Gr 9 Science Exam - physics

Law of electric Charges

Like charges repel each other // unlike charges attract each other

But also, both positive and negative attract neutral

The strength of electric charges

• The amount of charge the objects have

• The distance between the objects

Induced Charge Separation

A shift in the position of electrons in a neutral object once a charged object is brought close to it

E.g. if a negative rod is brought close to a neutral ball, the electrons will shift to the further side of the ball

If the rod was positive, the electrons would move to the closer side of the ball to the rod

The object is still neutral as subatomic particles are still in the object

Protons do not move

Charging by friction

The transfer of electrons of neutral objects that occurs when they are rubbed together

There is always one object that is likely to attract electrons (negative) and one that loses electrons (positive)

E.g. clothes rubbing together in the dryer

Static occurs more in winter because in those months, the air is dry which means there is less water molecules so the dry objects cannot be attracted by water

Electrostatic series

A list of materials placed in order based on whether they will gain or lose electrons

Objects near the top will lose electrons, becoming positive

Objects near the bottom will gain electrons, becoming negative

Charging by conduction

Objects of different charges come in contact and electrons move to another object

Can be a charged object with a neutral charge or 2 opposite charges

Moves from the object with more electrons to the object with less to produce a more even distribution of charges

Grounding

Removes excess electric charge (positive/negative) from charged objects by providing an electron flow path

A positively charged object: the electrons from a source/sink (such as earth/ ground) travel to the object until it is neutral

A negatively charged object: electrons from the object travel to the sink/earth until it is neutral

We receive a shock if we are charged and touch a neutral object which is grounded

Charging by Induction (temporary)

When a charged object is brought near a neutral object, the electrons of the neutral object will shift further back, creating a positive surface but a negative object since the electrons did not change

Electrons move back once the object moves away

Charging by Induction (Permanent)

Same as grounding, bring a negatively charged object to a neutral one so the electrons will move away. Touch the side of the object that is negative and some electrons will travel into your hand, leaving a positive charge

Conductors

A material that allows electrons to move or flow through it

E.g. metals like copper, nickel, gold and tungsten

Fair conductors: allow electrons to move through them, but with some difficulty

E.g. semiconductors such as, earth, water

Insulator

A material that does not allow electrons to move through it

Causes the electrons to build up in one spot

E.g. non metals such as plastic, wax, wood, oil and fur

Keep electrons from moving from one thing to another

Conventional Current flow

protons flowing from the positive to negative terminal of the energy source

The direction that current was thought to flow when electricity was discovered

Electron current flow

negative charges flowing from negative to positive terminal

what actually happens

Direct current

electrons to flow in one direction only, produced by an electric cell

Alternating current

Electrons move back and forth, produced by generators at electric generating stations

Better with distributing energy over long distances

Wall outlets produce for things like lights, ovens, clothes dryers

Generating electricity

Electric generating stations convert mechanical energy into electrical energy

An external energy source (falling water) is pushing on a turbine to make it turn. It is then connected to a coil of wire between a magnet. The turbine turns the coil which starts the flow of electrons.

Its production is controlled and can travel long distances

Can turn into other forms of energy like motion, thermal, or light

Efficiency

a measure of how much useful energy an electrical device produces compared to the amount of energy that was supplied

E.g. if 2 devices require different amounts of of electrical energy but produce the same outcome, the one that requires less is more efficient

Fluorescent lights are more efficient than incandescent because they need less input for almost the same amount of output

LEDs are the most efficient

EnerGuide label

a label that gives a yearly estimate of how much electrical energy the product will use

Energy star symbol

shows that the product is highly efficient

Series Circuits

All electrons push on ahead electrons resulting in a smooth even flow

Electrons have one path

They flow from one load to the next; must pass through every load before returning

Loads share the energy provided by the source

The current level is exactly the same at any points in the circuit

Parallel Circuits

More than one pathway for a current to flow

Electrons can flow through 2 or more loops/paths

Loads do not share the energy provided from the battery

The current level at any point is different in level at another point

Current

Current (symbol = I) refers to the number of charges (# of electrons per second) passing through a point in a circuit

Ammeter: a device used to measure electric current (I)

Always connected in series in a circuit.

Too much current is dangerous so a circuit breaker/fuse is used

If too much, the fuse will trip/blow, resulting to something like an open switch

Potential difference

Electrons move from one point in a circuit to another point there is a change in electric potential energy

Current will flow through an object when there is a potential difference across that object

A battery creates the potential difference and provides them with energy to flow

There is always a drop in voltage across a load

A voltmeter is connected in parallel with a load to detect the drop in voltage

Electrical Resistance

The ability of a material to oppose the flow of electrons as they flow through a circuit

electrons colliding with the material that they are moving through.

The push on the charge (potential difference) is the same, but the pathways aren’t

The more difficult the path, the more opposition there is to flow

Ohmmeter is similar to a voltmeter where it connects parallel to a load but the circuit does not need to be closed because it’s individually powered and provides a electric current

Factors of resistance

Type of material: Certain materials are better at conducting electricity - low resistance

Cross-sectional area: Thicker wires have less resistance because they allow electrons to move freely in them

Length: The longer the material, the more resistance, more to move through and long wires are usually thicker to give more space

Temperature: When a material gets hotter, the atoms move faster and collide more

Ohm’s Law

The straight line relationship among voltage and current.

R = V/I

Resistance (Ω) = Voltage / Current

Ohm found that the ratio/slope V/I was constant for a particular resistor

Kirchhoff’s law - series

All electrons must pass through all lamps

Current is the same through each load

The potential difference is divided up between all the loads

T = total

IT = I1 = I2 = I3…

VT = V1 + V2 + V3…

Kirchhoff’s law - parallel

Electrons have a choice of path

The current is different at each load

The potential difference is constant

IT = I1 + I2 + I3…

VT = V1 = V2 = V3…

Relating Current, Voltage, and Resistance

The electrons are able to move more freely

Since V is constance, if R decreases, I will then increase

When the current increases the temperature of the wire increases

The current is the same within the current but may change if other factors such as resistance are different