Electromagnetic Induction

J.J. Thomson was the first to measure the charge-to-mass ration of electron

Mass SpectrometersMass spectrometry is an analytical technique that identifies the

chemical composition of a compound or sample based on the q/m of charged particles.

Area 1 of Mass Mass Spectrometers – The Velocity

Selector

• The sample need to travel STRAIGHT through the plates. The electric field and the magnetic field will apply a force in such a way as to CANCEL out the electric force caused by the electric field.

Example problem

• Protons passing without deflection through a magnetic filed of 0.60 T are balanced by a ( 4.5x103 N/C) electric field. What is the speed of the moving protons?

• Known variables: • B = 0.60 T • E = 4.5x103 N/C• v = ?

Protons passing without deflection through a magnetic filed of 0.60 T are balanced by a ( 4.5x103 N/C) electric field. What is the speed of the moving protons?

• Bqv = Eq

• v = Eq / Bq

• v = E/B • (4.5x103 N /C) / (0.60 T) = 7.5x103 m/s

M.S. – Area 2 – Detector Region

• After leaving region 1 in a straight line, it enters region 2, which ONLY has a magnetic field. This field causes the ion to move in a circle separating the ions separate by mass. This is also where the charge to mass ratio can then by calculated.

Electromagnetic Induction

• Wind pushes the blades of the turbine, spinning a shaft attached to magnets. The magnets spin around a conductive coil, inducing an electric current in the coil, and eventually feeding the electrical grid.

Can magnetic fields cause currents?

• we know that a current creates a magnetic field

• Can magnetic fields cause currents?

• Michael Faraday (1791–1862) and the American scientist Joseph Henry (1797–1878) independently demonstrated that magnetic fields can produce currents

Creating current-electromagnetic induction

• 2 ways to generate current –

• 1- conductor can move through a magnetic field

• 2- magnetic field can move past conductor

• Relative motion between between the wire and the magnetic field will produce the current (I).

The galvanometer is used to detect any current induced in the coil on the bottom

No current flows through the galvanometer when the switch remains closed or open

Current is Only detected during the instance when the switch is being closed or opened.

Electromotive Force – (NOT A FORCE)

• EMF - When a voltage is generated by a battery, or by the magnetic force according to Faraday's Law, this generated voltage has been traditionally called an "electromotive force” emf

• Note : Emf is not a force.

• .

EMF

• It is potential difference and is measured in volts (V ).

Moving a wire through a magnetic field

• A force is exerted on the charges

• Charges move in the direction of the force

• Work is then done on the charges

• (EPE) and electric potential(V) is increased

• The difference in potential(V) is called induced EMF

• EMF - measured in volts

• Depends on magnetic field B

• The length of the wire in the magnetic field L

• Velocity- v of the wire in the field

• EMF = BLv

Practice problem

• A strait wire .05m long, is moved straight up at a speed of 20 m/s through a 0.4 T magnetic field pointed in the horizontal direction.

• What is the EMF induced in the wire?

• The wire is part of a circuit of total resistance of 6.0Ω. What is the current in the circuit?

A strait wire .05m long, is moved straight up at a speed of 20 m/s through a 0.4 T magnetic field pointed in the horizontal

direction.

• Known:• B magnetic field = (0.4 T)

• L wire length = (0.4 T)

• v speed of wire = (20.0m/s)

• R resistance = 6.0Ω

• EMF (V) = ?• Current (I) = ?

• EMF = B L v

= (0.4 T) (0.05m) (20.0m/s)

=.4 V

I = V/R

= .4V/ 6.0Ω =.067A

magnetic flux, =BA

• Magnetic flux F is defined by F=BA

• B is the magnetic field

• A is the area perpendicular to the magnetic field

• Any change in magnetic flux (BA) induces an emf

• emf is directly proportional to the change in flux

• emf is greatest when the change in time• Δt is smallest—that is, emf is inversely

proportional to Δt

• coil has N turns, an emf will be produced that is N times greater than for a single coil, so that emf is directly proportional to N

• EMF = −N (ΔBA/ Δt)

• EMF = −N (ΔBA/ Δt)

Variations of Faraday's Law