Physics Unit 1Electricity Revision

Charge• + charges are carried by Ions

• - charges are carried by Electrons.

• An excess of electrons causes an object to be negatively charged, and deficit in electrons will mean an object is positively charged.

• Charge is conserved

• Like charges repel and unlike charges attract. The closer the charges are to one another the stronger the force.

• The elementary charge is the charge the smallest amount of charge, +e (proton) –e (electron), it is equal to

• Charge (q) has the units of Coulombs (C)

Conductors & Insulators• Conductors allow the movement of charge through their structure:

o Metalso Watero Eartho Skin

• Insulators do not allow the flow of charge:o Plasticso Polystyreneo Glasso Porcelaino Woodo Papero Ice

Coulomb's Law & Electric Force

• For the electric force between two charges and at a distance of :

• The force on a charge in an electric field is given by

• Force () has units of Newtons ()

Electric Field

• The amount and direction of the force on one unit (coulomb) of (positive) charge.

• The electric field in any region of space is defined as the electric force per unit charge:

• Vector quantity, (magnitude and direction)

• Electric Field () has units of Newtons per Coulomb ()

Electric Current• The rate of transfer of charge or

movement of charge along or through a conductor per time.

• Current () has the units Ampere or Amp () where 1 amp is 1 coulomb per second ()

• The direction of an electric current is the direction of transfer of positive charge.

• It is important to realise that to move a positive charge to the right, can be achieved by moving a positive charge to the right, or moving a negative charge to the left (electron & conventional current).

Kirchhoff's Current Law (KCL)

Ammeters• Ammeters are used to

measure the current flowing in a circuit.

• It is placed so that the current flows through the ammeter as well as the circuit element.

• Never place an ammeter across a circuit, because it would effectively become a short circuit, and could damage the meter, circuit or both.

• Ammeters can be analogue or digital.

EMF & Electric Potential

• EMF is the (ElectroMotive Force) however this term is an out dated and inaccurate term because the EMF involves energy given to charges rather than the force on them.

• EMF has later been defined as the amount of work done for each unit of charge in the process of charge concentration. There EMF actually is Electric Potential Energy per unit charge.

• The Electric Potential is equal to the work which would have to be done, per unit charge, against a static electric field to move the charge between two points.

• EMF () & Electric Potential or Voltage () has the units Volts () where 1 Volt is 1 joule per coulomb ()

Voltmeters

• Voltmeters are used to measure to potential difference across any two points, any circuit element or source of EMF

• Voltmeters are placed across the circuit element.

• Voltmeters can be analogue or digital, a meter that contains both an ammeter and a voltmeter is called a multimeter.

Potential Difference

• When a battery or EMF source is connected to a circuit, it produces a Potential Difference, that is a change in potential energy of the charges in the conductors connected to its terminals.

• A charge moving through a Potential Difference () will lose potential energy given by:

•

I-V Graph

• An current-voltage graph can be used to predict the behaviour of different devices.

• The voltage is plotted on the x-axis because it is usually the independent variable, and the current on the y axis, as its normally dependent on the current.

• The graphs can be divided into two catagories, Ohmic and Non-Ohmic, graphs with a straight I-V caracteristic are Ohmic, and graph that are not are Non-Ohmic.

Resistance

• The electrical resistance of an electrical component or circuit is the opposition to the passage of an electric current through that component or circuit.

• The resistance is the inverse gradient from an I-V graph.

• Resistance () has the units Ohm () where 1 ohm is 1 volt per amp ()

Ohm’s Law

• Geog Ohm (1789-1854) found that if the temperature of a metal wire kept constant, the current flowing through it was directly proportional to the potential placed across it:

• or • is only correct if the resistance is constant.

Electrical Energy

• Electrical potential energy is produced whenever charges are pushed close together.

• As charges move through a circuit they lose energy given to them by the source.

• Electrical energy () has the units Joules ()

Electric Power

• Power is the rate of energy use:

• Power () has the unit Watt, where 1 watt is 1 joule per second ()

• These relationships applies in all electrical situations. Fundamentally it is another expression of the principle of conservation of energy: the energy we can obtain from an electric circuit (each second) is equal to that put into it by the source of voltage.

• Energy companies calculate energy consumption of households in terms of Kilowatt hours () this comes from the amount of power multiplied by how long it has been used for.

AC & DC• When electric power is generated on a large scale it

is almost always AC, or alternating current. An electric current that continually reverses direction. The mains electricity supply is AC and, in Australia, alternates direction 50 times per second. power. What does this mean?

• Direct current (DC) is an electric current that continually flows in the same direction. Batteries provide DC. Compare alternating current (AC), but could just as easily mean direct voltage in many situations. A DC source of EMF always pushes charges in one direction.

FORMULAS!!!!

• • or

Some Questions

• Calculate the force on a and charges, placed 50cm apart.

Some Questions

• A charge of 10 µC is placed near a system of charges and experiences a force of 8 N. What is the strength of the electric field at that point?

Some Questions

• Find the missing current, and label its magnitude and direction.

𝐼 1=1 𝐴

𝐼 2=4 𝐴

𝐼 3=2𝐴

𝐼 4=?

Some Questions

• In the following circuit the current flowing through is . The resistors are of equal value. Find the current through .

Some Questions

• Ammeters have very low resistance, why?

• Voltmeter have very high resistance, why?

Some Questions

Some Questions

• The I-V graph describing a reversing lamp is show.

A veranda is fitted with two of these, but due to faulty wiring , one light receives only 75V rather then than the 240V expected from the mains.

1. How much current flows through the faulty light?

2. What is the resistance of this lamp under these conditions?

3. What is the resistance of the other lamp?

4. State the value of the ratio:power of dim lamp/power of brighter lamp.

Some Questions

• A refrigerator operates from a domestic supply of 220V. During a typical 24 hour period, its motor runs for 3 hours, 5 nights a week for a period of 4 weeks. The motor is rated at 440W.

1. What current will it draw when its operating at 440W?

2. What is its resistance when operating at 440W?3. How many kilowatt-hours are used in this period?4. Convert the amount of kilowatt-hours into joules?5. The electricity company charges $0.20 per kWh, how

much does the fridge cost to run for one month?