Electric charge   

• Electric charge is caused by an imbalance between positive (protons) and negative (electrons) particles in matter.
• Like charges repel each other, while opposite charges attract.
• Neutral objects can be charged in two ways:
  • Charging of neutral objects through contact is called conduction.
  • Charging of neutral objects without direct contact is called induction.
• Conductors are substances that allow electrons to move easily through them. Metals are examples of good conductors.
• Insulators are substances that don’t allow electrons to move easily through them.  Plastics, wood, and glass are good insulators.
• Cool demo: Rub a balloon against your hair and then touch then touch a wall with it. The balloon should cling to the wall because it now charged and inducing the opposite charge in the wall, as discussed a bit more here.

Coulomb's Law

• The force of attraction or repulsion between two charges can be described mathematically by Coulomb's Law:
  • The magnitude of the force is directly proportional to the product of the two charges.
  • The magnitude of the force is inversely proportional to the square of the distance between the two charges.
• Example: If the distance between two protons increases from 5 cm to 15 cm, the force of repulsion is reduced by a factor of 3² = 9.

Power vs energy

• Power is the rate of energy production or consumption.
• The most common unit of power in the US is watt (W) or Joule per second (J/s).
• A typical lightbulb consumes 50-100 W.
• Over time, running devices consume energy: Energy = Power times Time.
• In Physics, the most common unit of energy is the Joule. In the power industry, the unit of energy is the kilowatt-hour, which is equivalent to 1000 W *3600 s = 3.6 million Joules (W*s=J).
• This website offers a few examples dealing with power and energy.

Cost of running appliances

• Despite the name, the power company charges customers for energy, not power.
• The standard unit of electric energy in the US power industry is kilowatt-hour (kWh).
• The cost of electricity charged by the power companies varies somewhat with demand but is roughly $0.10 per kWh.
• This website helps you calculate the cost of running various appliances.

Example: Estimate the cost of running a TV all night.
Solution: A TV consumes about 250 watts (estimates can vary). In the course of an 8-hour night, the TV would consume about 0.250 kW* 8 h = 2 kWh. Since 1 kWh costs about $0.10, 2 kWh should cost about $0.20.

Return to class notes TOC.

Page last modified: