Magnetism and Electromagnetism Notes (Pages 582-9)

17.1 The Magnetic Force- Another Force at a Distance

• A magnetic field is the distribution of a magnetic force in the region of a magnet. 
• There are two magnetic characteristics, lebelled north and south, that are responsible for magnetic forces. Similar magnetic poles repel one another while dissimilar poles attract one another.
• Magnets attract certain metals such as iron, nickel, and cobalt, which are not magnetic. These are called ferromagnetic metals.
• Domain Theory- All large magnets are made up of many smaller and rotatable magnets, called dipoles, which can interact with other dipoles close by. If dipoles line up, then a small magnetic domain is produced.

17.2 Electromagnets

• Oersted's Principle- Charge moving through a conductor produces a circular field around the conductor.
• Scientists have developed several hand signals to predict how magnetic forces act, known as right hand rules because they involve using your right hand.

• RHR#1: Grasp the conductor with the thumb of the right hand pointing in the direction of conventional, or positive (+), current flow. The curved fingers point in the direction of the magnetic field around the conductor.
  
  
• RHR#2: Grasp the coiled conductor with the right hand such that curved fingers point in the direction of conventional, or positive (+), current flow. The thumb points in the direction of the magnetic field within the coil. Outside the coil, the thumb represents the north (N) end of the electromagnet produced by the coil.
  

Notes: Textbook pages 553-63

16.5 Resistance- Ohm's Law

• The amount of energy transferred to a device depends on both the potential difference of the power supply and the nature of the pathway using the electrical potential energy.
• Resistance is a measure of the opposition to current flow. the formula is:
  
  R=V/I
• The amount of current flowing through a resistor varies directly as the amount of potential difference applied across the resistor.
• Properties such as length, cross-sectional area, material, and temperature also affect the resistance of a conductor.
  

16.6 Series and Parallel Circuits

• In a series circuit, the loads are connected one after another in a single path. In a parallel circuit, they are side by side.
• Kirchhoff's current law: the total amount of current into a junction point of a circuit equals the total current that flows out of the same junction.
• Kirchoff's voltage law: the total of all electrical potential decreases in any complete circuit loop is equal to any potential increases in that circuit loop.
• There is no net gain or loss of electric charge or energy in a circuit.

Pre Lab: Using Voltmeter and Ammeter

NAME	SYMBOL	UNIT	DEFINITION
Electrical Potential Difference	V	Volt (V)	Electrical Potential energy for each coulomb of charge.
Current	I	Ampere (A)	Rate of charge flow.
Resistance	R	Ohm  (Ω)	The opposition to current flow.
Power	P	Watt (W)	The rate at which electrical energy is passed on the various circuit loads.

In Class Assignment- Circuits

In Today's class we worked in groups of 4 to learn more about circuits. We played around with a "special" ping-pong ball that would light up and hum if its circuit was completed. Here is what we found out:

     1.Can you make the energy ball work? What do you think makes it flash and hum?

Yes, we could make the ball flash and hum by completing the gap in the circuit using our hands.

     2.Why do you have to touch both metal contacts to make the ball work?

As mentioned in the previous question, touching both metal contacts bridges the gap in the circuit. Since humans do conduct electricity, the flow is able to resume, thus completing the circuit.

     3. Will the ball light up if you connect the contacts with any material?

No. Only electrical conductors will make the ball light up.

     4. What materials will make the energy ball work? 

When tested, the materials that make the energy ball work were all conductors of electricity, such as metal and the human body.

     5. This ball does not work on certain individuals, what could cause this to happen?

This question had the whole class thinking. We were unsure because the ball worked on everyone in our class. The only reason I could think of was that some people's bodies may not conduct electricity as well as the rest of ours. With further research I have found that water and salt in our bodies cause the conduction of electricity. Maybe some people have a condition where they don't have as much salt/ water in their bodies?

     6. Can you make the energy ball work with 5-6 individuals in your group? Will it work with the entire class?

We experimented with this question and found that it did work with 5-6 people, as long as we were all connected to each other. We also tried this out with the entire class and once again, the ball began to flash and hum. However I do think the strength is weakened when more people stand between the completed circuit.

     7. What kind of circuit can you form with one energy ball?

A series circuit.

     8. Given two balls can you create a circuit where both balls light up?

Yes, to complete this we simply connected our hands as before except the people who had a ball beside them touched a metal connector.

     9. What do you think will happen if one person lets go of the other person's hand and why?

The energy ball stops working. The gap in the circuit now makes this an open circuit.

     10. Does it matter who lets go?

It does not because everyone in the group played a part in the completed circuit.

     11. Can you create a circuit where only one ball lights up? Use both.

This can be attained by forming a parallel circuit. Much like the ones we have in our homes (if one system fails everything else still works). creating a break in the circuit wouldn't affect both balls, just one.

     12. What is the minimum number of people needed to complete this?

A minimum of five people would be needed. 

In conclusion, what is the difference between a series and parallel circuit?

In a series circuit, all the loads are connected one after another in a single path. It is possible for some of the loads to be supplied with current while others are not.

In a parallel circuit, the loads are connected side by side. Thus, any break in the circuit would cause a loss of current to all the loads.

 

Challenge 1: Physics of Tall Structures

Yesterday in class we accomplished a mini challenge. In small groups we were told to build the tallest, free- standing structure we could with 5 sheets of newspaper and a length of tape. The class immediately started planning and got to work on their structures. Within the given period of about 25 minutes many of the groups had completed their challenge. However not all of the structures worked. Some collapsed while some were unable to stand freely. Many of the structures were extremely unstable. So, what was the best way to build the structure? How much height has to be compromised in order to make the structure stable?

Physics of Tall Structures

Several aspects can contribute to a stable tall structure. Mainly, having most of the mass located at the base would help the construction to stand. Other ways to make sure of an effective structure would be to: have a wider base and/ or attaching tripod- like extensions to distribute the mass.

What Makes a Tall Structure Stable?

Obviously the most important thing to consider when building a tall structure is the base. It must be wider, and able to support the weight of the rest of the building.It is also better that the base is as flat as possible

Center of Gravity

The center of gravity is a point where the total weight of a structure is thought to be concentrated.  A structure should be built with the lowest center of gravity possible, this means that it is most stable at this point.

Overall, i think my group did a good job building our structure. We stacked our newspapers pyramid- like so that the base was the widest part. In addition, we incorporated the tripod design for additional stability.

Our Structure

Notes: Current Electricity

Current

·         In an electric circuit, an energy source provides electrons with energy.

·         Conductors transport the electrons to where the electron energy is transferred, then back to the source to be re-energized.

·         Current can be defined as the rate of charge flow and is given the symbol I. The formula for current is the total amount of charge moving past a point in a conductor divided by the time taken.

I=Q/T

I is the current in amperes (A), Q is the charge in coulombs (C), and t is the time in seconds.

·         Current is a flow of negatively charged electrons repelling one another.

·         On the subject of measuring current, an ammeter must be used. It must be rewired so that all current runs serially through the ammeter.

·         In direct current (DC) the current flows in a single direction from the power supply through the conductor to a load, while in alternating current (AC), the electrons periodically reverse the direction of their flow.

·         A path for the electric current to flow to and from the power supply is called a circuit and is required for any electrical device to work properly.

Example of an electrical circuit (This is the familiar circuit of a torch battery

Electrical Potential

·         An electric charge has a certain amount of electrical potential energy because of the electric field set up by the power supply. The power supply has to increase the electrical potential energy of each coulomb of charge from a low to a high value. As the charge flows through the load, its energy decreases.

·         The electrical potential energy for each coulomb of charge in a circuit is called electric potential difference (Voltage)

V=E/Q

E is the energy required to increase the electric potential of a charge, Q. 

·         A volt (V) is the electrical potential difference between two points if one joule of work (J) is required to move one coulomb (C) of charge between the points.

·         Potential difference between any two points can be measured using a voltmeter.

Example of a voltmeter