Physics Topic 12: Electromagnetic Induction

A changing magnetic field induces an emf — current within a conductor. Such a current is called an induced current. Motion or change is required to induce an emf. If does not matter whether the magnet or the coil moves.

If the rod is made to move at a speed v, it travels a distance of ∆x = v∆t. Therefore the area of the loop increases by an amount ∆A = l ∆x = lv∆t.

•Magnetic Flux is the product of the flux density normal to the surface and the area of the surface. Φ=BAcosθ •Magnetic Flux Linkage is the magnetic flux multiplied by the number of coils. Φ=NBAcosθ

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Faraday’s Law The magnitude of the induced emf in a circuit is directly proportional to rate of change of magnetic flux or flux- linkage. Lenz’s Law The direction of the induced emf is such that the current it causes to flow opposes the change producing it.

•The emf is alternating or sinusoidal.

A generator consists of many loops of wire wound on an armature that can rotate in a magnetic field. The axle is turned by some mechanical means and an emf is induced in the coil.

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Rms values are used to find the average voltage as they are sinusoidal

Peak value = rms value x ,-2. V_peak = V_rms x,-2.I_peak = I_rms x ,-2.

A useful device that makes use of electromagnetic induction is the ac transformer as it can be used for increasing or decreasing ac voltages and currents.

Power loss in transmission line · Heat loss due to current · Resistance of the metal used · Dielectric losses · Self-inductance Power loss in real transformers · Eddy currents · Resistance of the wire in the winding · Hysteresis · Physical Vibration · Dielectric losses · Electromagnetic radiation · Flux leakage

The voltage is step-up from the power plant to have a higher power output due to power dissipating. The voltage is then step-down when the power has reached its consumer source (where the power is being consumed). There is no ideal valve of step-up voltage due to economic reason. From the power plant, a step-up transformer is used to increase voltage (so that current is decreased) from 11,000 V to 250,000 V. from the transmission line to city storage, a step-down transformer reduces voltage from 250,000 to 4500 V. then in front of a house, a step-down transformer reduces that to 200-240V.

A human body is a conducting medium, so when it is moving in an alternating magnetic field at extra-low-frequency, then electric field is induced, hence inducing current in human body.

Current experimental evidence suggests that low-frequency fields don’t harm genetic material. There has been evidence that this may lead to infant cancer and infant leukemia. These risks are likely to be dependent on current (density) frequency, and length of exposure.