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Phillip would like to know:Would it be possible to create, generate, and control a static electromagnetic field?What does everyone think?
Your average AA battery from the corner store generates a static electromagnetic field while it is sitting on the shelf.There is no current, so the magnetic component is zero. And the electric field is about 1.5 Volts.
That's an EMF and as such there is no electric field associated with it. The units of the electric field is not volts, its volts per meter.
Could you clarify that for me, please, Pete?The length of an AA battery is around 4cm. That makes it around 1.5V/4cm = 40V/m; is it now an electric field?
Sorry Pete; I didn't follow that.
chiralSPO - Great explanation. Please note that a capacitor does not have an EMF, it has an actual difference in potential across it due to the separation of charge across the capacitor plates. When applying Kirchhoff's around a circuit a capacitor is treated as a difference in potential, not an EMF.
I believe that what Pete is saying is that a battery does not necessarily have an electric field about it. An electrochemical (galvanic) battery is not a capacitor, which has an EMF that is determined purely by a macroscopic electric field. Batteries instead have an EMF that is determined by microscopic electric fields (atomic scale), and so when completely isolated from a circuit, a battery is unlikely to have a significant dipole moment (uneven distribution of charge within the battery) Imagine a simple electrochemical cell, containing a piece zinc metal at one end of a tube, and a piece of copper at the other end. The copper end of the battery contains an electrolyte solution composed of copper chloride dissolved in water (Cu2+ and Cl– ions), while the zinc side is dissolved sodium chloride (Na+ and Cl– ions), and a thin membrane separates the two electrolytes, allowing chloride ions to cross, but not copper. When the two electrodes are not connected to anything, no electrochemical reaction occurs, and the tube shouldn't have any significant electric field about it. (you could measure the amplitude and direction of the field anywhere around the cell, and would probably not observe any--if you touch a meter to the two ends of the cell it would detect the EMF, but the potential difference between points in space each a micron away from the electrode is probably pretty close to zero).Once a circuit is completed, electrons flow from the zinc electrode to the copper electrode, which dumps the electrons into the empty orbitals of the Cu2+ ions, reducing them to copper metal, and chloride ions move to the zinc end of the cell. So negative charge (moving either as electron or chloride) has moved all around the circuit, but the cell itself doesn't have any change in charge distribution on a macroscopic scale, only changes in the composition of matter within.
You've lost me.What distinction are you drawing between EMF and PD?
If there's a voltage between the terminals of a cell, then there's an electric field in the space surrounding the cell, which is what I assume evan_au was getting at.
I believe that what Pete is saying is that a battery does not necessarily have an electric field about it. An electrochemical (galvanic) battery is not a capacitor, which has an EMF that is determined purely by a macroscopic electric field.
Then suspend (the rubber rod) by a thread. When you've done that place a battery next to the charged rod. If there was any charge on it then it would move. You'll find that it won't.
Doesn't this just reflect the fact that the insulating rod is charged to thousands of volts, while your typical battery only has an EMF of 1.5V?
There's a chemical force that is responsible for the emf of the battery. This chemical force will drive positive charge to the + >terminal, and/or minus charge to the - terminal. This accumulation of charge will (of course) set up an electric field (pointing, incidentally, in the opposite direction to the chemical force, so the net force on charges inside the battery is zero, if it's not connected to a circuit, and hence no current flows). So the answer to your question is YES: there WILL be an electric field inside the battery. If you now connect the battery up to a circuit, it is this electric field that drives charge around the loop (only indirectly the chemical force, which after all is localized within the battery, and cannot directly push charge through the distant resistor).
There's a difference in potential between two points in a static electric field but such a field cannot produce a steady current in a resistor whereas an EMF can.
This seems to me a bit like arguing that there is a difference, in kind, between the metre used to measure the position of a point in empty space, and the metre used to measure the height of a reservoir that can generate power in a hydro plant. Distance is distance.
FWIW, and digressing a bit from electrostatic fields, my usage of the two terms as a radio engineer
The electric charge that has been separated creates an electric potential difference that can be measured with a voltmeter between the terminals of the device.