capacitance meter, a file, a steady hand and a great deal of patience! Although this can be done, it is not a very practical way to get a non-standard value of capacitance. Instead, we can produce non-standard capacitance values by combining standard values in parallel or series.

Figure 2.26a shows two capacitors, $C_1$ and $C_2$ in parallel. We require to find the equivalent capacitance of this arrangement. Let us connect a d.c. source, $V_s$, to the capacitors. Now both capacitors will have the same voltage across them, but store different charge. Thus, we can write
$$Q_1=C_1 V_s$$
and
$$Q_2=C_2 V_s$$

If we replace the two capacitors by a single equivalent one of value $C_t$, the charge on the new capacitor, $Q_t$, must be the same as on the parallel combination. Thus,
$$Q_t=C_t V_s$$
As $Q_t$ must be the sum of the individual charges, in order to be equivalent, we can write
$$Q_t=Q_1+Q_2$$
or
$$C_t V_s=C_1 V_s+C_2 V_s$$
Thus,
$$C_t=C_1 \mid C_2$$
So, we can increase capacitance by adding another capacitor in parallel with the original.

## 物理代写|电动力学代写electromagnetism代考|SOME APPLICATIONS

We will look at two applications of electrostatics: the electron gun and the electrostatic precipitator. Electron guns are often found in high-school physics experiments to study diffraction of electrons amongst other experiments.

Figure $2.27 \mathrm{a}$ is a cathode ray tube, CRT, that uses an electron gun (Figure 2.27b) to produce an image on the screen. (Although such technology has now been largely superseded by LCD and plasma screens, electron guns are still used in physics experiments to study electrons.) The electron gun is capable of accelerating electrons to speeds approaching that speed of light. At the very rear of the device is a heater that causes the cathode to emit electrons. (These electrons are thermally excited because the cathode reaches temperatures in excess of $1800^{\circ} \mathrm{C}$. This is thermionic emission.) Of course, if the gun contains air, the electrons will lose momentum and may fail to reach the target. Thus, it is important that the gun has a vacuum inside and this is usually done by burning magnesium in the tube after it has been sealed.

Now for some simple calculations, let us assume that the anode is grounded and that the cathode is at a negative voltage of $V$ volt. Let us also assume that the distance between the anode and cathode is $x$ metre. When the cathode produces electrons by thermionic emission, they are accelerated by the $E$ field. So, the force acting on one electron is
$$\boldsymbol{F}=q \boldsymbol{E}$$
This must be equal to the mass of the electron, $m$, times the acceleration due to the field. Thus,
$$\boldsymbol{F}=q \boldsymbol{E}=m a$$
giving
$$\boldsymbol{a}=\frac{q}{m} \boldsymbol{E}$$

## 物理代写|电动力学代写electromagnetism代考|Combinations of Capacitors

$$Q_2=C_2 V_s$$

$$Q_t=C_t V_s$$

$$Q_t=Q_1+Q_2$$

$$C_t V_s=C_1 V_s+C_2 V_s$$

$$C_t=C_1 \mid C_2$$

## 物理代写|电动力学代写electromagnetism代考|SOME APPLICATIONS

$$\boldsymbol{F}=q \boldsymbol{E}$$

$$\boldsymbol{F}=q \boldsymbol{E}=m a$$

$$\boldsymbol{a}=\frac{q}{m} \boldsymbol{E}$$

myassignments-help数学代考价格说明

1、客户需提供物理代考的网址，相关账户，以及课程名称，Textbook等相关资料~客服会根据作业数量和持续时间给您定价~使收费透明，让您清楚的知道您的钱花在什么地方。

2、数学代写一般每篇报价约为600—1000rmb，费用根据持续时间、周作业量、成绩要求有所浮动(持续时间越长约便宜、周作业量越多约贵、成绩要求越高越贵)，报价后价格觉得合适，可以先付一周的款，我们帮你试做，满意后再继续，遇到Fail全额退款。

3、myassignments-help公司所有MATH作业代写服务支持付半款，全款，周付款，周付款一方面方便大家查阅自己的分数，一方面也方便大家资金周转，注意:每周固定周一时先预付下周的定金，不付定金不予继续做。物理代写一次性付清打9.5折。

Math作业代写、数学代写常见问题

myassignments-help擅长领域包含但不是全部: