Electricity

1
Electricity Magnetism

• Seb Oliver
• Lecture 3 Vectors, Forces, and Units

2
Summary Lecture 1

• Equipment and Techniques
• Electroscope Measures charge
• Earthing Earth as a limitless supply or sink of
  charge
• Van de Graaff Generator Generates charge
• Introduced the concept of charge

• Properties of charge
• Two types ve -ve
• Like repel
• Opposites attract
• Charge always conserved
• Charge quantised
• Methods of charging
• Friction
• Conduction
• Induction

3
Summary Lecture 2

• Why study electromagnetism?
• It is fundamental, in abstraction and practically
• Model of atom has electrons orbiting nucleus
• Charge is carried by electrons

• Properties of matter
• Conductors
• non-conductors
• semi-conductors

4
Vectors, Forces Units

• Much of this may be revision all this material
  will come from other courses
• Vectors
• Maths course
• CD-ROM Module 2
• Forces
• Mechanics course
• CD-ROM Module 4 (3)
• Units

5
Vectors
6
Scalars Vectors

• A scalar is a number
• it may have units e.g. kg (it may not)
• it does not have any direction
• it may have a sign ve or -ve
• it may be real, or integer
• A vector has both
• magnitude/size
• direction

7
Ways to add vectors

• Graphically
• Using components

Add vectors movie
Sub. vectors movie
Sub. vectors movie
Add vectors movie
8
Vector Summation
B (parallel)
Note B has direction and size, but starting
point is arbitrary
9
Two special Triangles45º Triangle
b
a
45º
a
10
Two special TrianglesEquilateral
Triangle30º,60º
2a
2a
30º
c
60º
ba
11
Unit Vectors
Unit vectors have length 1, i is in direction of
x-axis, j is in derection of y-axis and k is in
direction of z-axis
A nice way to think about unit vectors is as the
equivalent as a or sign for scalars. Any
vector is the product of a magnitude and a unit
vector. Just as any scalar is a magnitude with a
sign. The unit vector supplies the direction
information
y-axis
x-axis
12
Vector Components
Unit vectors have length 1, i is in direction of
x-axis, j is in derection of y-axis and k is in
direction of z-axis
y-axis
A
x-axis
13
Vector Summation
14
Forces
15
Revision of Force

• Newton's Laws
• 1. In the absence of external forces, an object
  at rest remains at rest and an object in motion
  continues in motion with a constant velocity
  (i.e. A constant speed in a straight line)
• 2. The acceleration of an object is directly
  proportional to the net force acting on it and
  inversely proportional to the mass
• 3. If two objects interact, the force exerted by
  object 1 on object 2 is equal in magnitude and
  opposite in direction to the force exerted by
  object 2 on object 1

16
Revision of Force

• Newton's Laws
• 1. if F 0, a 0
• 2. F m a (N.B. F, a are vectors)
• 3. F12-F21

17
Motion in a circle

• Velocity is a vector it has direction
• Object travelling in a circle at constant speed
  is continuously changing direction
• Therefore it is accelerating
• Thus requires a force

18
Force of Gravity
19
Units
20
SI Units in Forces Motion

• Displacment, Unit m, Vector
• Time, Unit s, Scalar
• Velocity, Unit ms-1, Vector
• Acceleration, Unit ms-2, Vector
• Mass, Unit kg, Scalar

21
Units in Forces Motion

• Force
• From Newton's third law we can derive a unit for
  Force
• F m a
• F kg ms-2
• SI Unit of force Newton N is kg ms-2

22
Summary Lecture 3

• Newtons Laws of motion
• 1. if F 0, a 0
• 2. F m a (N.B. F, a are vectors)
• 3. F12-F21
• Vectors Vector addition
• Units