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Electric Fields
N/A
75
Physics
Not Applicable
10/02/2012

Term
 ELECTRIC FIELD STRENGTH
Definition
 The electric field strength - E - at a point in the field is defined as the force per unit charge on a test charge placed at that point
Term
 UNIT OF ELECTRIC FIELD STRENGTH
Definition
 Newton per coulomb so NC-1
Term
 POSITIVE TEST CHARGES AT A CERTAIN POIN IN THE ELECTRIC FIELD
Definition
 It will be acted by a force - F - due to the electric field. The electric field strength is then given the equation   electric field strength = force / charge   E = F / Q
Term
 ELECTRIC FIELD STRENGTH NOTES
Definition
 Electric field strength is a vector, it is in the direction of the force on a positive test charge. So the direction of field lines at any point is the same as the direction of the electrical field strength at that point.   Force = Same direction as electric field if positive charge Opposite direction to field if negative charge   The test charge must be less than 1 coulomb so it doesn't alter the electric field strength
Term
 LIGHTNING CONDUCTOR
Definition
 Metal rod at the top of a tall building. It's connected to the ground by a metal conductor which is very thick. When a charged cloud is overheated, a strong electric field is created near the conductor which ionises the air molecules. These ionised air molecules discharge the cloud which reduces the risk of lightning.   *Air is an insulator and is ionised by strong electric fields which pull elctrons out othe air molecules.
Term
 ELECTRIC FIELD BETWEEN TWO PARALLEL PLATES*
Definition
 Field lines between two oppositley charged flat plates are parallel to each other and at right angles to the plates. The lines go from the positive plate to the negative plate. This means the field is uniform not radial because the electric field strenght has the same magnitude and direction at all points
Term
 ELECTRIC FIELD STRENGTH USING VOLTAGE
Definition
 Electric field strength can be calculated using potential difference;   electric field strength = voltage / distance(seperation) between the two plates   E = V / d   This unit is Vm-1
Term
 PROVING E = V / d (1)
Definition
 The force - F - on a test charge - Q -  Force = Charge x Electric field strength F = QE   If charge - Q - is moved from positive plate to negative; work is done to move it. The field does this work on Q. Work done = force x distance moved   W = Fd
Term
 PROVING E = V / d (2)
Definition
 The definition of potential difference - V - is the work done per unit charge when a small charge is moved through it.   Voltage = Work done / Charge V = W / Q   Work done = Charge x Electric field strength x distance W = QEd
Term
 PROVING E = V / D
Definition
 This means that: Voltage = (Charge x Electric field strength x distance) / Charge   V = QEd / Q   The charges appear on both sides of the divide sign so they cancel out   V = Ed  so E = V / d
Term
 FIELD FACTORS
Definition
 Electric fields are near any charged object or body. The greater the charge, the stronger the electric field.   - Charged metal conductors have thier charge spread across the surface, the more conecntrated the charge is, the greater the strength of the electric field
Term
 ELECTRIC FIELD PATTERN BETWEEN A V-SHAPED CONDUCTOR AND FLAT PLATE: *
Definition
 When a constant pd is applied, The field lines are more contcentrated (closer together) at the tip of the V plate, becasue this is where most of the charge is
Term
 ELECTRIC FIELD BETWEEN TWO OPPOSITELY CHARGED PARALLEL PLATES
Definition
 This depends on the concentration of charge on the surface of the plates. The charge on each plate is spread evenly across the surface of the plate facing the other plate   The elctrical field strength is proportional to the charge per unit area   E ≈ Q / A
Term
 STATIC ELECTRICITY
Definition
 Like charges repel and unlike charges attract
Term
 ELECTRONS
Definition
 Electrons charge in most situations. Uncharged atoms contain equal numbers of protons and electrons. ADDING electrons, negatively charges the atom, REMOVING electrons positively charges the atom. e.g when a perspex rod is rubbed with a dry cloth, the electrons from the rod transfer to the cloth. This removes electrons from the rod making it positive and adds electrons to the cloth making it negative
Term
 ELECTRIC CONDUCTORS
Definition
 Metals contain lots of free electrons. These move about inside a metal and are not attatched to any one atom. They are the charge carriers. To charge a metal is must be isolated from the earth otherwise the charge will be neutralised by electrons transferring between the earth and metal.
Term
 EARTHING
Definition
 If the conductor is charged postively, electrons from the earth transfer to the conductor to neutralise the charge. If negatively, the electrons from the conductor will transfer to the earth, discharging the conductor.
Term
 INSULATING MATERIALS
Definition
 Don't contain free electrons, so all the electrons are attatched to individual atoms. Some insulators are easy to charge because thier surface atoms lose or gain electrons easily
Term
 THE SHUTTLING BALL EXPERIMENT
Definition
 Shows elecric current is a flow of charge Conducting ball is hung by insulating thread between two vertical plates of opposite charge When high voltage is applied, the ball bounces back and forth between the two plates Everytime it touches the negative plates, electrons are transferred to the ball (so it's negatively charged) and repelled by the negative plate yet attracted by the positive plate. Here electrons transfer to the positive plate and the ball becomes positive and repelled again
Term
 CURRENT CHARGE AND FREQUENCY
Definition
 Current = Charge x Frequency  I = Qf    Frequency = 1 / time    Therefore current = charge / time for one cycle   I = Q/Time for one cycle
Term
 GOLD LEAF ELECTROSCOPE
Definition
 Used to detect charge  If charged obejct is in contact with metal cap of electroscope, some charge transfers to electroscope which then flows to the gold leaf and metal stem causing them both to have the same charge so they repel each other. The leaf is light so as being repelled by the metal stem, it rises. If another object with the same charge is bought close too, this will cause the leaf to rise further as more charge is forced to transfer to the leaf and stem
Term
 CHIPS
Definition
 The small amount of charge on pins of a microchip is enough to destroy the circuits inside a chip.  If the pins are touched by a charged near a charged object, they will be earthed. This means the electrons will transfer from the pins to the earth. Microchips are stored in antistatic packets which allow charges to flow across the surface
Term
 FIELD LINES
Definition
 Any two objects exert equal and opposite forces;  Electric fields surround each charge If a small positive test charge is placed near an object with a much bigger charge that is also positive, the test charge will follow a path AWAY from the big charge. The path is called field lines
Term
 FIELD LINES OF 2 OPPOSITELY CHARGED POINTS*
Definition
Term
 FIELD LINES OF A POINT NEAR A PLATE*
Definition
Term
 FIELD LINES OF TWO OPPOSITELY CHARGED PLATES*
Definition
Term
 OPPOSTIELY CHARGED POINTS
Definition
 Create field lines which are concentrated at each point. Positive test charges would follow a curved path to the negative point charge
Term
 POINT OBJECT NEAR AN OPPOSITELY CHARGED FLAT PLATE
Definition
 Field lines concentrated at point object but at right angles to the plate; field is strongest where the field lines are most concentrated
Term
 TWO OPPOSITELY CHARGED PLATES
Definition
 Field lines run parallel from one plate to the other at right angles to the plates. This is a uniform field because the field lines are parallel
Term
 THE VAN DE GRAAF GENERATOR
Definition
 Produces sparks in the air Charge created when a rubber belt rubs against a metal pad and is carried up the belt to the metal dome. As charge increases on the ome, the pd between the dome and earth increases until sparking occurs.  Sparks transfer energy from the dome. Work is done to charge the dome because a force is needed to move the charge on the belt up to the dome. Electric potential energy of the dome increases as it charges up. Some of this energy is transferred from the dome when a spark is created
Term
 TWO OBJECTS OF THE SAME CHARGE
Definition
 Work must be done to move a charged object towards another charged object of the same charge. Thier electric potential energies increases as they move toward each other
Term
 CONSIDERING TWO CHARGED OBJECTS OF THE SAME CHARGE
Definition
 The electrical potential of object 1 increases from zero at infinity, as it moves towards object 2. The electrical field of object 2 causes a repulsive force which acts on object 1 . Object 1 must overcome this force to move closer to object 2
Term
 ELECTRIC POTENTIAL AT A CERTAIN POSITION IN ANY ELECTRIC FIELD
Definition
 The work done per unit positive charge on a positive test charge when it is moved from infinity to that position.
Term
 ZERO POTENTIAL ENERGY
Definition
 By definition, the position of zero potential energy is infinity
Term
 UNIT OF ELECTRICAL POTENTIAL
Definition
 Volt (V) = 1JC-1
Term
 THE RELATIONSHIP BETWEEN ELECTRIC POTENTIAL ENERGY (Ep) AND ELECTRIC POTENTIAL (V)
Definition
 Electric potential = electric potential energy / charge   V = EP/Q   So    EP = QV
Term
 EQUIPOTENTIALS
Definition
 Lines of constant potential Test charges moving along an equipotential has constant potential energy. No work is done by the electric field because the force due to the field is at right angles to the equipotential -- lines of force of the electric field cross the equipotential lines at right angles
Term
Definition
 The potential gradient at any position in an electric field is the change of potential per unit charge of distance in a given direction
Term
 POTENTIAL GRADIENT OF A NON - UNIFORM FIELD
Definition
 Potential gradients vary with position and direction. The closer the equipotentials are, the greater the potential gradient is at right angles to the equipotentials
Term
 POTENTIAL GRADIENTS IN A UNIFORM FIELD
Definition
 If the field is uniform e.g two parallel plates that are oppositely charged, the equipotentials between the plates are equally spaced lines parallel to the plates.
Term
Definition
 The potential(negative) is proportional to the distance The potential gradient is constant The potential gradient increase in the opposite direction to the electric field The potential gradient = electric potential / distance   = V / d
Term
 ELECTRIC FIELD STRENGTH AND POTENTIAL GRADIENT
Definition
 The electric field strength is equal to the negative of the potential gradient
Term
 COULOMB'S LAW  - LINK OF F AND r
Definition
 The force is proportional to 1 / radius squared From here Coulomb law states that: Force = Constant of proportionality x Charge of one object x charge of second object / distance between them squared   Constant of proportionality = 1 / 4πεο F = Q1Q2 / r2
Term
 SALT CRYSTALS DISSOLVING IN WATER
Definition
 Salt crystals are ionic Sodium ions and chlorine ions are oppositely charged Electrostatic forces between them hold them together Water weakens the electrostatic forces between the ions at the surface so they break free from the surface of the crystals, so they dissolve The force in water is 80 x weaker than if the crystals were in air
Term
 COULOMBS LAW APPLIED TO THE FORCE ON A TEST CHARGE (q)
Definition
 Force = Constant of proportionality x point charge x test charge / distance sqaured   F = kQq / r2    Electric field strength at distance (r) = Force / test charge = constant of proportionality x point charge / distance squared   E = F / Q = kQ / r2
Term
 POINT CHARGES
Definition
 A charged obejct which effects distances much greater than it's own diameter
Term
 TEST CHARGES IN A FIELD
Definition
 A test charge in an electric field is a point charge that doesn't alter the electric field in which it is placed - if the object has a charge that is greater than one coulomb, it would change the distribution of charge which creates the field
Term
 POSITIVE POINT CHARGE AND POSITIVE TEST CHARGE
Definition
 Field lines radiate away from point charge because the test charge in the field experiences a force directly away from the point charge wherever it's placed.  Coulombs law states that force = (one / four x pi x epsilon) x (point charge x test charge / distance squared) F = (1/4πεο) Qq/r2 Electric field strength = force / test charge so electric field strength = point charge / 4,pi,epsilon x distance2 E = Q / 4πεο x r2
Term
 WHAT IF THE POINT CHARGE IS NEGATIVE
Definition
 If the point charge is negative using the formula; E = 4πεο x r2, the value of E will be negative so the field lines will  point towards the point charge
Term
 ELECTRIC FIELD STRENGTH AS A VECTOR
Definition
 If a test charge is in an electric field with 2 or more point charges, each charge exerts a force on the test charge.  The resultant force on the test charge gives the resultant electric field strength at the test charge
Term
 FORCES IN THE SAME DIRECTION
Definition
 For example a positive test charge between a negative point charge and positive point charge have forces in the same direction because the positive point charge repels the test charge and the negative point charge attracts the point charge This means the resultant force is equal to the sum of both forces    F = F1 + F2
Term
 RESULTANT ELECTRIC FIELD STRENGTH FOR FORCES IN THE SAME DIRECTION
Definition
 Force = electric field strength x charge F = qE Rearranging this gives electric field strength = force / charge E= F/q  Resultant force = force from negative + force from positive  F = F1 + F2 So resultant electrical field strength is equal to (charge x electric field strength1 + charge x electric field strength 2) / charge  E = (qE1 + qE2) / q Charge,q, cancels out to leave E = E1 + E2
Term
 FORCES IN OPPOSITE DIRECTIONS
Definition
 A test charge between two positive point charges. The forces will be in opposite direction because the forces repel each other so the resultant force =    F = F1 - F2    Electrical field strength     E = E1 - E2
Term
 FORCES AT RIGHT ANGLES TO EACH OTHER
Definition
 Pythagoras theorum   F2 = F12 + F22   Electric field strength    E2 = E12 + E22
Term
Definition
 Electric field lines of force surrounding a point charge are radial - the equipotentials are circles around the point charge
Term
 THE ELECTRIC FIELD STRENGTH EQUATION
Definition
 E = Q/4πεο r2 Shows that the electric field strength is inversely proportional to the square of the distance. This is an inverse square law,  so shows a curve on a graph because  Electric field strength is proportional to1/r2
Term
 NEGATIVES
Definition
 Negative electric field strength means that the field is acting towards a negative charge   Negative electric potential means a value below zero   *E varies more sharply with distance than V
Term
 I
Definition
 CURRENT
Term
 Q
Definition
 CHARGE (Usually for point charges)
Term
 e
Definition
 CHARGE OF ELECTRON    1.6 X 10-19
Term
 f
Definition
 FREQUENCY (1/time taken) therefore 1/f = time taken
Term
 E
Definition
 ELECTRIC FIELD STRENGTH     E = F / Q (With test charges)      E = V / d (For two parallel plates)
Term
Definition
Term
 F
Definition
 Force due to electric field strength   F = QE
Term
 W
Definition
 Work done   W = QEd      W = QV(Volts)
Term
 εο
Definition
 Epsilon nought   8.85 x 10-12 Fm-1 (Farads per metre)
Term
 CONSTANT OF PROPORTIONALITY COULOMB'S LAW
Definition
 1 / 4πεο 9.0 x 109mF-1                                                          K
Term
 COULOMB'S LAW
Definition
 F = K x Q1Q2 / r2   Force = constant x charge 1 x charge 2 / distance sqaured
Term
 ELECTRIC FIELD STRENGTH
Definition
 Inversely proportional to the square of distance r
Term
 ELECTRIC POTENTIAL
Definition
 Inversely proportional to distance r it's not an inverse square alw becasue v is proportional to 1/r.   V curve is always LESS steep than E curve
Term
 NEGATIVE E AND NEGATIVE V
Definition
 Negative E = field acts towards negative charge Negative V = value less than 0  E varies with distance more sharply than V
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