Monday, May 4, 2009

Reviewed concepts from electricity for those students who missed Friday's class due to IB testing.

Reviewed formula for resistance: R = resistivity * length/Area
Resistivity is temperature dependent. In general, the lower the temperature, the lower the resistivity. Superconductors occur at very low temperatures.
Reviewed Ohm's Law
Reviewed resistors in series and parallel. Derived the equations for finding equivalent resistance. Showed how to do a complicated problem.

Reviewed using voltmeters and ammeters in circuits.
To use an ammeter, you break the circuit and insert the ammeter to measure the current. Ammeters are connected in series. Ammeters must have low internal resistance to not change the resistance of the circuit.
Voltmeters measure potential difference across a circuit element or between points in a circuit. Voltmeters must have high internal resistance so as not to increase the resistance of the circuit.

Meters and power supplies have internal resistance. If you connect a voltmeter across a battery, it may read the EMF because the current is very low and you do not get much of a voltage drop across the internal resistance. In a circuit, the current may be greater and you will have a voltage drop across the internal resistance decreasing the output voltage.

Did some problem on request from the second electricity worksheet.

Started magnetism.
Four forces. Magnetism has now been combined with electricity.
Currents create magnetic fields.
Changing magnetic fields create currents.

Demonstrated producing current in a coil by relative motion of a magnet and the coil.
Demonstrated magnetic field lines with iron filings.

Magnets have two poles, north and south. Opposite poles attract.
Discussed magnetism of Earth and that the geographic north is magnetic south since the north end of the compass needle points north and opposite poles attract.
Showed maps of magnetic declination.

Used right hand rule to find electric field lines around a current carrying wire. Intensity of the field is given by B = uo I/2 pi r
Force on a moving charge is given by F = qv X B where X is the vector cross product.
You cannot do magnetism in two dimensions, unlike the electric field where F = qE and the force and electric field are co-linear.

If v and B are perpendicular, then F = q v B. If v and B are co-linear, then F = 0.
Showed how to use the right hand rule to find the direction of the force.

Worked problem of parallel current carrying wires and showed that if the currents are in the same direction the wires attract and if in opposite directions they repel. Measuring the force of attraction gives the definition of the ampere.

Quiz on direction of magnetic fields around a current carrying wire.
Handed out both worksheets on magnetism.