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## Electric Force and Electric field

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**Electric Force and Electric field**1. There are two types of electric charge (positive and negative)**Electric Force and Electric field**2. Static charges can be produced by the action of friction on an insulator**Electric force and electric field**3. Conductors contain many free electrons inside them (electrons not associated with one particular atom)**Electric Force and Electric field**4. Charge is conserved. The total charge of an isolated system cannot change. I’m indestructible! So am I!**Coulomb’s law**F = kq1q2 r2 The constant k is sometimes written as k = 1/4πεo where εo is called the permittivity of free space.**Calculations using Coulomb’s law**The force between two charges is 20.0 N. If one charge is doubled, the other charge tripled, and the distance between them is halved, what is the resultant force between them? q2 q1 F = 20N r 2q1 3q2 F = ? N r/2**Calculations using Coulomb’s law**F = kq1q2/r2 = 20.0N x = k2q13q2/(r/2)2 = 6kq1q2/(r2/4) = 24kq1q2/r2 x = 24F = 24 x 20.0 = 480 N q2 q1 F = 20.0N r 2q1 3q2 x = 480 N r/2**Electric field**An area or region where a charge feels a force is called an electric field. The electric field strength at any point in space is defined as the force per unit charge (on a small positive test charge) at that point. E = F/q (in N.C-1)**Electric field around a point charge**If we have two charges q1 and q2 distance r apart F = kq1q2/r2 Looking at the force on q1 due to q2, F = Eq1 F = kq1q2/r2 = Eq1 E (field due to q2) = kq2/r2 q1 q2 NOT in data book**Electric field**Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition. Field due to q1 Field due to q2 Resultant field q1 q2**Electric field patterns**An electric field can be represented by lines and arrows on a diagram , in a similar ways to magnetic field lines. The closer the lines are together, the stronger the force felt. This is an example of a radial field**Field around a charged metal sphere**E = 0 inside the sphere**Field between charged parallel plates**NOT in data book V Uniform field E = V/d d “Edge effects”**Remember!**The force F on a charge q in a field E is F = Eq**Gravitational Force and Field**We already know that; • Masses attract each other**Gravitational Force and Field**We will know that; 2. Mass/energy is conserved (E = mc2)**Gravitational Force and Field**The force between masses was formulated (discovered?) by Isaac Newton in 1687**Newton’s law of universal gravitation**F = Gm1m2 r2 The constant G is known as “Big G”and is equal to 6.667 x 10-11 Nm2kg-2**Newton’s law of universal gravitation**F = Gm1m2 r2 For large objects like the earth, r is the distance to the centre of mass**Calculations using Newton’s law**What is the force of attraction between Pascal and Chris? 2 m 63kg ? 70kg ?**Calculations using Newton’s law**F = Gm1m2= 6.667 x 10-11 x 63 x 70 = 7.3 x 10-8 N r222 2 m 63kg ? 70kg ?**Force of gravity due to earth on Pascal?**F = Gm1m2= 6.667 x 10-11 x 63 x 6 x 1024 = 615 N (= mg) r2(6400 x 103)2 Pascal’s weight 63kg ? R = 6400 km, m = 6 x 1024 kg**Force of gravity due to earth on Pascal?**F = Gm1m2= 6.667 x 10-11 x 63 x 6 x 1024 = 615 N (= mg) r2(6400 x 103)2 In other words, for any planet; g = Gmp rp2**Gravitational field**An area or region where a mass feels a gravitational force is called a gravitational field. The gravitational field strength at any point in space is defined as the force per unit mass (on a small test mass) at that point. g = F/m (in N.kg-1)**Gravitational field around a point mass**If we have two masses m1 and m2 distance r apart F = Gm1m2/r2 Looking at the force on m1 due to m2, F = gm1 F = Gm1m2/r2 = gm1 g (field due to m2) = Gm2/r2 m1 m2**Gravitational field around a point mass**I told you, for any planet; g = Gmp rp2 Don’t forget that for a non point mass, r is the distance to the centre of mass If we have two masses m1 and m2 distance r apart F = Gm1m2/r2 Looking at the force on m1 due to m2, F = gm1 F = Gm1m2/r2 = gm1 g (field due to m2) = Gm2/r2 m1 m2**Gravitational field**Gravitational field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one mass) must use vector addition. Field due to m2 Field due to m1 Resultant Field m1 m2**Gravitational field patterns**A gravitational field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines.**Gravitational field patterns**A gravitational field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines. The closer the lines are together, the stronger the force felt. Note, gravity is ALWAYS attractive This is an example of a radial field**ALL magnets have two poles**NORTH seeking pole SOUTH seeking pole**Magnetic materials**Iron (steel), Cobalt and Nickel**N**S Magnetic induction When a magnetic material is close to a magnet, it becomes a magnet itself We say it has induced magnetism magnet S N**N**S Soft Magnetism Pure iron is a soft magnetic material It is easy to magnetise but loses its magnetism easily before after N S N S N Not a magnet Iron nail**N**S Hard Magnetism Steel is a hard magnetic material It is harder to magnetise, but keeps its magnetism (it is used to make magnets!) before after S N N S N N It’s a magnet! S Steel paper clip**Magnetic field**Magnets and electric currents produce magnetic fields around them. In a magnetic field, another magnet, a magnetic material or a moving charge will experience a magnetic force. www.physchem.co.za**Magnetic field lines**The arrows show the direction a compass needle would point at that point in the field. Note that magnetic field is a vector quantity The closer the field lines are, the stronger the magnetic force felt**Moving charges (currents)**Moving charges (electric currents) also produce a magnetic field Conventional current – electrons flow in the opposite direction http://www.sciencebuddies.org**Magnetic field around a straight wire**Stronger field closer to wire**Magnetic field around a flat circular coil**http://physicsed.buffalostate.edu**The Motor Effect**When a current is placed in a magnetic field it will experience a force. This is called the motor effect.**The Motor Effect**The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule. Thumb = Motion First finger = Field direction Centre finger = Conventional Current**D.C.Motor**Commutator ensures that every half rotaion the current direction reverses in the coil**Defining Magnetic Field B**The size of the force on a wire in a field depends on the size of the field (B), the length of wire in the field (L) and the current in the wire (I)**Defining Magnetic Field B**In other words , F α BIL, or F = kBIL