# I Electron properties

1. Mar 21, 2016

### Mihai Dinu

We can deflect a moving electron using an electric field or using a magnetic field. In order to obtain the same deviation, when the energy we should use is higher? Or, in other words, the "electric" or the "magnetic" property is stronger?

2. Mar 21, 2016

### Staff: Mentor

3. Mar 21, 2016

### Khashishi

Take a look at https://en.wikipedia.org/wiki/Lorentz_force
For electrons moving less than the speed of light (always the case), the electric force is stronger. But for relativistic electrons, the forces are about equal.

4. Mar 21, 2016

Staff Emeritus
Only if B = E in those particular units.

5. Mar 22, 2016

### Mihai Dinu

I try to choose the "cheapest energetic" way to deflect an electron, based on its known interactions. The electron has, also, a mass. Hypothetically, I could obtain same deflection as in previous case using this time gravitational attraction of another mass. How can this mass be compared with previous cases in terms of energy effort?

Last edited: Mar 22, 2016
6. Mar 22, 2016

### Staff: Mentor

Let me repeat myself: What about conservation of energy?

7. Mar 22, 2016

### Mihai Dinu

I understand. To obtain a certain same deviation, I have use the same energy, no matter what form it has.

8. Mar 22, 2016

### Staff: Mentor

Yes

9. Mar 22, 2016

### sophiecentaur

I need a bit of help with this. No work need be done on an object if its motion is circular. If we had a fixed pivot and a length of string, the Force X Distance would be zero. Where is the difference with circular motion in a magnetic field? Or are we discussion the Energy needed to set up the field?

10. Mar 22, 2016

### Khashishi

Yeah. That's the case in any reasonable system of units, in which electricity and magnetism are unified. Kind of hard to talk about an electromagnetic field, when the electric and magnetic fields have different dimensions.

11. Mar 22, 2016

### David Lewis

As far as deflecting electrons, if you have high voltages handy, electric fields are usually a good option. But if it's easier to provide high currents then electromagnets might be a better choice. Neither type of force field is intrinsically stronger.

12. Mar 22, 2016

### Mihai Dinu

When the electron pass between the plates of a polarized (ideal) capacitor, the electric field modifies the trajectory of this electron. One billion electrons can follow and the capacitor field will be unchanged. Some work is done. Where comes this energy from?

13. Mar 22, 2016

### ZapperZ

Staff Emeritus
Why isn't this obvious that it comes from the E-field? Turn off the field, no deflection.

Zz.

14. Mar 23, 2016

### Mihai Dinu

I can not turn off the field, because it comes from the electrons of one plate and the ions of the other plate, and their number is not changed, no matter how many free electrons cross this field.

15. Mar 23, 2016

### ZapperZ

Staff Emeritus
You missed the entire point of my post!

Zz.

16. Mar 23, 2016

### Mihai Dinu

I tried to understand: Free electrons that cross the capacitor static field change trajectories, so work is done, but the capacitor field rest unchanged forever. Where comes the new energy from?
Now I think at this scenario: the incoming kinetic electron starts to interact with the capacitor field and transferes energy to capacitor field by disturbing one by one the electrons from the plates, upto the moment when the capacitor field starts to restore its minimum state and pushes out the disturbing electron. So the electron gets no new energy, only a new trajectory - if case.

Last edited: Mar 23, 2016
17. Mar 23, 2016

### David Lewis

Correct. No energy need be consumed in deflecting electrons. You could use a permanent magnet, for example. There may be an energy input required to maintain an electric field (replace charge leakage) or magnetic field (resistance of the coil).

18. Mar 23, 2016

### ZapperZ

Staff Emeritus
This is incorrect for electrons in electric field, and certainly not true for uniform electric field such as that found in between parallel plate capacitors. The electrons gain kinetic energy from the E-field. This is similar to projectile motion in uniform gravitational field. So it isn't just a change in trajectory, as is the case for uniform magnetic field. There IS KE change! It is how we accelerate charged particles in particle accelerators.

Zz.