If the magnetic field is created by an electromagnet, what happens if we keep it stationary but vary its strength by changing the current through it?

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1 If a moving electron in a magnetic field experiences a force pushing on it at right angles to its motion, what happens when we take a copper wire (with lots of easily dislodged electrons in it) and move it through a magnetic field? If it cuts the field at 90 degrees, the electrons are pushed to one end of the wire(neg.) from the other (pos.) thus creating a VOLTAGE difference, or EMF - generating electric current if we attach the ends of the wire to a load (e.g. light bulb) What if we leave the wire alone, and move the magnetic field? The electrons don't "know" the wire isn't moving, but if they cut across magnetic field lines they feel the same pull, electric voltage generated again. If the magnetic field is created by an electromagnet, what happens if we keep it stationary but vary its strength by changing the current through it? Growing and collapsing or reversing magnetic fields from an electromagnet act on the electrons in the same way as a moving magnet getting stronger/weaker with distance - electric voltage is generated

2 Moving the magnet into the coil caused current to flow in one direction in the meter, pulling it out again caused it to flow backwards. When the magnet was stopped in the coil, no current flowed. Reversing the magnet reversed the current direction also.

3 The idea that the wire sweeps through the field, cutting across an area of magnetic lines leads to the concept of magnetic flux. If the field (B) is stronger, or the area (A) is larger, there is more flux (Greek letter phi), and if we get through it faster ( in less time t) we generate more electricity, induce a higher voltage difference in the ends of our moving conductor. And if pulling a single loop of wire through a field sweeps out area and induces voltage, why not increase the voltage by using many loops? This is the N in the formula. The negative sign in the formula relates to Lenz's law about the direction of the induced voltage, creating a magnetic field opposing the change in the magnetic field.

4 The coil has a small resistance, and if current flows in it there must be a voltage difference between the two ends of the coil. Both the V and I are called 'induced'. The induction of current works best if the conductor cuts across the magnetic field lines at right angles. Also easy to show experimentally is the fact that faster movement means more current, and no movement means no current. If the conductor is not moving, or moves parallel to the magnetic field lines, no induction.

5 is the voltage difference from one end of the moving wire to the other - note explanation at top of p658, for a wirre pulled in a magnetic field, if it is connected to a circuit, the current flowing in the wire in a magnetic field causes a force on the wire that opposes the applied force pulling the wire in the first place. This equally applies to coils of wire moving in magnetic fields (generators - if more electricity is being used, more current flows and a bigger magnetic field is created in the coil opposing the magnetic field used to generate the electricity, making the coil harder to turn.)

6 A motor spins because of the applied voltage causes current to flow in the coil, creating a magnetic field opposing the stationary magnet. The current direction automatically reverses in the coil as it rotates, so the fields always push against each other. But coils spinning in magnetic fields must generate an induced voltage at the same time, opposite to the incoming voltage. This effectively reduces the net voltage acting in the coil's resistance, decreasing the current flowing. At startup, there is no spin, and no counter voltage (back emf or V back ) so the current is just applied voltage divided by coil resistance. This high starting current creates a very strong magnetic field pushing (and high start-up torque) but as the coil quickly spins up to speed, it is creating V back and the current is decreasing. When the forces balance, the motor spins at a constant speed, creating constant V back and I. If a larger load (restrictive force) is placed on the motor, slowing it, less V back causes I to increase, creating more magnetic field and thus torque.