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## AP®︎/College Physics 2

### Course: AP®︎/College Physics 2>Unit 5

Lesson 2: Magnetic field created by a current

# Induced current in a wire

Sal determines the current and EMF induced in a wire pulled through a magnetic field. Created by Sal Khan.

## Want to join the conversation?

• What is the difference using slip rings in AC generator and split rings in DC motor?
It's not exactly related, but I find it very confusing.
• In a DC motor you have to use split rings, otherwise it's not a DC motor but another type of motor. Likewise, in an AC generator you have to use slip rings, because then it wouldn't be called an AC generator. The split rings in a DC motor allow the electrical source to be in contact with the coil/loop so that electrical current can flow through it as it turns within the magnetic field. The split rings allow the current to flow in one direction in the loop due to a gap (hence the name 'split rings') which allows the arms of the loop not to get twisted and intertwined, and also so that the current in the loop doesn't change direction every half cycle, because then the loop wouldn't rotate at all (the source of the current is DC). In the AC generator mechanical energy is converted to electrical energy. Because of the rotational motion of the coil/loop within the magnetic field, the current in the loop changes direction every half cycle (use Fleming's Right Hand Rule) - the slip rings simply make sure that the same arm of the coil is connected to the same terminal of the external electric circuit so that the current can be used to, say, light a light bulb. This generates an Alternating Current (AC). Hope this helps.
• I have heard from my professors that a magnetic field cannot do work. Can someone explain?
• Not 100% correct. Magnetic, electric and even gravitational fields are conservative fields. This means that regardless of the path you take through the field if you start and end at the same place in the field the difference in energy, potential and total work is 0.

Using gravity as an example if you have water at the top of a cliff and drop it on a turbine it will spin the turbine and you can get work out of it. This is only half of the story, how did the water get to a higher potential? Work was done on it to lift it up to a higher potential. So if you look at the whole cycle without any energy losses you would have exactly 0 work. The same goes for any conservative field.

Because of this in most cases magnetic, electric and gravitational fields are used to transmit work from one system to another.
• since the charge started moving up, that means there is another force pointing to the right, correct?
• Yes, you are exactly right.
However, compared to the velocity of the wire being moved to the right, the force on the wire to the right (because of the charges moving up) is infinitesimally small and insignificant (in most cases).
• When the charge starts moving up the wire it's going to slow the movement of the wire (or start pushing the wire in the direction of the magnetic field, right?
• Yes, it slows the movement of the wire.

Because of related effects, you can sometimes drop a magnet down a copper pipe, and the magnet will fall much more slowly than a normal object would - almost as if it is parachuting.
• Doesn't it have to be a loop of wire for there to be induced current?
• No one seems to be mentioning or noticing the fact that there is no closed circuit, so if it is not an infinitely long wire, a charge must be building up on both ends of the wire because if the charges have nowhere else to go, they will "bunch up". But, in that case, the capacitance between one end of the wire and the other would discharge an the current would have to reverse, causing a reverse voltage. The current between the "capacitor" and the wire, which is an inductor, would be a damped oscillation at a very high frequency since the inductance of a short wire and the tiny capacitance would resonate at a high frequency. Whatever the case, this should be addressed in the video or another following video. Of course, another way, energy could be released is by radiation, which would happen as the charges accelerate through the wire.'

Of course, in the previous videos, no current flows when the motor loop is not connected to a load.

I too, would like to see some other comments on this.
• what is the difference between potential difference/voltage and e.m.f?
• EMF is in essence a superset of voltage. For e.g. If a battery of EMF 6 V is hooked up across a resistor it doesn't necessarily produce a voltage difference of 6 V across the resistor, since there is an internal resistance in the battery itself, caused due to its internal mechanism.
Hence Voltage = EMF - (Current in the battery 'i' )*(Internal Resistance of battery)
V = E - iR
• What are carbon brushes?
• A brush is a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft.
• In self inductance when current is increased the induce emf will be opposite to that of battery and if current is decreased the induce emf will aid rather than opposing the battery. How?
• When current is flowing through an inductor, the situation is analogous to a mass gaining momentum, or a spinning object with rotational inertia. The magnetic field created by the inductor stores a bunch of energy, and you can think of this like an object that is rotating, and thus has both angular momentum and rotational kinetic energy. Trying to suddenly change the current in an inductor is like trying to suddenly change the rate of rotation of a spinning object -- it will "resist" you. If you really did stop the current in a very short time, the magnetic field would disappear in that time (the current is the source of the magnetic field), but the magnetic field represents a bunch of energy, and that energy has to go somewhere.

The same idea works in the opposite direction. If you suddenly try to increase the current in an inductor, you are trying to suddenly create a significant magnetic field, but that requires energy, and so your battery will be pushing with all its voltage, but it will appear that the inductor "pushes back" with some opposite emf, since otherwise you'd be extracting a large amount of energy out of a small power source in a very short time.