PHYS110-How Things Work
Notes on Induction Lab
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.
- 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).
- 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.
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.