# SPH3U Grade 11 Physics Electricity and Magnetism Test

Electricity Unit Test Study Notes

• Electrostatics
• Static Electricity: build up of stationary electric charge on a substance
• Fundamental Laws of electric charges: likes repel, opposites attract
• Charging: to create an imbalance of electric charges on either substance
• Charging by Friction: rubbing rips out the electrons and charging it
• Charging by Contact: touching will affect the movement of the charges
• Charging by Induction: the electric fields can move the charges in the object.
• Electric Charges
• Coulomb’s Law: the magnitude of force between 2 charged particles are directly proportional to product of charges and are inversely proportional to the distance squared
• F = kQ1Q2/D^2
• F is Force, K is 9e9
• Calculating Electric Charges
• Relationship between number of electrons and charge
• Q=Ne
• N is number of electrons, e is a constant of elementary charge 1.60*10^-19
• Electric Current
• Defined as the rate of movement of electrically charged particles past a point
• Flows from Positive to negative (long side to short side of terminal)
• Cathode to Anode
• I = Q/delta T
• Q is Coulombs, I is current, T is time in seconds
• 1A = 1C/s
• Electric Potential difference
• V = w/q
• Amount of work it takes to move a certain amount of charge of electricity
• 1V = 1J/C
• Kirchoff’s Law of electric circuits
• Law of conservational charge: electric charge is neither lost or accumulate at any point in a circuit
• Kirchoff’s Voltage Law: around any complete path, the sum of voltage increases is equal to the sum of decreased in electric potential
• Kirchoff’s Current Law: At any junction in an electric circuit, the total electric current into the junction is equal to the total electric current out
• Solving Circuit Equations:
• Parallel
• V0 = V1 = V2 = V3..= VN
• I0 = I1 + I2 + I3.. + IN
• 1/R0 = 1/R1 + 1/R2 + 1/R3.. + 1/RN
• Series
• V0 = V1 + V2 + V3.. + VN
• I0 = I1 = I2 = I3… = IN
• R0 = R1 + R2 + R3.. + RN
• Electrical Resistance
• The amount of impedance something has against a current in a circuit
• Ohm’s Law
• Unifies voltage, current, and resistance
• V = I*R
• Electric Power
• Measured in Watts, is how much energy is consumed / time
• P = I * V
• Using Ohm’s Law, I or V can be substituted with their equivalence.
• When paying, it’s measured in Kilowatt Hours, instead of watts per second
• Electromagnetism
• Laws of Magnetic forces
• Opposites attract, like repel
• Magnetic fields always travel from the North side to the south
• Magnetic Materials
• Domain Theory
• In magnetic materials, it is said that there are small polar dipoles grouped into domains
• these domains will move and attract to eachother, forming the material
• The orientation of these dipoles can also be affected by nearby magnets as well
• Permanent Magnets: magnets where the dipoles are in a fixed position to define their poles
• Ferromagnets: magnets with random ordered dipoles until they are affected by permanent magnets nearby.
• Magnetic Induction: the process of altering the order of the dipoles by adjusting their positions and having it temporarily stable
• Demagnetization: Magnets which’ve lost their direction of dipoles
• Reverse Magnetization: If strong magnets are placed near an end of a magnet, it has the potential to switch poles and reversed
• Magnetic Saturation: when the strength of the magnet has increased to it’s maximum and become saturated.
• Magnets from electric current
• a straight conductor carrying a current creates a magnetic field around it
• 1st Right hand rule
• Thumb points in direction of current
• Fingers, in direction it’s pointing, indicates direction of magnetic field
• (X) indicates current inside page, (.) indicates current coming out
• Magnetic Field from a solenoid
• a coil of wire, coiled up, can create an magnetic field in the ends with current
• 2nd Right hand rule
• Thumb indicates direction of magnetic field
• Fingers point in direction of current flowing in the solenoid
• Factors that affect magnetic field of coil:
• Current in coil, more coil is directly proportional to it
• Number of loops, also directly proportional
• Type of core material: different materials have different Magnetic permeability
• Magnetic Permeability relative: the factor by which a core material increases the magnetic field strength
• Ferromagnets: usually have high K as they help increase magnetic field strength
• Paramagnetic: materials that only magnetize slightly (air, aluminum)
• Diamagnetic: materials that reduces magnetic field of the coil (copper, water)
• Electromagnetic Relay: using a soft iron core called an Armature, the first circuit’s electromagnet will lift or lower the armature thus, enabling and disabling the second circuit
• Motor Principles
• With magnetic fields created by an electric current, there can be movement or force obtained by the wire
• Using third right hand rule, we can find that out
• thumb points in the direction of the current
• fingers point in direction of magnetic fields
• palm points out in direction of the force acting on it
• 2 wires side by side will have their own movements too based on 1st right hand rule
• Motors
• Using a solenoid and 2 permanent magnets on each side, the current can be used to move the solenoid
• When it switches over to the other side, the current’s direction is flipped in reverse, allowing the same attraction to the incoming magnet
• Electromagnetic induction
• Law of Electromagnetic induction: from Faraday
• An electric current induced in a conductor whenever the magnetic field in the region of the conductor changes
• Lenz’s law of induced current
• when a magnet is placed closer to a coil, the coil can create a current
• And in creating a current, it will also create it’s own field
• According to Lenz, that field will always be opposite to that pointing towards it
• Therefore, to figure out direction of current, right hand rule, with thumb pointing north, will work
• When looking at current based on the direction wire is moving, use third right hand rule, and it’s the opposite direction of current as the right hand rule suggests.
• If direction is given, current will be opposite of what right hand rule suggests.
• More random stuff with Electricity and magnets
• Transformers
• Based on Faraday’s ring’s theory, a Transformer can be used to Up or lower the voltage in a current of electricity
• Because magnetic fields are created by solenoids, that field can carry itself to other solenoids through an iron ring
• In ratio to the number of windings in the primary and secondary solenoid, a transformer can up scale or down scale a voltage accordingly.
• Step down Transformer has lesser secondary windings
• Step up Transformer has more secondary windings