A plate capacitor usually consists of two round or rectangular conductive plates. These have an area A and are located at a distance D from each other. Both the area and the distance between the plates are two important parameters that geometrically characterize a plate capacitor. So far, there are only two plates. Only when you put positive and negative electric charges on the two plates, the whole setup becomes a plate capacitor. Charge one plate with positive charge and the opposite plate with the same amount of negative charge. So the total charge on one plate is plus Q and on the other plate minus Q. The amount Q is the same on both plates. The positive and negative electric charges on the separated plates now attract each other. If they were free, they would simply move towards each other. But since the plates are spaced at a fixed distance from each other, they cannot do that. From this, you probably already recognize the first possible application of a plate capacitor. If you connect the two charged plates with a conducting wire and a small lamp, an electric current flows from one plate to the other and causes the lamp to light up until there is no more difference in charge on the plates. So with a capacitor, you can store electric energy. But you can do much more with it. For example, creative frequency filter, which is built into the charging cable of your smartphone, and is there to protect the microelectronics from external electromagnetic interference. But that is by far not all. Capacitors are all around us. Even now in this very room. So it's worth getting to know them a little better. If you place a small charge Q that is freely moving positive test charge, directly on the positive plate, positive plate repels the positive charge and the negative plate attracts it. The free charge experiences an electric force F inside the plate capacitor, which accelerates the test charge straight ahead. The charge accelerates until it reaches the opposite negative plate. Before it hits the plate, it has received a velocity V from the acceleration and thus also a kinetic energy W. This energy, which the charge has gained by moving from one plate to the other, is characterized by the electric voltage U between the plates and is shown in a drawing, for example, like this. Electric voltage U between two plates is the energy W that is small sample charge gains when it moves from one plate to the other, divided by the charge Q. So voltage is energy per charge. You can influence the voltage between the plates and thus also the energy gain from the test charge by charging the plates even more, that is increasing the charge on both plates. Then the electric force on the test charge will increase. The test charge would then accelerate even more and thus achieve a greater speed at the end, so gain a greater energy. If you double the electric charge Q, then the electric voltage U doubles as well. A test charge would then gain twice as much energy when it moves from one plate to another. Charge and voltage are proportional to each other, where the constant of proportionality C is the so-called capacitance. Its unit is called per volt, or for art for short. The capacitance indicates by how much the voltage U changes when the charge Q on the plates is changed. Capacitance is an important characteristic quantity of a capacitor, which depends mainly on its geometry, that is on the distance D, and on the plate area A. The capacitance also depends on the material, with which the space between the plates is filled. Here we assume that between the plates there is a vacuum or at least only air. You can calculate the capacitance of a plate capacitor as follows. Capacitance C is the electric field constant epsilon zero, multiplied by the plate area A, and divided by the distance D. So to find out the capacitance of a plate capacitor, you only have to determine the plate area and measure the distance between the plates. To get the largest possible capacitance of the plate capacitor, you have to make the plate area as large as possible, and the plate distance as small as possible. No matter where you place the charge inside the plate capacitor, it will always move straight ahead to the other plate everywhere and experience the same force F. A force field, that is the entirety of all force vectors in space, is homogeneous here. Homogeneous means that it doesn't matter where you place the test charge. The test charge experiences the same electric force everywhere in the plate capacitor. You can calculate the force on a test charge. The force F is without deriving the formula here, test charge Q, multiplied by the voltage Q, and divided by the distance D between the plates. The force on a test charge is therefore greater if the test charge Q and the voltage U are greater and the plate distance is smaller. If you divide the force F by the test charge small Q, you get the quantity electric field E. So the E field is nothing else than force per charge. The electric field E in the plate capacitor is determined by the voltage and the play distance. The greater the voltage and the smaller the distance, the greater the electric field. Since the force field is homogeneous, the electric field in the plate capacitor is also homogeneous. Instead of drawing the vector arrows, the electric field is often illustrated with field lines. On such a straight line, the test charge then moves.