17.1: Electric Charge Like and unlike charges ● Like charges repel ● Unlike charges attract During charging process ● We are not creating charge we are transferring it from one object to another ● Ie; rubbing 2 things together; ○ One loses electrons while the other gains electrons the one which loses electrons is positively charged while the one which gained electrons is negatively charged ○ Total electric charge on both objects does not change The physical basis of electric charge ● Electrical interactions are responsible for the structure and properties of atoms and molecules ● Atoms have ○ Neutron ■ uncharged ○ Electron ■ Negatively charged ○ Proton ■ Positively charged ● Proton and neutrons make up the nucleus ○ Masses; ○ ■ Magnitude of Negative charge of electron = magnitude of positive charge of proton Atomic number ● Number of protons or electrons in neutral atoms of any element Neutral atom ● # of protons= #of neutrons To charge an object ● Add or remove negative charge Ion ● An atom that has lost or gained one or more electrons ● ● Positive if; ○ Lost electrons Negative if; ○ Gained electrons Ionizaation ● Gaining or losing of electrons 17.1 examples 17.2; COnductors and Insulators Conductor ● A material that can transfer charge , permit movement of charge through them ○ Metals Insulator ● A material that cant transfer charge, don’t permit charge through them ○ Non metals Semiconductors ● Intermediate in properties between good conductors and good insulator ● Can be engineered to have a controllable conductivity In a liquid or gas ● Charge can move in the form of positive or negative ions ○ Ionic solutions are usually good conductors Induction ● A caharged object can give another object a charge of opposite sign without losing it’s own sign by moving close to the neutral object Polarization ● Charged object exerting forces on objects that are not charged causing molecules to develop induced charges 17.2 examples 17.3 ; Conservation and quantization of charge Conservation of charge ● The algebraic sum of all electric charges in any closed system is constant ● Charge can neither be created or destroyed it can only move from one place or object to another Quantization of charge ● Since the magnitude of chage of the electron of proton is a natural unit of charge ever observable electric charge is always an integer multiple of this basic unit ● Electric charge cant be divided into smalled amounts than the charge of one electron or proton 17.3 examples 17.4; Coulomb’s law Proportionality ● The electric force between 2 point charges is proportional to the inverse square of the distances between the charges Point charges ● Charged bodies that are very small in comparison with the distance r between them Force ● Depends on the quantity of charge on each object ( denoted by q) ● The forces that 2 point charges q1 and q2 exert on each other are proportional to each charge and therefore are proportional to the product of the 2 charges Coulombs law ● The magnitude of the force that each of two point charges a distance r apart exerts on the other is directly proportional to the product of the charges and inversely proportional to the square of the distance between them ● ● ● ● Units ; ○ Q1 and q2 are in couloumbs ○ F is in newtons K is a fundamental constant of nature ○ ● ● ● ● ● F represents only the magnitude of the force The direction of the force; ○ Is determined by using the fact that like charges repel and unlike charges attract ○ Always along the line joining the charges The t10 forces are allways equal in magnitude and opposite in direction R is the distance between the 2 charges Only used for point charges in vacuum or air Force acting between charges with matter present ● The net force acting on each charge is altered because charges are induced in the molecules of the intervening material The constant k ● The charge of an electron or proton ● ● ● Electron charge; ○ -e Proton charge ○ +e The principle of Superposition ● When 2 charges exert forces simultaneously on a third charge the total force acting on that charge is the vector sum of the forces that the 2 charges would exert individually Problem solving strategy ; coulombs law ● Set up ○ Consistent units ■ Distances in meters ■ Charges in coulombs ■ Forces in newtons ● Solve ○ When forces acting on a charge are caused by 2 or more other charges the total force on the charge is the vector sum of the individual forces ○ Use components in an x-y coordinate system 17.4; Examples 17.5 Electric Feild and Electric forces Electric feild ● A charged object creates and electric feield in the space around it ● The electric field causes the force on another charged particle in the feild ● The electric feild is the force per unit charge exerted on the test charge ● ● ● The charged object A produces or causes an electric field at point P and all other points in the neighbourhood When a point charge is placed at point b it is acted upon by the force by the electric field at p The point charge b also sets up an electric field which exerts a force on A To determine whether there is an electric field experimentally ● To determine whether theres an electric field at a particular point we place a charged object, called da test charge, at the point ● If the test charge experiences a non zero electric force then there is an electric field at that point Definition of an electric field ● ● ● ● ● When a charged particle with charge at a point P is acted upon by an electric force F, the electric field E at that point is defined as ○ The test charge q can be positive or negative ○ If positive; ■ E and F have the same direction ○ If negative; ■ E and F have opposite directions Units of the electric field; ○ N/C The force acting on the test charge varies from point to point so the electric field is different at different points The electric field is not a single vector but an infinite set of vector quantities associated with each point in space ○ This is a vector field ■ A vector quantity associated with every point in a region of space, different at different points Electric field within a conductor ● The field exerts a force on every charge in the conductor, causing free charges to move Electrostatic situation ● A situation in which the charges do not move ● The eclectic field at every point within the material of a conductor must be 0 An electric field that is uniform in a region ● The magnitude and direction of the field are constant throughout a certain region 17.5 examples; 17.6; calculating electric fields The principle of superposition ● The total electric field at any point due to two or more charges is the vector sum of the field that would be produced at the point by the individual charges FInding the field caused by several charges or an extended distribution of charge ● Imagine the source is made up of many point charges ● a source point ○ the location of one of these points ● A field point ○ The pont where we want to find the field ● We calculate the fields at point p caused by the individual point charges and take their vector sum to find the total field at that point ● Force due to point charges ● If source distribution is a single point charge q ● The source point (S)- the location of the charge ● P- the location of the field point P ● If we place a small test charge q at the field point P, at a distance r from the source point the force magnitude is given by; ○ Electric field due to a point charge ● The magnitude E of the electric field at point P due to a point charge q at point S, a distance r from P is; ● ● ○ If point charge is positive; ○ The electric field produced by it points away from it Id point charge is negative; ○ The electric field produces by it points toward the point charge Spherical charge distributions ● ● The electric field produced by any spherically symmetric charge distrobution, at all points outside this distribution, is the same as though all the charge were concentrated at a point at the center of the sphere The field outside and spherical charge distribution can be obtained by replacing the distribution with a single point charge at the center of the sphere and equal to the total charge of the sphere Problem-solving strategy ● Set up ○ Use a consistent set of units ■ Distances in meters ■ Charges in coulombs ○ Use components to compute vector sums ○ Indicate coordinate axess clearly on diagram ● Solve ○ While working out directions of E vectors be careful to distinguish between the source point and the field point 17.6 examples; 17.7; Electric field lines Electric field lines ● An imaginary line drawn through a region of space so that at every point it is tangent to the direction of the electric field vector at that point ● ● The direction of the electric field at any point is tangent to the field line through that point Electric feil lines show; ○ The direction of the electric field at each point ○ Spacing gives a general idea of the magnitude of the electric field at each point ■ Where the field is strong; ● Lines are drawn bunched closely together ■ Where the field is weak ● The lines are drawn far appart ○ Field lines never intersect because ■ One field line can pass through each point of the field Characteristics of electric field lines ● At every point in space , the electric field vector E at that point is tangent to the electric field line through that point ● Electric field lines are close together in regions where the magnitude of the electric field is large and are farther appart where it is small ● Field lines point away from the positive charges and toward the negative charges The direction of a field line at a given point determines the direction of the particles acceleration, not it’s velocity Thus the charged particle does not follow the path of the field line
