One Chapter 4

Current Electricity

• Electric Current - The movement of electric charges from one place to another.
  • The amount of charge that moves past a given point in a conductor per unit of time.

I = Q/T

• I = Current, Q = charge, and T = time.
• The unit of current is the Ampere (A).
• In practice, the ampere is defined in terms of the force between current carrying wires. For now, 1A is the current present when 1C of charge passes a point in a conductor in 1s.
• 1 A = 1 C/s

Measuring Electric Current

• Ammeter - An instrument used to measure current directly.
  • It must be inserted in a circuit so all the electrons flow through the device.

Electric Potential

• A small positive charge next to a negative charge has no potential energy.
• Work must be done to pull the positive charge away from the negative charge (i.e.: potential energy increases).
• If released, the positive charge will move back towards the negative charge and gain kinetic energy while losing potential energy.
• Whenever charged particles move in an electric field and convert electric potential energy to other forms, a current is present.
• In conductors, if an electric field is set up from one side to the other, electrons move in response to the field to produce a current.
• In fluids and plasmas, both positive and negative ions move in response to electric fields to produce currents.
• Many devices have been built to provide charges with electric potential energy.
• The amount of work done on an electric charge to move it through an electric field equals the increase in electric potential energy.
• Electric Potential Difference - is the energy available per unit charge.

V = E/q or V = W/q

• W in the equation is work needed to move the charge q from A to B. V then is the potential difference between A and B, and E is energy.
• This allows us to define the volt formally.
• Volt - the electric potential difference between two points if it takes 1 J of work per coulomb to move a positive charge from one point to the other.
• Electric potential difference is often referred to as voltage.
• A 12 V battery does 12 J of work on each 1 C charge flowing through it.
• As charges flow, they lose energy therefore experiencing a decrease in electric potential (referred to as electric potential difference between two points).
• Electric Potential Difference - the decrease in electric potential
• Energy lost or work done by a charge going through a potential difference is given by:

E = qV

• Note that q = IT, therefore:

E = VIT E is energy

T is time

I is current

V is potential difference

Measuring Electric Potential Difference

• Voltmeter - An instrument used to measure electric potential difference between two points.
  • It is connected in parallel.

Producing Electric Potential Energy

• Any device that can separate electric charge acts as a source of electric potential energy.

Electrochemical Cells

• Voltaic Cell - uses the principle that when two different metals are placed in an acid or base solution, separation of electric charge occurs.
• Electrodes - The two metals in a voltaic cell.
• Electrolyte - The solution.
• The electrolyte breaks into positive and negative ions which move and chemically react with the electrodes. During the chemical reaction, an excess of electrons is left at one electrode and a deficit at the other.
• A voltaic cell may contain copper and zinc put in a dilute sulfuric acid solution. This cell will produce a potential difference of about 1.1 V between the two electrodes, and would drive electrons through a wire connecting the electrodes.
• Dry Cell - Similar to a voltaic cell.

- The electrodes are commonly zinc and carbon.

- The electrolyte is often ammonium chloride in a moist paste.

• The advantages of a dry cell are:
  • unbreakable
  • easy to handle
  • can be connected together to produce a higher potential.
• Battery - Several cells connected together.
• Voltaic and dry cells operate as long as there is electrodes and electrolyte. When used up, the cell is "dead."
• Secondary Cells - Storage cells.

- These are cells in which the chemical reaction producing electrical potential

energy is reversible.

• In secondary cells, energy is supplied to reverse the chemical reaction so the cell can once again separate charge.

e.g.. Ni - Cad dry cells (nickel - cadmium)

Electromagnetic Generators

• Electrons in a conductor that is moved through a magnetic field experience a force that causes a current.
• Electromagnetic Induction - An electric current is set up in a conductor which moves in relation to a magnetic field.
• - Used to build generators.

Piezoelectricity

• Piezoelectricity - The reaction of some crystals like quartz, which results in the production of a small electric potential difference when a force is exerted on the crystal.
• The amount of potential difference depends upon the amount of force.
• Piezoelectricity was used on phonograph players. A vibrating crystal (vibrated by the grooves on the record) produces small pulses of electric potential.
• A diaphragm may also vibrate a crystal and therefore produce small pulses.
• Some lighters also use this effect.

Photo electricity

• Photo electricity - The emission of electrons from a surface as a result of light falling on the surface.
• Used in solar cells.
• Solar cells connected together form solar batteries.
• Used on satellites and space probes.

Using Electric Energy

• Most devices that use electric energy produce
  1. heat
  2. light
  3. motion

Heating Elements

• Electrons entering a heating element encounter opposition to their motion from the charges in the atoms making up the heating element. Collisions cause a transfer of kinetic energy from electrons to these atoms. The atoms begin to vibrate more rapidly (the element heats up). The electrons leaving the element therefore have less energy than those entering. This means there is a potential difference across the elements.
• Fuses and circuit breakers are installed in circuits so they will turn the flow of electrons off if too much current is present.
  1. Fuses - melt and open the circuit.
  2. Breakers - open a switch by using a bimetal strip which bends when heated.
• Using a larger fuse than the one specified causes the build up of too much heat and therefore fires can occur.

Light Bulbs

• Incandescent bults operate like heating elements. The temperatures produced are high enough so that atoms start giving off light. The greater the number of electrons passing through the filament, the more energy is supplied to the atoms in the filament, and therefore the greater the light.
• Neon lights and fluorescent bulbs have a gas instead of a solid. Collisions transfer energy to the gas, but the atoms don't vibrate much. The energy transferred to these atoms is radiated as light. Very little energy goes into heat.

Motors

• Electrons flow through wires in a coil inside the motor. This produces a magnetic field. This magnetic field interacts with other magnetic fields to cause work to be done on the coil to cause it to rotate. This uses up electrical energy and causes kinetic energy of the coil.
• Load - Devices that use electrical energy.
• Source - Devices that produce electrical energy.
• Loads and sources take part in a transfer of energy in a circuit.

Movember 30, 2013