PHYS132 Spring 2012 Magnetic induction March 30, 2012
PHYS132 Spring 2012 Organization of Part F Chapter 18 – Describing AC Circuits Chapter 19 – Impedance in an AC Circuit Chapter 20 – Capacitors Chapter 21 – Inductors Chapter 22 – Applications: Motors, Transformers and Magnetic Breaking
PHYS132 Spring 2012 What is a capacitor? Two “plates” separated by an insulator
PHYS132 Spring 2012 How does a capacitor work? • No charge ever crosses the “gap” • How long it takes for the plates to “fill” depends on the capacitance • For most capacitors, the time is very, very short (microseconds) • If the current oscillates quick enough, the plates never fill up and current continues to flow
PHYS132 Spring 2012 What is an inductor? An inductor is just a coil • When current flows through a coil – it becomes a magnet • Placing a metal “core” in it just strengthens the magnet
PHYS132 Spring 2012 How does an inductor work? An inductor does not like changes in the current it acts like a resistor when current increases and a battery when current decreases Where does the energy “go” (when the current increases) and where does the energy “come from” (when the current decreases)? magnetic energy
PHYS132 Spring 2012 Metal detectors • How does a metal detector work?
PHYS132 Spring 2012 Units • An inductor’s inductance (in henries) and a capacitor’s capacitance (in farads) depends only on its structure. • An inductor’s impedance (in ohms) and a capacitor’s impedance (in ohms) depends not only on its structure but also on the frequency of the applied voltage. • The impedance does not depend on the amplitude of the applied voltage.
PHYS132 Spring 2012 Three exploration stations • Generating electricity • Magnetic braking • Transformers
PHYS132 Spring 2012 Station 1 – Generating electricity Turn the crank – what happens? Does it matter how fast you turn it? Or which way you turn it? Examine how everything is connected. Be careful not to break the connections. You can remove the “top” of the N/S piece in order to look more closely at the coils. Just be careful since the magnets are strong.
PHYS132 Spring 2012 Station 2 - Magnetic braking The large magnet is particularly strong. Be careful with credit cards and ferromagnetic objects (once on you may not be able to get it off). Examine the aluminum pieces. Is aluminum ferromagnetic? Place the aluminum pieces between the poles. What happens?
PHYS132 Spring 2012 Station 3 - Transformers One coil is connected to the power supply. The other is connected to the bulb. Are the two coils connected via any wires?Turn on the power supply. Can you get the bulb to light? Feel free to open up the metal core to examine it and/or switch coils. Just turn off the power supply first.
PHYS132 Spring 2012 What is going on? If you change the magnetic field inside a coil, current is induced in the coil. The induced current produces its own magnetic field, opposite the change, such that the magnetic field “status quo” is maintained. This happens even though the coil itself is not attached to a power supply.
PHYS132 Spring 2012 Change Current is induced when the magnetic field changes. This means current is induced when the magnetic field increases. And it happens when the magnetic field decreases.No current is induced if the magnetic field remains the same.
PHYS132 Spring 2012 Directions The coil is only “sensitive” to magnetic fields directed along its axis.So, if the direction of the magnetic field changes, that will also induce current in the coil.
PHYS132 Spring 2012 Generating electricity Rotating a coil in the presence of a magnet will induce current to flow.
PHYS132 Spring 2012 Magnetic braking You don’t even need a coil. Current is induced in any conductor that allows current to flow in loops. Once current is set up, it will experience a force (since moving charge experiences a force when within the presence of a magnetic field).If one part of the metal is experiencing a different magnetic field, there is a net force pushing the metal backwards.
PHYS132 Spring 2012 Transformers The changing magnetic field can be produced by another coil.A common ferromagnetic core increases the effect.
PHYS132 Spring 2012 Inductors The changing magnetic field can be produced by the same coil. If current is already flowing, the induced current is in addition to whatever current would be flowing otherwise. So, current is less than it would otherwise be.
PHYS132 Spring 2012 For Monday • Read chapter 22 • Submit “reading review” question on chapter 22 • Quiz on part F is Monday, April 2 (not April 4) • Meet in SciTech