ELECTRICITYCharge and Electric Fields

Charge

• Electric charge, q, is measured in coulombs (C). • Like charges repel.• Unlike charges attract.• An electron has a charge of .• A proton has a charge of .

Field Lines

Any electric charge sets up an electric field. We show a field with lines. These lines will affect any charge in the vicinity with an electric force.

• Their direction is the way a positive charge would go (i.e. attracted to the negative).

• They are radial around a charge, like the Earth’s gravitational field.

• The closer the lines are, the stronger the field is.

Draw Electric Fields For:

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Gravitational Field Strength

• The further from the Earth you get the weaker the gravitational field is.• As an object you feel the force of the gravitational field. The size of the

force you experience depends on your mass and the strength of the field where you are.

• When you move up (against field lines) you do work against the gravitational field.

• Moving back and forth (perpendicular to the field): you do no work.

Electric Field Strength

• The further from a charge you get the weaker the electric field is.

• As a charge you feel the force of the electric field. The size of the force you experience depends on your charge and the strength of the field where you are.

• When you move against field lines you do work against the electric field.

• Moving back and forth (perpendicular to the electric field): you do no work.

ForceAs we said the force a charge experiences depends on the size of the charge and the strength of the field, E.The strength of the field is the ratio: E = Force/Coulomb Newtons per

coulomb

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Work and PotentialWhen you do work against gravity walking uphill (opposite to field lines) you gain gravitational potential energy.A charge will do work against an electric field travelling in the opposite direction to the field lines and gain electric potential.This is the same as voltage (V). It is thought of as the work done per coulomb.

Electric Potential = Voltage = Work/Coulomb of charge

Work done is stored as electric potential energy so…

𝑉𝑜𝑙𝑡𝑠(𝑉 ) 𝐽𝑜𝑢𝑙𝑒𝑠𝑝𝑒𝑟𝐶𝑜𝑢𝑙𝑜𝑚𝑏 ( 𝐽 𝐶− 1)

Uniform Electric FieldYou can create an electric field easily by connecting the terminals of a battery to two plates. The potential difference creates an electric field.

A uniform field means that the electric field is constant, no matter where the charge moves…

Example:If I have this 9V battery, connected to these two plates, 1cm apart. We can calculate the strength of the uniform electric field set up:

Example:We can also find the force an electron would feel in between those plates.

Cathode Ray

The work done by the electric field is converted into kinetic energy of the electrons,

Electron GunThe electron gun is the first part of a CRT and is what initiates the cloud of electrons and focuses them into a beam. They can be quite small for such a dangerous-sounding mechanism.

Millikan’s Experiment

Forces Acting On The Oil Drop

𝑭 𝒈=𝒎𝒈

𝑭 𝒆=𝑬𝒒

Millikan adjusted the voltage applied to the plates which adjusts the strength of the electric field between them.When the oil drop is suspended between the two, not moving, then and

in this case q is the charge on an electron.

With the same charge as an electron

ELECTRICITYDirect Current Electricity

How do electrons flow?Electric current is the flow of electrons. Electrons pass on energy in a circuit by vibrating. A component in a circuit receives this energy and converts it into…

LightHeat

Movement

Essentially any kind of energy

CurrentWe know what charge is. • Current, , is a measure of the amount of charge

passing a point per second.

• Current is measured in Coloumbs per second or Amps .

• Notice that this doesn’t mention anything about what kind of charge (could be positive or negative).

Fun FactIt’s not the voltage that can kill you in an electric shock,

it’s the amps.100 mA will stop your heart.

Conventional vs. Electric• As we’ve said, electric current is the flow of

electrons.• Conventional current is the flow of positive

charges in a circuit. (this is the current we use in electricity in physics)• Conventional current flows from the positive terminal of a cell to the connected negative terminal (obviously if it is not connected no current will flow).

VoltageVoltage is a measure of the energy carried by the charge. Strictly: voltage is the “energy per unit charge”.

Like mechanical potential energy, the zero of potential can be chosen at any point, so the difference is what we’re interested in. The difference in voltage measured when moving from point A to point B is equal to the work which would have to be done, per unit charge, against the electric field to move the charge from A to B.

Joules per Coloumb

SeriesA circuit connected in series has components connected side-by-side.

If one of the lamps blows, no current will flow through the circuit and so even the un-blown lamp will not glow.

Series RulesA series circuit has only one path for current to flow through. The following rules apply to a series circuit:• The sum of the potential drops equals the potential rise of

the source.

• The current is the same everywhere in the series circuit.

• The total resistance of the circuit (also called effective resistance) is equal to the sum of the individual resistances.

Ohm's Law may be used in a series circuit as long as you remember that you can use the formula with either partial values or with total values but you can not mix parts and totals.

Parallel In parallel, components are connected across each other in branches.

Even if one lamp blows, current will still flow through the other and it will glow.

Parallel RulesA parallel circuit has multiple (parallel) paths for current to move along.The following rules apply to a parallel circuit:• The potential drops of each branch equals the potential rise of the

source.

• The total current is equal to the sum of the currents in the branches.

• The inverse of the total resistance of the circuit (also called effective resistance) is equal to the sum of the inverses of the individual resistances.

Ohm's Law may be used in a parallel circuit as long as you remember that you can use the formula with either partial values or with total values but you can not mix parts and totals.

Don’t Blow Yourself Up

One important thing to notice from this last equation is that the more branches you add to a parallel circuit (the more things you plug in) the lower the total resistance becomes.

Remember that as the total resistance decreases, the total current increases. So, the more things you plug in, the more current has to flow through the wiring in the wall. That's why plugging too many things in to one electrical outlet can create a real fire hazard.

Sum up• Voltage is supplied by the battery (or power

supply). • Voltage is used up in components, but not in

wires. • We say voltage across a component. • Voltage is measured in volts, V. • Voltage is measured with a voltmeter,

connected in parallel. • The symbol V is used for voltage in equations.

PowerJust as in mechanics, Power is the rate at which work is done. Each coulomb of flow is carrying energy.

As always, units for power is or .

But is the current, I

ResistanceResistors were made to control the flow of electricity. The higher the resistance in a circuit, the less current will flow.

Resistance is measured in Ohms, Ω.One ohm is a relatively small amount, so typically you will see large numbers for resistance.

Ohms LawAs we said: The higher the resistance in a circuit, the less current will flow. This is shown by the formula/ relationship discovered by Ohm.

Basically; increase the voltage acrossa component and you will increase The current.

𝑽¿𝑰𝑹

Internal ResistanceAll components in a circuit have some inherent internal resistance because the materials they’re made of have resistance. Most manufacturers do their best to limit this internal resistance.

Internal resistance is usually manifested as heat in a component. Hence, why most computers have cooling mechanisms in them, though they still get hot.

Circuit SymbolsThe positive terminal is the longer one, it’s useful to write this in to get used to which is which.

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Voltage DividerA voltage divider does exactly that: It divides the voltage. Setting up two resistors in series means that, depending on the values of resistance, we can control how much voltage flows through each branch.

Diodes

Resistors

• Twinkle Twinkle Little Star Power equals I squared R

• WATT is the unit of power?

ELECTRICITYElectromagnetism

Magnetic Fields

Right Hand Rule

Solenoid

Motor Effect

Right Hand Slap

Magnetic Field Strength

DC Motor

Moving Charge

Electromagnetic Induction

Voltage Induced

Generator