Electrostatics – Forces, Fields, and Energy

1. Electrostatics – Forces, Fields, and Energy Unit 8

2. Electric field – a field of force surrounding a charged particle • Electric force – an attractive or repulsive electrostatic force described by Coulomb's law • Coulomb's law – an equation describing the electrostatic force between electric charges • Electromagnetic force – force associated with electricity and magnetism • Magnitude – an amount, a quantity, or a size • Conductors – material through which heat and electrical charges can be transferred • Insulators – material that is a poor conductor of heat and electrical charges • Law of conservation of energy – the fundamental principle of physics that the total energy of an isolated system is constant, despite internal changes

3. Charges will move due to these “Electrical Forces” Electrical Charge – Charge is a fundamental property of matter, like mass. Objects are charged either positive, negative, or neutral. The unit for electrical charge: The “coulomb”. It represents how strong a charge is. Electrical Forces are very strong if 1 negative couloumb were 1 meter away from 1 positive coulomb the force would be 9 billion Newtons! These forces are so strong because they have to be! They are the forces that hold together molecules! Like charges repel Opposite charges attract • What is our model of matter? _ + + + ATOMIC MODEL OF MATTER Electrical charges work like magnetic poles. Attraction and repulsion of electrical forces causes electrons to move! Protons – Positive in the nucleus. POSITIVE charges are represented in RED. ELECTRON – Negative, orbits the nucleus. NEGATIVE charges are represented in BLUE We always speak in terms of the electrons, NOT the protons moving. Protons CAN’T move because they are in the nucleus. + When electrons move we get : NEUTRON – Neutral, in the nucleus. ELECTRICITY!

4. Conductors vs. Insulators Net Charge – How to calculate it. If it had one more electron, our sum would have been zero. (11-11 = 0) and the atom would have been neutral. (Net charge = 0) Conductors Insulators Allow electricity to flow Resist the flow of electricity Sodium Ion Metal Water Plastic Air Wood Glass Sand Dirt Styrofoam 1. Count number of protons 11 2. Subtract number of electrons -10 3. The resulting sum is the net charge 1 Conductors of electricity also conduct heat, aka thermal energy. More electrons than protons and our atom has a negative ‘net charge’. Have weak attraction to valence electrons Have strong attraction to valence electrons + + + + + + + + + + + We would now say, “This atom has a positive charge”, because it has one more proton than neutron.

6. Ancient Greeks - discovered static electricity when they rubbed animal fur and amber together Our word electron comes from the Greek word for amber which was elektron Benjamin Franklin – “coined” the terms positive and negative in his investigation of electric charge He imagined electricity as an invisible sort of fluid Coulomb – Measured the force of attraction/repulsion between two charges • Faraday – Faraday proved that the electricity induced from the magnet, voltaic electricity produced by a battery, and static electricity are all the same. • Millikan – Measured the fundamental unit of charge to be e = 1.602 x 10-19 C. Amber

13. Electric Fields Source charge = the charge that is producing the magnetic field that we want to measure the strength of. Variable for the magnitude (strength) of the electric field. Units are Newtons per Coulomb (N/C). This is the amount of force per charge Test charge = the charge that is acted on by the source charge. We will test the electric field at its location. E=F/q Example Problem = Calculate the magnitude of the electric field produced by the positive charge at the negative charge. Postive charge = 1 C Negative charge = -2 C Distance between the two charges = .003m • Use coulomb’s law to find (F) the amount of force between these two charges. • Figure out what is the test charge and what is the source charge. Since we are calculating the magnitude of the electric field PRODUCED by the POSITIVE CHARGE, it is the source charge. This leaves the other charge to be the test charge. The test charge will go on the bottom of our E=F/q equation. • Plug the numbers into the E=F/q equation and solve. The amount of charge (in coulombs) of the test charge. Usually given to you in the problem. The force between the test charge and the source charge. It could be a repulsive or an attractive force. Might be given to you in the problem. Might have to be found using Coulomb’s Law. Electric Field Lines – The arrows above represent electrical field lines. The arrows always point towards negative charges and away from positive charges.

14. Electrophorus Important Electrostatic Instruments

15. Calculating the PE of an Electric Field – Just like Coulomb’s Law, but without squaring the distance Source charge = the charge that is producing the magnetic field that we want to measure the strength of. Coulomb’s Constant Test charge = the charge that is acted on by the source charge. We will test the electric field at its location. Variable for PE of an electric field Value of charge #1 Example Problem = Calculate the magnitude of the electric field produced by the positive charge at the negative charge. Postive charge = 1 C Negative charge = -2 C Distance between the two charges = .003m • Use coulomb’s law to find (F) the amount of force between these two charges. • Figure out what is the test charge and what is the source charge. Since we are calculating the magnitude of the electric field PRODUCED by the POSITIVE CHARGE, it is the source charge. This leaves the other charge to be the test charge. The test charge will go on the bottom of our E=F/q equation. • Plug the numbers into the E=F/q equation and solve. U=k(q1q2)/r The distance between charges Value of charge #2