Physics Notes: Electricity

Free Web Hosting Provider - Web Hosting - E-commerce - High Speed Internet - Free Web Page

Conduction: a charged object touches another object; the amount of charge equally divides between the two objects; the same sign charge is acquired by each object.
Induction: a charged object is brought near, but not touching, another object; it attracts charges opposite to it and repels charges like it; when a ground is used, the opposite charge is acquired on the other object; it is thus charged without being touched

Net charge is caused by the difference in the number of protons and electrons which creates the net difference in charge. If an atom loses an electron, it gains a net charge of +1. If an atom gains an electron, it gains a net charge of -1.
You can discharge an object by neutralizing the charge on it.

Insulators do not allow electricity to flow through them, whereas conductors do. Semiconductors are modern materials whose conductivity can be controlled. Charge will remain where placed onto an insulator material such as a perspex rod, but will distribute itself across the surface of a conductor material such as metallic solid. Charge accumulates along edges or points at a higher density than on smooth surfaces.

2. Two states of charge exist for any material, positive if it has lost electrons and negative if it has gained electrons. Opposite charges attract and like charges repel as they aim to become neutral species.

3. An electric field is a region in 3D space surrounding an electrically charged object. An electric field is said to exist anywhere a force is felt on a positive test charge. Placing a positive test charge in a field and observing its path creates a segment of a "field map." By tracing of the path of the positive test charge, Electric field lines of Flux lines define the direction and strength of the electric field in terms of the force on the test particle.

The flux lines have a direction to them. The direction represents the motion of the positive test charge when placed at different points around the field. Notice that the field lines never cross each other. It is important to know that the closer the field lines are to each other, the greater the field intensity or field strength.

4. Any two charged objects will create a force on each other. Opposite charges will produce an attractive force while like charges will produce a repulsive force. The greater the charges, the greater the force. The greater the distance between them, the smaller the force.

5. Coulomb's inverse square law describes the nature of the force between electrically charged objects. The force is proportional to the charges Q1, Q2 and inversely proportional to the square of the separation distance (d). The constant of proportionality (k) in Coulomb's law has a value of 9.00 x 10^9 N m2 C-2 in a vacuum. Mathematically this is written as

If you use the electrostatic constant (k) given above for a vacuum, you will notice that it is given in terms of Coulombs so that forces us to have to measure the quantities of charge in Coulombs.

Stawa Set 33

Physics Text book pg 320, 323

Physics Study Guide pg 142

6. Electric Current (I) is the number of coulombs of charge that passes through a circuit in a second. It can be said the be the rate of flow of the electric charge. It is measured in the Ampere (Amp) by the use of an ammeter.

7. Direct Current (DC) is a current that only flows in one direction. eg a battery. Alternating Current (AC) is a current that flows in both directions, alternating on a regular basis. eg mains power.

8. The Si unit for current is the Ampere. The Ampere is the number of coulombs of charge that passes through a circuit in a second.

9. Charge (q) is a quantity of electricity measured in coulombs. 1 Coulomb = 6.3x1018 Elementary charges. 1 Elementary charge = 1.6x10-19 Coulomb.

10. The link between current, charge and time is that if you know two of these, you can work out the last one using this formula: current = charge/time
I = q/t where:
I = the current going through a component (A)
q = charge (C)
t = time (s)

11. Conventional current is defined as being in the direction of positive flow. It may be considered as a current opposite in direction to a flow of electrons. It flows positive to negative.

12. Moving a charge in an electric field involves work (energy) and hence a change in electrical potential. Electrical Potential refers to the amount of energy that can push a charge through a circuit. It is deemed potential as the actual movement of the charge will only occur when a circuit is established. It is measured in the unit volt. If one joule of electrical energy is possessed by one coulomb of charge, then that net charge has and electrical potential energy of one volt.

13. The potential difference between any two points in an electric field is defined as the work done per unit charge as the charge is moved from one point to the other. It is the difference between two points in electrical energy per unit charge. It is measured in volts using a voltmeter. If one joule of electrical energy is possessed by one coulomb of charge, then that net charge has and electrical potential energy of one volt. The Potential difference (V) = Work (J) / Charge (C).

14. Electrical Work equals the number of charges pushed through and the voltage at which they were pushed. This can be expressed as W=qV where:    W= Work Done (J)
                                               V= Potential Difference (V)
                                                q= Charge transferred (C)

Since we know that charge equals current multiplied by time, when can derive the formula W=VIt where: W= Work Done (J)
                                                                                                                                                     V= Potential Difference (V)
                                                                                                                                                      I = Current (A)
                                                                                                                                                      t = time (s)
Work Done = Energy Transferred = VIt

15. Electrical power is the rate at which electrical work is done in a circuit and is measured in watts. It is also the amount of energy that is transferred every second. It can be calculated using the formula P=W/t where: P = Power (W)
W = Work Done (J)
t = Time (s)

Since work equals VIt, you can derive the formula P=VI where:
P = The power transferred by the component (W)
I = The current going through a component (A)
V = The voltage across a component (V)

16. An electroscope is a charge measuring device which can be used to detect the quantity and type of charge present on insulators or conductors. An electroscope consists of a plate (near the top), a support stand (which connects to the plate and extends through the center of the scope), and a needle which rests upon the support stand and is free to rotate about its pivot.
The plate, support stand, and needle are all made of a conducting material which allows for both the free flow of electrons and a uniform distribution of any excess charge in the electroscope. By observing any deflection of the needle, the presence of charge in either the electroscope or any nearby object can be determined.

Physics Text book pg 316, 323

17. A common demonstration involving the induction process. A charged object is brought near to but not touching the electroscope. The presence of the charged object above the plate of the electroscope, induces electrons within the electroscope to move accordingly. With the charged object still held above the plate, the electroscope is touched. At this point electrons will flow between the electroscope and the ground, giving the electroscope an overall charge. When the charged object is pulled away, the needle of the electroscope deflects, thus indicating an overall charge on the electroscope. The process of charging an electroscope by induction using a negatively charged balloon is shown below.

As shown above, the presence of the negatively charged balloon above the plate of the electroscope will induce the movement of electrons from the plate of the electroscope to the support and needle of the electroscope. This is explained by the "like charges repel" principle. The negatively charged balloon repels the negatively charged electrons, thus forcing them to move
downwards. Once the electrons leave the plate and enter the needle, both plate and support/needle acquire an imbalance of charge. The plate acquires an excess of positive charge (since electrons have left this once neutral region) and the support/needle acquires an excess of negative charge (since electrons have entered this once neutral region).

Once charge within the electroscope has been "polarized" (i.e., separated into opposite types), the bottom of the electroscope is touched by a finger. Being positively charged, electrons from the electroscope exit and enter into the ground. Once more, this process is driven by the principle that "like charges repel"; the electrons, having a mutual repulsion for one another, choose to exit the electroscope and enter into the larger region. By doing so, the electrons are able to distance themselves and so minimize the repulsive interactions. It is at this point in the induction process that the electroscope acquires an overall charge. Since electrons have left the electroscope, the overall charge on it is positive. In general, the induction process will always place a charge on the object which is the opposite type of charge possessed by the object used to charge it.

18. The standard International unit for power is the Watt (W). However in Australia, electrical power is measured in kilowatts hours. (kWh). 1 Kwh = 3.6 MJ = 3.6 x 10^6 J.

To calculate Kilowatt - Hour we multiply power (KW) by time (Hours). ie KWh = P(kw) x t(hr). To determine the total cost for electrical power we multiply the number of units (kWh) used by the per unit cost. Therefore we can say that Cost = kWh x U.C (note 1 W/sec = 1 joule.)

19. Electronic Symbols:

Wire
Wire Crossed
Wire Joined
DC power supply
AC power supply
Ammeter
Voltmeter
Fuse
Switch
Resister
Variable Resister
Unspecified Component
Globe
Earth
Diode

Physics Text book pg 348

Physics Study Guide pg 146

20. A Simple Circuit has one possible path and the current flows through all parts of the circuit.
A Parallel Circuit has more than one path.
A Complex Circuit has a combination of both of the above.

A series circuit has more than one resistor and has only one path for the charges to move along. Charges must move in "series" first going to one resistor then the next. If one of the items in the circuit is broken then no charge will move through the circuit because there is only one path. There is no alternative route.

Below is an animation of a series circuit where electrical energy is shown as gravitational potential energy (GPE). The greater the change in height, the more energy is used or the more work is done.
The symbolisation in the animation includes:
The battery or source is represented by an escalator which raises charges to a higher level of energy.
As the charges move through the resistors (represented by the paddle wheels) they do work on the resistor and as a result, they lose electrical energy.
The charges do more work (give up more electrical energy) as they pass through the larger resistor.
By the time each charge makes it back to the battery, it has lost all the energy given to it by the battery.
The total of the potential drops ( - potential difference) across the resistors is the same as the potential rise ( + potential difference) across the battery. This demonstrates that a charge can only do as much work as was done on it by the battery.
The charges are positive so this is a representation of Conventional Current (the apparent flow of positive charges)
The charges are only flowing in one direction so this would be considered direct current ( D.C)

A parallel circuit has more than one resistor (anything that uses electricity to do work) and has multiple (parallel) paths to move along . Charges can move through any of several paths. If one of the items in the circuit is broken then no charge will move through that path, but other paths will continue to have charges flow through them. Parallel circuits are found in most household electrical wiring. This is done so that lights don't stop working just because you turned your TV off. Below is an animation of a parallel circuit where electrical energy is shown as gravitational potential energy (GPE). The greater the change in height, the more energy is used or the more work is done.
The symbolisation in the animation includes:
More current flows through the smaller resistance. (More charges take the easiest path.)
The battery or source is represented by an escalator which raises charges to a higher level of energy.
As the charges move through the resistors (represented by the paddle wheels) they do work on the resistor and as a result, they lose electrical energy.
By the time each charge makes it back to the battery, it has lost all the electrical energy given to it by the battery.
The total of the potential drops ( - potential difference) of each "branch" or path is the same as the potential rise ( + potential difference) across the battery. This demonstrates that a charge can only do as much work as was done on it by the battery.
The charges are positive so this is a representation of conventional current (the apparent flow of positive charges)
The charges are only flowing in one direction so this would be considered direct current ( D.C. ).

21. Ohm's Law States: Provided that the temperature remains constant, the current through a resistor is proportional to the potential difference applied to it. V = IR It only applies when the temperature of wire is constant
Ohmic conductors are those that follows ohm law, voltage and current vary at a constant rate (gradient of a V-I graph is constant)
Non-ohmic conductors are those that as voltage varies, current doesn't vary at the same rate. ie resistance varies (gradient of a V-I graph changes)

22. The current in a wire is equal to voltage divided by resistance. Current = voltage / resistance. The constant of the relationship between V and I is called resistance. It can be defined as the ratio of potential difference across a conductor to the current running through it. It can also been seen as the ability to impede electron flow.
Resistance is the opposition to the flow of electric current through a conductor and is measured in units called ohms.
A resistor is an appliance eg heating element.

Stawa Set 35

Physics Text book pg 336

Physics Study Guide pg 147

23. Factors effecting resistance include:
The Nature of the material
The Length of the Conductor
The Cross sectional area of the material:

Notice that the electrons seem to be moving at the same speed in each one but there are many more electrons in the larger wire. This results in a larger current which leads us to say that the resistance is less in a wire with a larger cross sectional area.

The Temperature of the material : The temperature of a conductor has a less obvious effect on the resistance of the conductor. A Higher temperature means more kinetic energy. As a result, the higher the temperature, the higher the resistance. A prime example of this is when you turn on a light bulb. The first instant, the wire (filament) is cold and has a low resistance but as the wire heats up and gives off light it increases in resistance. As a result we can say that Ohm's law holds true unless temperature changes.

In general it is important to realize that:
If you double the length of a wire, you will double the resistance of the wire.
If you double the cross sectional area of a wire you will cut its resistance in half.

Stawa Set 35

Physics Text book pg 336

Physics Study Guide pg 148

24. Due to the work of George Ohms, Resistance is seem as the ratio between potential difference and current. It was originally expressed as R = V / I where:
R = The resistance of a component (ohms)
V = The voltage across a component (V)
I = The current going through a component (Amps)

Ohmic Resistors are those that obey Ohm's Law. The current will increase in proportion to the voltage. If you double the voltage, the current will also double. The following graph shows this.

Non-Ohmic Resistors are those that do not obey Ohm's Law. Some devices work in slightly different ways. Examples include:
a) Thermistor: Thermistors have a lower resistance at higher temperatures. They let more current flow through them at higher temperatures.
b) LDR Light Dependent Resistor: Light dependent resistors have lower resistance when there is more light.
c) Diode: A diode lets the current flow one way only, in the direction of the arrow. This means that it has a low resistance when the current is flowing in the direction of he arrow, but a very high resistance when the current tries to flow the other way.

Stawa Set 35

Physics Text book pg 339

Physics Study Guide pg 147

25. In a series circuit, the total resistance of the circuit is equal to the sum of the individual resistances. It can be calculated using the formula:

26. In a parallel circuit, the inverse of the total resistance of the circuit is equal to the sum of the inverses of the individual resistances.
It can be calculated using the formula:

Notice from this last equation is that the more branches you add to a parallel circuit 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.

27. In a Series Circuit: The sum of the potential drops equals the potential rise of the source and the current is the same everywhere in the series circuit. This is shown in the formulas:

Adding more globes to a series circuit will cause them to dull as voltage doesn't stay constant

28. In a Parallel Circuit: The potential drops of each branch equals the potential rise of the source and the total current is equal to the sum of the currents in the branches. This is shown in the formulas:

Adding more globes to a parallel circuit will have no effect as voltage stays constant.

29. Complex circuits are a combination of resistors in parallel and in series.
Steps in simplifying complex circuits:
1. determine the equivalent resistance of each set of resistors in parallel
2. determine the total resistance of the circuit
3. determine total current
4. calculate voltage drops across all series resistors
5. calculate currents in each parallel branch

30. Ammeters measures current in Amps. It is connected in series as current is the flow of electrons.
A Voltmeter is used to measure potential difference in volts. The voltmeter is connected across a resister in a circuit as it measures the drop in electrical potential energy.
A multimeter is an electrical device that allows us to measure potential difference, current or resistance. It must still follow the same rules.

32. Resistivity (p) refers to the inherent resistance that a material possesses. This is a constant value measured in the Ohm meter ((ohm)m).

34. Resistance=Resistivity*Length/Area^2·

where:
R is the resistance of the conductor in Ohms
A is the cross sectional area in m2
L is the length of the wire in meters
p is the resistivity of the material in Ohm(meters)

The resistivity is a value that only depends on the material being used. For example, gold would have a lower value than lead or zinc, because it is a better conductor than they are.
Insulators - materials such as wood, glass and plastic have extremely high resistivities, and are therefore useful insulators. They
are very necessary for the safe operation of electrical devices and the prevention of electrocution. ·

Stawa Set 35

Physics Text book pg 336-338

Physics Study Guide pg 148

35. Appliances are Resistors. eg heating elements in kettles.

Physics Text book pg 349

Physics Study Guide pg 153

36. Sources of EMF in circuits include Generators, Solar Cells, Chemical Cells and Batteries. These transform kinetic, potential, chemical or heat energies into electrical energy.

37. The electromotive force (EMF) of a battery is the energy transferred per coulomb of charge within the battery. EMF is measured as a voltage in volts. In a battery, chemical energy is transformed into electrical energy.
in an appliance, often resistors are used as heat coils.

38. Wiring a plug:
These are the wire colours you will find in a plug.
Live wire: Brown
Neutral wire: Blue
Earth wire: Green and yellow stripes

Fuses: The fuse does two jobs. It protects the wiring if something goes wrong, and it can also protect us. The fuse contains a piece of wire that is designed to be thinnest piece of conductor. In event of short circuit, a heavy current flows through all conductors. eg when broken active conductor coming contact with the neutral or earth wire. If your circuit is attacked by some high-energy-impulses, the only thing which becomes disturbed is the fuse. If the current through the fuse is too great, the wire melts and breaks the circuit. Each time it is burnt out, it is replaced. Fuses will not save a person from electrocution if they touch live wire because a fuse blows at an amperage way above a dangerous level to humans.

Switches: Switches should always cut the active conductor. A switch in the neutral will turn the element off, but will leave it connected to active wire. This very dangerous if the element is touched. For safety reasons the appliance needs to be completely unplugged.

Earth Wire: An Earth Wire is a wire that is parallel to the neutral wire and aims purely to provide protection against a fault occurring in the appliance. If an active wire disconnects and touches the case, the case would become active, but if earth wire connected, a short circuit will blow a fuse. The symbol for the ground is a horizontal lined arrow aiming down.

If the live wire inside an electric cooker comes loose and touches the metal casing, you will get an electric shock. The earth terminal is connected to the metal casing, so the current goes through the earth wire instead of through you.
The earth wire route has a very low resistance, so a big current flows which blows the fuse, disconnecting the cooker.

Double Insulation: This provides us with two effective barriers between the active wire and a person using appliance. You can't get a shock off the casing even if the wires inside become loose. This is because the casing is made of plastic, or because the wire can't touch the casing. It is safer than using an earthed case because the earth wire could break and touch active wire.
Some devices such as vacuum cleaners and electric drills don't use an earth wire. A no earth wire is characterised by two pin plugs and special symbol.

Earth Leakage Devices: Earth Leakage Devices are also called residual current device. The current in an active wire and a neutral wire should theoretically be equal. RCD detects any current loss from the active and neutral circuits and uses the magnetic effect of electric current to detect difference. Magnetic effects should cancel each other out. If there is a fault, this is not the case. These Devices can switch off supply within 20 milliseconds and therefore save lives.

39. Electric Shock is caused by the change in electrical impulses that control our bodies through nerves. Another potentially fatal result of the interference with nerve functions of an electric current is that the involuntary contraction of the muscles may make it impossible to let go of the object causing the shock.

The likely effect of a half-second electric shock:
Current (mA) - Effect
1 - able to be felt
3 - easy felt.
10 - painful.
20 - muscles paralyzed - cannot let go.
50 - severe shock.
90 - breathing upset.
150 - breathing very difficult
200 - death likely
500 - serious burning, breath stops, death inevitable.

The likely effect on the body of a 50 mA shock for various times.
Time - Likely effect
Less then 0.2 s - noticeable not dangerous
0.2s to 4s - significant shock, possibly dangerous
Over 4s - severe shock - possible death.

40. Preventing electrical shock
Never walk barefoot, particularly outdoors.
Take extreme care when using appliances near water with eg Hair dryer.
If you suspect an appliance of being faulty, use the back of your hand with the other hand held well away.
Never tamper with electrical appliance, keep them in good working order.

To help a person with electrical shock:
Look for reasons of shock, pull out plugs and turn off switches.
Push the person away from the appliance using an insulated object, wooden pole or thick wad of clothing.
Do not touch the person until you are sure that the voltage is removed. When checking, check with the back of your hand.
If Power lines have fallen, keep well away, do not touch car if the lines have fallen on the car. You should tell all occupants to stay put so they don't touch an object with a potential difference. Unless they touch something at earth potential they will be safe.
Check consciousness, by talking loud or shaking the victim gently. Treat burns with cold water and if the victim is not in the coma position, put them in the position and check breathing and pulse.
If the breathing or pulse are missing, get urgent medical aid and begin mouth to mouth and cardiac massage.

41. Any chemical cell will have internal resistance due to the fact current must flow through various chemicals and the electrodes. The flatter the battery, the increased internal resistance. Internal resistance can be calculated using the formula
Ri=(change in V)/I where
Ri = The internal resistance of the component (Ohms)
V = The voltage across a component (V)
I = The current going through a component (A)

Created : 28 October 2000
Last modified : 30 October 2000
Author : Chad Silver email:Chaddysi@start.com.au
Site maintained by : Chad Silver