Electricity and Magnetism Physics 12 16.1 in textbook

Magnetic Force Magnetic forces have properties that are similar to electric and gravitational force in that: The force varies following an inverse square relationship due to the separation of poles The force varies directly with the strength of the poles However, since we don’t have magnetic monopoles there is no equation for magnetic force

Magnetic Forces are… Similar to electric forces because: Like poles repel and unlike poles attract Both are strong forces (unlike weak gravitational force) Different to electric forces because: Can’t have a monopole (like a single charge) Magnets don’t discharge when touched (like conductive charging)

Magnetic Field Lines It is possible to map a magnetic field in a manner similar to that of electric or gravitational fields

Magnetic Field Lines Arrows go into the South poles and away from North poles Lines are perpendicular to poles Increase in lines means increase in pole strength

What happens if you break a magnet in half? You will get 2 magnets with North and South poles You always have to have dipoles; never single poles (monopole)

Common Magnets Compass needle (points towards Earth’s North magnetic pole) ***Note: What we consider the “North Pole” in the Arctic is really a South pole… explain! North poles repel North poles so if the compass needle points there it must be South! If the Earth had a North “North pole” this is what would happen with a magnet.

Magnetic Materials Materials that are strongly magnetic: - they can be turned into magnets called ferromagnetic materials (iron, steel, nickel, cobalt) Materials that are not strongly magnetic can show signs of magnetism but they are small and usually unnoticeable.

Domain Theory On a small scale, ferromagnetic materials are actually made up of tiny regions known as domains. Each domain behaves like a tiny magnet with a North and South pole. In an unmagnetized piece of iron, these domains are arranged randomly pointing in all directions. The magnetic effects end up cancelling each other out.

Magnetic Domains If the domains are aligned, the material will be magnetic

Diamagnetism All materials are diamagnetic in nature. Diamagnetism is a very weak form of magnetism, exhibited only in the presence of a very strong external magnetic field. It is difficult to observe in everyday life. Materials like ferromagnetic materials have magnetic properties that overshadow the diamagnetic ones. Diamagnetic is used to refer to non-magnetic materials like water, wood, plastic, etc.

Magnetic Domains Some materials will easily align the domains in the presence of a magnetic field; however they will usually return to a random arrangement after the field is removed; these are called temporary magnets (like iron) – kind of like induction for charges Other materials will not easily align the domains, however once aligned they will remain aligned; these are called permanent magnets (like steel) Even permanent magnets can “demagnetize” if heated above a certain point (Curie point – see page 755) If you heat a magnet and then “freeze” it by cooling it to room temperature, you get a more permanent magnet.

Electric Charges and Magnetic Poles In 1819, Hans Oersted noticed that a current carrying wire (moving charged particles or electrons) would create a magnetic field when a compass needle was near a current the compass needle deflected He also noticed that static charges did not create this. This was the first step into the realm of electromagnetism.

Magnetic Field around a Current It was observed that a compass needle placed near a straight current carrying wire will align itself so that it is perpendicular to the wire. Magnetic field lines are actually CIRCLES around the wire. The direction of the magnetic field can be found using the first right hand thumb rule. The idea that all magnetic fields are a result of electric currents supports the idea that North and South poles must always exist in pairs since an electric current will always produce both.

Right Hand Thumb Rule 1 Used to find polarity in a straight wire or single coil Steps: a) Point your thumb in the direction of the current (positive flow). Curl your fingers (as if making a fist) Fingers will point in the direction of the magnetic field

Right Hand Thumb Rule

Example 1 The picture below shows a cross-sectional view of a wire carrying a current away from you (into the page – note the x). Sketch the magnetic field around the wire.

Solution Using the first RHT rule, make a slightly open fist with your thumb pointed outward. Point your thumb into the page (note the x). Your fingers will curl around the wire in a clockwise pattern, shown below.

Example 2 – Draw in the magnetic field (is it into or out of the page http://ocw.mit.edu/courses/physics/8-02t-electricity-and-magnetism-spring-2005/lecture-notes/prs_w06d1.pdf

Magnetic Field of a Current-Carrying Coil If you take a straight wire (like in previous example) and form a single loop, the first RHT rule can be applied to show that the field inside the loop is the same direction everywhere. The field outside the loop is in the opposite direction.

Magnetic Field in a Coil We can use RHT Rule 1 to determine the magnetic field in a coil of current carrying wire We end up with a strong magnetic field inside the soil of wire

Coil The field here is into the page inside the loop and out of the page outside the loop. Note: The field lines are more concentrated inside the loop… so the field is stronger here. Think about how the field should get weaker the further from the current (distance increases, field intensity decreases). Inside the loop, the field is uniform unlike outside the loop.

Worksheet Check Your Understanding 1, 3 Question 1

Solenoid Solenoid: A coil of wire containing many loops. The solenoid behaves as a magnet, with a North pole at one end and South pole at the other. This is called an ELECTROMAGNET.

Solenoid To increase the strength of the electromagnet: add more loops increase the current use a ferromagnetic core inside the coil (so the domain is aligned)

Solenoid

Second Right Hand Thumb Rule Used to find polarity in a solenoid Steps: a) wrap your fingers around the coil in the direction of the + current flow b) your thumb will point toward the North pole (or the direction of the magnetic fields INSIDE the coil)

Notes Field lines are always CLOSED loops Field lines go from North to South poles

Example 1 – Which pole is North?

Example 2

Example 3 Which compasses are pointing in the wrong direction?

Worksheet Remaining questions Check: 2 Question: 2, 4

Practice Problems Section Review Page 767 question 2 (read pages 766 – 767)