Electromagnetic Induction and Electromagnetic Waves!

Chapter 20

Emf

Recall that Emf stands for “electromotive force” which is a voltage (potential difference) capable of creating an electrical current.

Emf is measured in Volts!

Magnetic Flux Magnetic flux is the

magnetic field that passes through a given area.

Magnetic flux is similar to ‘magnetic field density’ – it gives the number of magnetic field lines passing through a given area.

Magnetic flux depends on the magnetic field and the area of the wire loop.

Magnetic Flux

Flux can be increased by increasing the density of magnetic field lines, or by increasing the area.

Φ = BAcos θ [Tm2] = [Weber] = [Wb]

Changing Magnetic Flux Shape of area

doesn’t matter. Orientation does

matter.

Changing Magnetic Flux Motion of the

magnet changes flux

Also motion of the wire changes flux

Faraday’s Law / Lenz’s Law A current (voltage)

is induced in a wire when magnetic flux is changing! (Faraday)

The current induced opposes the change in magnetic flux! (Lenz)

Faraday’s Law for Induced Emf Faraday’s Law:

Emf = -N(ΔΦ/Δt)

Where N gives the number of turns in a wire loop and

Φ = BAcos θ

Example

The south end of a bar magnet is pulled far away from a small wire coil. Looking from behind the coil towards the south end of the magnet, what is the direction of induced current? Clockwise or counterclockwise?

Example In rural areas where electric power lines

carry electricity to big cities, it is possible to generate small electric currents by means of induction in a conducting loop. The overhead high tension power lines carry currents that periodically reverse direction 60 times per second (60 Hz). How would you orient the plane of a conducting loop to maximize induced current if current in the power lines run north-south.

Example: Electrical instruments can be damaged if

they are in a rapidly changing magnetic field. If the induced currents are large enough, they could damage the instrument. Consider a computer speaker that is near an electromagnet. Suppose the electromagnet reverses direction every (1/120) s and exposes the speaker to a magnetic field of 1.0 mT. Assume the speaker coil consists of 100 circular loops of radius 3.00cm with a resistance of 1.00 Ω. Calculate the magnitude of the average induced Emf. Will it damage the speaker?

Homework

Read Sections 20.1 and 20.1

Do # 2 – 6, 9, 11- 13, 16- 18, 20, 21, 24, 28

Example An external force does work

to move the bar; the work is converted to electrical energy. Since there is a magnetic field present, flux changes.

What is the direction of induced current in the resistor?

If the bar is 20cm long and pulled at a steady speed of 10 cm/s, what is the induce current if B = 0.25 T and R = 5.0 Ω

Electric Generator A generator is a device

that converts mechanical energy to electrical energy.

Φ = BAcos θ

Emf = -N(ΔΦ/Δt)

For uniform circular motion, θ = ωt

So Φ = BAcos ωt

Electric Generator Φ = BAcosωt

Emf = -N(ΔΦ/Δt)

= -N(ΔBAcosωt)/Δt

In the generator, B and A are constant

Emf = -NBA(Δcosωt/Δt)

Emf = (NBAω) sinωt

Recall ω = 2πf

Generator If natural processes

can create a turning motion, then current can be generated from the turning motion!

Motor A motor is a device that

converts electrical energy into mechanical energy.

Current sets up a magnetic field in the coil; the magnetic field aligns with the magnet

Brushes then reverse the current and the coil rotates to align again.

The process continues because every half turn, the brushes switch the current.

Electromagnetic Waves Vibrating electric

charges create a varying electric field and a varying magnetic field.

This variation in electric and magnetic fields is known as an electromagnetic wave.

Consider an electron moving up and down in an antenna…

Electromagnetic Waves Electromagnetic waves are varying electric

and magnetic fields. The travel in vacuum at the speed of light,

and have a characteristic frequency (and wavelength).

The frequency of the wave depends on the frequency of vibration of the oscillating charge.

Electromagnetic waves transfer energy and have no mass!

Electromagnetic Spectrum

Homework

Read and record example 20.5 on page 665.

Read carefully pages 672 – 677.

Do # 32, 35, 42, 43, 44, 70 – 73, 78, 79, 82, 84 page 684