PHYS110-How Things Work

Notes on Induction Lab

Notes
When the handheld generator is connected to one coil and the oscilloscope is connected to the other coil, the generator produces changing current in the first coil, which generates a changing magnetic field. The second coil experiences this changing magnetic field and thus current is induced in the second coil. This is the principle behind the transformer. The current in the second coil is observed as the signal seen on the oscilloscope. If the second coil has less turns than the first coil, the pair make a step-down transformer. Likewise, if the second coil has more turns than the first coil, the pair make a step-up transformer. When the steel rod is inserted it becomes magnetized by the first coil, and thus the second coil has an even greater current induced.
Misconceptions
  1. Current is not produced in the generator by the friction in the brushes.
    The brushes serve only to make an electrical connection between something external (like a light bulb or a galvanometer) and the ends of the rotating coil wires (which are connected to two parts of the rotating shaft).
  2. The rod inserted in the coils does not carry charge.
    The rods become magnetized by one of the coils and thus make the total magnetic field stronger. This means the second coil will experience an even greater magnetic field than it would without the rod. Thus the rod helps one coil to induce current in the second coil. There is no electrical connection made with the rod. There is no current in the rod. The rod is not charged.
Additional Question
1. If a battery is connected to a coil of wire, and a compass is held near the coil, how will the compass be affected?

A coil with current generates a magnetic field, therefore the compass will be affected by the field. How much it is affected and in what direction it points, depends on the placement of the compass and the strength of the current.

2. In the above question, if the battery is replaced by an AC source, how will the compass be affected?

Since the current is changing, so is the magnetic field. If the field is changing slowly enough, the compass will change its direction in response. If it is changing too quickly the compass may not be able to respond quickly enough to be noticable. If a compass is placed near an electrical cord plugged into a 120V AC outlet, and the current is on, no deflection will be observed for two reasons. The first is because the field is changing too quickly (60 Hz). The second reason is that the wires carry current is opposite directions, so the magnetic field due to each wire cancels out.

3. Why does a compass not work reliably when around some metal objects?

A compass is a magnet. So if it is near a ferrous piece of metal like iron, the metal will be magnetized and thus deflect the compass. Another reason is that often metal has some magnetization to start with, which will aslo affect the direction of the compass.