Electric Fields Unit 5: Module 1: Electric and Magnetic Fields What is a field? A field is a region in which a force is exerted on an object at a distance There is an electric field around any charged object Like charges repel each other (+ve and +ve or –ve and –ve) Unlike charges attract each other (+ve and –ve) If a charged object is placed in an electric field it will experience a force

Coulomb’s Law In 1785 French physicist Charles Coulomb carried out a series of experiments and discovered that the force F between two charges was: Directly proportional to each charge Inversely proportional to the square of their separation The force of attraction between two point charges can be calculated using Coulomb’s Law: If charges are opposite, then the force is attractive and F will be negative. If charges are like charges, then the force is repulsive and F will be positive

Electric Field Strength Electric field strength, E is defined as the force per unit positive charge, (the force that +1C would experience if it was placed in an electric field). E = F/Q units of E = NC-1 E is a vector quantity (points in the direction that a +ve charge would move) For Radial fields Combining the above equation and Coulomb’s Law gives the Electric field strength around a radial field: E = kQ/r2 +

Uniform Electric Field For a uniform field the field strength is the same at all points. E = V / d Units of E = Vm-1 Charged particles follow a curved path in a uniform electric field. Just like a thrown ball – the horizontal velocity remains constant but the vertical velocity is altered

Magnetic Fields Starter questions Draw the magnetic field around a bar magnet Draw the magnetic field between and around two north poles brought close together. Draw the magnetic field between and around a north and a south pole brought close together. Draw the magnetic field through and around a solenoid (coil of wire) Use the right hand grip rule to draw the magnetic filed around a straight current carrying wire if the current is flowing for top to bottom. From these diagrams, what tells us if a magnetic field is strong or weak? What happens to a current carrying wire if it is placed in a magnetic field? What do you know about Fleming's left hand rule? What is magnetic flux and magnetic flux density?

Magnetic Fields Magnetic field lines can be represented by field lines. The field lines go from north to south and the closer together they are the stronger the field.

Current Carrying Wire When current flows through a wire, a magnetic field is created around the wire. This magnetic field forms concentric circles around the wire. The closer the circles are together, the stronger the field. To identify the direction of the magnetic field around a wire we use the RIGHT HAND RULE

Solenoid Magnetic Field If a current carrying wire is wrapped around to make a solenoid then the magnetic field is the same as that for a bar magnet.

Looking Through a Solenoid

Magnetic Field directions

Flemings Left Hand Rule If a wire carrying a current is placed in a magnetic field, the wire experiences a force on it. If the wire is at right angles to the magnetic field then it receives a maximum force. If the wire is parallel with the magnetic field then it receives no force. The direction of the force is identified by using Flemings left hand rule.

Let's practice!

Calculating the Force F = BIL sin θ The size of the force on the wire can be calculated by: F = BIL sin θ Where F = force on wire (N), B = magnetic field strength (T), I = current (A), L = length of wire(m) and θ = angle between magnetic field lines and wire. If wire and magnetic field are at right angles sin θ = 1 TASK : Using F = BIL derive a word definition for the Tesla

Definitions One Tesla, T is the magnetic flux density when a wire of length one metre and carrying a current of one ampere at a right angle to the field experiences a force of one newton. Magnetic Flux Density,B is defined by the equation F = BIL sin θ: Where F = force on wire of length L carrying a current I at an angle θ to the field. Magnetic field strength is also called Magnetic Flux Density It is a vector as it has magnitude and DIRECTION http://www.magnet.fsu.edu/

Practice Questions 1) A 4cm length of wire carrying a current of 3A runs perpendicular to a magnetic field of strength 2 x 10-5 T. a) calculate the force on the wire b) If the wire is rotated so that it is at 30 degrees to the direction of the field - what would the size of the force be? 2) Two wires are placed in a uniform magnetic field of 0.25T. Both wires are 0.5 m long and carry a current of 4A. One wire is at right angles to the direction to the magnetic field and the other is at an angle of 40 degrees. Calculate the size of the force on each wire.

Charged Particles in a Magnetic Field Current is the flow of negatively charged particles, therefore if a current carrying wire experiences a force in a magnetic field so to will charged particles. This force can be calculated by: F =BQv Where F = force (N), B = magnetic field strength (T), Q = charge (C), v = velocity (m/s) The force on a moving charge in a magnetic field is a circular path. BQv = mv2/r (centripetal force)

Practice Exam Questions Qu 4 on page 109 of course text book

Electromagnets 1) List things that you can think of that use electromagnets 2) What is the basis of an electromagnet? 3) How can the strength of an electromagnetic be increased?

What is Electromagnetic Induction   Whenever an electric current flows through a conductor, a magnetic field is created. When electrons are in motion, they produce a magnetic field. The reverse is also true i.e. when a magnetic field moves relative to a conductor, it produces  a flow of electrons in the conductor. This phenomenon whereby an e.m.f.  and hence current is induced in any conductor which is cut across or is cut by a magnetic flux is known as ELECTROMAGNETIC INDUCTION http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr/electric_circuits/mainsproducedrev1.shtml

Why does electromagnetic induction occur?

Magnetic Flux Ф = NO = BAN Magnetic flux tells us how much magnetic field has been 'cut. It depends on the strength of the magnetic field B (magnetic flux density) and the area A Therefore total magnetic flux is calculated by: O = BA Where A = area, m2, the unit of O = weber If the magnetic field is not perpendicular to B then the magnetic flux can be found by: O = BA cos θ where θ is the angle between the field and the normal Flux Linkage Ф is calculated by multiplying the magnetic flux by the number of turns of the coil. Ф = NO = BAN

Definitions Magnetic flux through an area A is defined as the product of the magnetic field density B and the projection of area A onto a surface at right angles to the flux. O = BA cos θ The unit of magnetic flux is the weber. One weber (Wb) is the magnetic flux when a magnetic field of magnetic flux density one tesla passes at right angles through an area of one square meter

Questions How much flux is linking a 200 turn coil of area 0.1m2, when it is placed perpendicular to the magnetic field of flux density 2.5 x 10-3 T? A coil of 100 turns and area 0.2m2 is at 90 degrees to a flux density which decreases from 0.5T to 0.2T. What is the change in flux linkage of the coil?

EM Induction Laws Faraday’s Law concentrates on the EMF produced during electromagnetic induction. FARADAY’S LAW of electromagnetic induction states that the magnitude of the induced e.m.f. is equal to the rate at which magnetic flux linkage is cut. Lenz’s Law concentrates on the direction of the induced current. LENZ’S LAW of electromagnetic induction states that the direction of any induced current is in the direction that opposes the flux change that causes it.

Induced e.m.f. = - rate of change of magnetic flux linkage EM Induction Equation Combining Faraday’s and Lenz’s law gives us the following equation: E = - Δ Ф / t Where E is the induced e.m.f, Δ Ф is the change in flux and t is the time interval Or in words: Induced e.m.f. = - rate of change of magnetic flux linkage

Questions A coil of wire has 5000 turns and an area of 1.0 x 10-4m2. It is placed in a long current carrying wire, so that it's face is perpendicular to the lines of magnetic flux inside the coil. What e.m.f is induced across the coil when the flux density inside it changes from 2.5x10-3T to 1.3 x 10 -3 T in 0.4 s A coil of wire has 2500 turns and an area of 1.5 x 10-4m2. It is placed between the poles of a horseshoe magnet as the coil is rapidly pulled out of the field in 0.3 s, the average value of the e.m.f induced in the coil is 0.75V. Calculate the flux density between the poles of the magnet.

AC Generators When a wire is tuned in a magnetic field a current is induced. One side of the wire goes down whilst the other goes up. This means that every half turn means the induced current changes direction - AC is produced. http://www.walter-fendt.de/ph14e/generator_e.htm

Transformers Vp/Vs = Np/Ns Transformers are used to step up or step down voltage. They only work on AC because an alternating current in the primary coil causes a constantly alternating magnetic field. This will “induce” an alternating current in the secondary coil. Vp/Vs = Np/Ns