Electricity – Exam Review

A. Static Electricity Static = Fixed. An imbalance of electrical charges (+≠ -)

B. Law of electric charges :

C. Electrostatic List It is a list of materials classified according to their tendency to hold electrons. It is used to predict the charges of materials once rubbed together .

D. Insulators vs. conductors   Insulators Conductors Definition Substances that are non-metals, they don’t allow e- to flow easily. Substances that are metals that allow the e- to flow easily. There is no accumulation of electrical charges. Examples/ uses Furniture  wood Plates  ceramic windows glass Electric cords (exterior)  rubber Electric cords (interior)  Cu, Al Dry Air is low in humidity (not enough water in the air), since e- can’t move freely, there are more problems with static electricity. Moist air, has a lot of humidity (water vapour)  therefore there is no accumulation of e -, no static electricity.

E. Methods to charge an object. Charge by friction. Charge by contact. Charge by induction.

1. Charging by friction (Rubbing) Two neutral objects (different materials) are rubbed together. One will lose e- ( + charge ) & the other will gain e- (- charge ) (Electrostatic Series). Both objects have opposite charges and will attract. Examples: wool sweater and cotton blouse. Application: Bounce (covers all clothes in wax) so there will be no static electricity.

2. Charging by contact (conduction) A charged object transfers its charge to a neutral one. There is a transfer of e- when the 2 objects touch. Causing an electrical choc = Electrons move to get discharged  ground

3. Charging by induction Transfer of e- without direct contact. a neutral object is attracted to a charged object. E.g. a charged balloon sticks to a wall, dust on a TV screen.

4. How to discharge objects ‘ eliminate static charges’ To Ground You need a good conductor or a semi-conductor to take the excess e- making it neutral again “discharging it”.

F. Electroscope

Temporary charge Permanent charge Electroscope (induction) http://www.shep.net/resources/curricular/physics/P30/Unit2/electroscope.html Temporary charge Permanent charge

G.Lightning charge separation in the storm cloud. The negative charges in the cloud repel the electrons in the ground. There is a strong attraction  spark (lightning) It strikes the ground more than 100x per second  fires

H. Lightning rod Is a metal sphere, connected to the ground via copper wires. allows the e- from the cloud to be safely conducted to the ground. (by induction)

I. Current Electricity Symbol (I) Movement of e- throughout an electric circuit (- terminal to +terminal) in a one direction (DC direct current) Ammeter. Unit : amps (A or mA) 1A = 1000 mA

Where do we place an ammeter?

What happens to the current when you add a second light bulb in series?

J. Potential Energy Symbol : V Each e- has a potential energy. A cell has the potential energy caused by the two electrodes. The potential difference causes the electric current in a circuit. Unit : volt (V) Voltmeter

Where do we place the voltmeter?

K. Resistance  Resistance is the opposition to the flow of electrons. It is used to control the flow of a current in an electrical circuit. Symbol : R Unit: Ohm (Ω) Ohm’s Law: V= R I

L. Key Concepts Symbol Unit Instrument Current I A or mA Ammeter Voltage V Voltmeter Resistance R Ω Ohm’s law R = V/I

M. Types of Circuits Series circuit Parallel circuit All in a row 1 path for electricity 1 light goes out and the circuit is broken Example: Old Christmas lights Parallel circuit Many paths for electricity 1 light goes out and the others stay on

Series circuit Parallel circuit

N. Circuits Summary Series Parallel Current current is the same at all points in the circuit. IT = I1=I2=I3=…. current is shared between the components. IT = I1+I2+I3 Voltage voltage is shared between the components VT = V1 + V2 voltage is the same in all parts of the circuit. VT = V1 = V2 Resistance RT= R1+ R2+…. RT= R1/#of light bulbs *(assuming that they are identical)

Circuit Construction http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

O. Generating & Distributing Electricity Renewable vs Non-renewable energy

Energy = power (kW) x time (h) How do we calculate the electrical energy used by an appliance? Energy = power (kW) x time (h) What unit do we use to measure electrical energy? kW.h How do we calculate the cost of electrical energy? Cost = Power x time x the price How do we calculate percent efficiency? Percent efficiency = useful energy output x 100% total energy input

How to conserve? EnerGuide Presentations