Electricity and Magnetism

ISCI 2002

1. Electricity is a Force – Vector – Electric charges (attract and repel) – Comb and Ball Example

2. Atoms – Protons and Electrons – “Atoms are electrically neutral” – Formation of ions

• Rub a rubber rod with fur

3. Insulators – Does not allow charge to run through it

4. Conductors– Allows for charges to run through it

• Metals

Van de Graff Generator

How it Works: • Belt – high RPM• Picks up charges• Distributes charges to

spherical dome surface• Conductors – Charges concentrate

(curvature of surface is greatest)

1. Coulomb’s Law – The force between charges is proportional to the product of

the charges and inversely proportional to the square of the distance between them.

2. Equation– Fe = k (q1q2)

r2

3. Newton’s Universal Law of Gravitation• Differences

– Charges may be positive or negative (mass always positive)– Electrical force is greater than gravitational force (k > G)

k = constant (9.0 x 109)q = chargesr = distance between

charges

1. Charged objects creates an electrical field around itself (space around the object)

2. Electrical Fields exert a force on any object placed into the field. – Fields may be weak or strong

3. Represented by vector lines (field lines)

4. Direction – field lines point; magnitude – lines closest together

5. If: – Positive charge – move in direction of lines– Towards negative charged area – Negative charges pushed in opposite direction

Lines point away from positive charges to negative charges

All field lines begin at the positive charge

More charge on an object, more field lines attached to it.

1. If you separate the positive and negative charges from each other, WORK must be done!

2. This creates Electrical Potential Energy (EPE). – A charge in a field has energy– External force pushes it the against the field – work

• Creates EPE

3. If Charges are brought together again – Energy is released

1. Work – Creates Electrical Potential Energy

2. How it is measured: – Electrical Potential = electric potential / charge

3. Unit – Volt – 1 Volt = 1 joule / charge (coulomb) – 1.5 – Volt Battery

• Gives 1.5 joules of energy to convert to 1 coulomb of charge flowing through a battery

1. Electrical Conductor – Two electric potentials separated – Charges flow from one to the other

2. How a Chemical Battery Works – Removes electrons from one terminal

and deposits them on another terminal• Adds energy to the charge

3. Where does the energy come from? – Chemical reaction – breakdown (lead,

zinc)

1. Metals– Electrons (free) flow through the

wires – Protons do not (bound to nuclei)

2. Non-Metals – Electrons and Protons both may

flow – Car Battery

1. Conductors – Diameter

• More narrow – greater resistance – Temperature

• Higher – more resistance – Material

• Metals (free electrons available)– Great Conductors!

• Rubber (no free electrons) – Poor Conductors or Insulators

2. Measured in Ohm’s (Ω) 3. Types of Current – AC vs DC

1. Current and Voltage – Proportional relationship (Current = Voltage)

2. Current and Resistance– Current = 1/resistance

3. Ohm’s Law – Current = Voltage / Resistance

I = current

V = volts

R = resistance

1. Path of Electron Flow through wires

2. Simple 3. Series– Current flows through each lamp (resistor) – From positive terminal to negative terminal – Easy to make; Disadvantages?

4. Parallel – Electrical devices connected to the same two points of an electrical

circuit– Three lamps

• Electrons travel to lamp one and return• Video Link

What type of Circuit is show at the Left?

How do you know?

What are the symbols used?

1. Moving charges do work.

2. Rate at which work is done 3. Formula – Power = current x voltage

4. Unit– Watt

1. Guess What? Magnetism is a force – vector

2. Where this force comes from– Metal atoms (Fe) – Domains – aligned atoms – Induced

3. Magnetic Fields – Space around a magnet – Magnetic Field – Field Lines – North pole to South Pole – Produced by moving electric charges – Video Link 1 – Video Link 2

Electric Currents and Magnetic FieldsA magnetic field is produced by the motion of electric charge.A magnetic field surrounding a current-carrying wire makes up a pattern of concentric circles.

1. Magnetic Poles ------ Magnetic Forces

2. Like poles repel; unlike poles attract

3. Break a bar magnet – Still have a north and south pole

1. Charges are moving – deflected by magnets

2. Particles moving through a wire are deflected– wire is also deflected

3. Electromagnetic Force – Electric Motors – Coil of wires turn - magnet – Current in motor – changes direction when coil makes a half-turn – Video Link

4. Meters – magnetic compasses detect electrical current– Compass in a coil of wire – current loop has effect on needle– Galvanometer (current); Voltmeter (V)