Coulomb's Law. Two like charges of 1 Coulomb placed 1 meter apart repel each other with a force of 9 x 10 9 Newtons

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1 Electricity Electricity is a force much like gravity but many times more powerful. Unlike gravity, electrical forces can be attractive or repulsive. Gravity has only one type of mass. Electricity has two types of 'charge' positive and negative. Like changes (two positive or two negative changes repel one another. Two unlike changes (one positive and one negative) attract each other. 1

2 Electricity Ordinary matter is made of atoms. At the center of the atom is a very small nucleus which contains positively charged protons and neutrons which have no charge. 'Orbiting' the nucleus are electrons which have a negative change. The proton (and neutron) have a mass of 1.67 x kg. While this is very small, the electrons have a mass of the electron is about 1800 times smaller (9.1 x kg). The proton and electron have exactly the same magnitude of charge but opposite signs. This makes atoms charge neutral. If an atom loses or gains an electron we call the charged atom an ion. It is the electrons which are lost or gained. This happens when you walk across a carpet on a day with low humidity. 2

3 Electricity Socks tumbling around in a cloths dryer can rub electrons off of other clothes and become negatively charged. (It depends on the type of cloth). If you have an initially uncharged shirt, the negatively charged sock will attract the positive charge from the shirt (actually push away electrons) making the side of the shirt positive. The positive side of the shirt will attract the negatively charged sock and cling together. 3

4 Coulomb's Law The force law which describes the force between two point charges is called 'Coulomb's law. It looks a lot like Newton's law of gravity. Coulomb's law is: q 1 q 2 F = k r 2 Where q1 and q2 are the charges, r is the distance between the charges and k is Coulomb's constant. Coulomb's constant is k = x 10 9 N m 2 /C 2. It is very large compared to G! The unit for charge is the Coulomb which is a large amount of charge. We will see how it is defined later when we discuss current. The electron has a charge of x Coulombs. Two like charges of 1 Coulomb placed 1 meter apart repel each other with a force of 9 x 10 9 Newtons 4

5 Induction and Dipoles A charge can be 'induced' on an initially uncharged object if a charged object is place near the uncharged object. This is known as charge induction. The charged object (plus) attracts the electrons to one side of the uncharged object. This makes the side near the positively changed object negative. The other side is then left with a positive charge. The arrangement of positive and negative charge on opposite ends of an object is called a 'dipole'. Many molecules have this arrangement. A water molecule is a dipole and can be deflected by a charged object. 5

6 Static Charges Making a static charge is more complicated than rubbing electrons off of some object. Making a static charge but rubbing two materials together highly depends on the materials. Some elements like chlorine tightly hold on to their electrons while others give them up easily. If they atoms are combined into a molecule, the molecule may tightly hold some of their electrons and other may give them up more easily. The process is self limiting. As one object builds up a charge by accumulating extra electrons, the negative charge repels additional electrons. 6

7 Photocopiers Photocopiers use static electricity to make copies. The key to the process is a photoconductor. A photo-conductor is normally an insulator. An insulator will hold a static charge. When a photoconductor is exposed to light, it becomes a conductor. A conductor allows the static charge to move and neutralize. 7

8 Photocopiers In a photocopier, a charge is uniformly applied to a belt of cylinder made of a photo-conductor. An image of the original paper is projected on the photoconductor. Where the paper is white, the charge is erased while the black areas (the typing) is not. Fine black particles in the toner are attracted to the charged areas on the belt. 8

9 Photocopiers The remainder of the process is transferring the image to the paper. The charge is neutralized on the belt and paper is pressed against the belt. Heat or chemical attach the toner particles to the paper. Any residual particles are removed and the belt is recharges so the process can be repeated. 9

10 Electric Fields Like gravity, electric charges is influenced by another charge or distribution of charge at a distance. We can use this idea of an electric field to visualize the effect of charges on one another. A field line at some point in space points in the direction of the force a small positive charge would feel. Electrical field lines begin at positive charge and end on negative charges. Electric field lines point away from postive change and towards negative charge. 10

11 Electric Fields The electric field is a vector and defined as: F E = q From Coulomb's law, the magnitude of the electric field for a point charge is: q E = k r 2 11

12 Force (or Action) at a Distance Gravity and electric forces are examples of force at a distance. The force (or field) pervades all of space. The earth and moon pull on each other without any direct contact or exchange. What if an object moves very quickly. How does the other object (mass or charge) know the object has moved. Some have called this very basic idea 'spooky'. How do we 'know' something is really there. In modern physics (quantum physics and relativity), the idea of all pervasive field has been replaced by the exchange of 'particles' (bosons) that carry the force between the two particles that 'feel the force'. In this way modern physics avoids the problem of 'action at a distance'. 12

13 Electric Potential Just as we have gravitational potential energy, we can have electrical potential energy. We have to do work to push two like charges together which will store energy. (This energy depends on position and not motion so it is potential energy.) A larger charge will have more potential energy in an electric field than a smaller charge. We define the electric potential as: Electric potential = Electric potential energy Amount of charge The unit of electric potential is the volt (V) = Joule/Coulomb. Electric potential determines the electrical effect at some point even if there are no charges present. 13

14 Electric Potential Normally we measure a potential difference between two points. If the potential difference is known between two point, we know how much work it takes to move a given amount of charge between the two point. We can relate the magnnitude of electric field to the electric potential by: V E = r r is the distance between between the two points and V is the potential difference between the points. The direction is given the direction of the largest change in the potential (the 'gradient'). 14

15 Electric Potential There are several ways to make a potential difference between two points. 'AA' batteries uses chemical energy to produce a potential difference of 1.5 volts between the ends. A car battery uses different chemicals to product 12 volts' across its terminals. Most of the electricity we use is produced by a power plant which used magnetic induction to produce 'alternating current' or AC at 120 volts (rms). We will discuss this in more detail when we discuss magnetic effects in the next chapter. 15

16 Electric Current When a potential difference is applied across a conducting wire, the (negatively charged) electrons move towards the positive terminal of the battery and away from the negative terminal. This movement of the electrons in the conductor is called a conductor. Some materials like metal allow the electrons to move easily. Other materials like plastic or glass do not allow the electrons to move easily and are called insulators (analogous to heat flow). By convention (due to Ben Franklin and his kite), the direction of a current is the way a positive change would flow. In other words, by convention the direction of a current is opposite to 16 the direction of the electrons.

17 Electric Current The unit of current is the Ampere. One ampere is one Coulomb moving past a point in the wire every second so 1 A = C/s. A common analogue to current moving through a wire is to think of water flowing through a pipe. The current is the flow rate (liters/minute) and the voltage is the pressure in the pipe. Resistance Some materials conduct electrical current better than others. Electrical resistance is how well a material conducts electricity. The unit for resistance (Ω) is the 'ohm' = V/A. This is contained in Ohm's Law: V = IR where V is volts, I is current and R is resistance. 17

18 Flashlights A flashlight is a simple electric circuit. Two 1.5 V batteries are placed in series giving a total voltage of 3.0 V. The positive battery terminal connects to one side of the bulb filament. The other side of the filament is connnect to the switch which Completes the circuit to the negative terminal of the batteries. The resistance of the bulb filament is about 20 Ω. Ohm's law then give the current as I = 3 V/ 20 Ω = 0.15 Α. 18

19 Electrical Power Power (energy/time) is normally expressed in watts. For an electrical circuit, the power consumed is given by: In terms of units this is Power = current x voltage = V I Watt = Ampere x volt A 60 Watt incandescent light bulb that operates at 120 volts has a current of ½ ampere. For the example of the flashlight, The power is 3 V x 0.15 A = 0.45 W. If you look at the electrical units, you see that an ampere is Coulomb/s and the volt is a Joule/Coulomb so the product is a Joule/s = Watt. 19