electricity

what happens when the electrons build up on a plate meaning it will become more negative

- plate becomes less attractive so the flow of electrons (ie current) reduces
- electrons on the 2nd plate are repelled from the negative plate
- As the charge builds up so does the pd across the plates. Charge stored is directly proportional to the pd across the plates

How to work out the time constant

increase/decrease whatever is remaining by 63.2%

Faraday’s Law

the induced emf across the conductor is directly proportional to the rate of change of magnetic flux

Lenz’s law

the direction of the induced current is such that the magnetic field of the induced current opposes the change creating it

Magnetic flux

the amount of magnetic field that you have swept through (but not just area but area x field strength) - weber wb

Circuit turning on - voltage across the inductor

as the current increases the mag. field strength builds up. FL says that an emf will be induced as there is changing mag field. LL says that the induced emf opposes the change creating a back emf. Therefore voltage across bulb builds less quickly

circuit turning on - current through an inductor

The back emf creates a back current which reduces the forward current. As the forward current stablises, the back current reduces until the current is maximum

circuit turning on - brightness of the bulb

starts off, slowly gets brighter

circuit turning off - voltage across the inductor

The current drops, so the mag field gets weaker. FL says a changing mag field induces an emf. LL says the induced emf opposes the change - tries to maintain the voltage across the inductor.

circuit turning off - current through the inductor

the back emf creates a back current which opposes the change ie keeps the current flowing until the energy across the inductor had dissipated

circuit turning off - brightness of the bulb

starts on, gets dimmer more slowly

what is back emf proportionate to?

change in I/change in t

what is AC current produced by

generators

purpose of AC current

can be transferred over larger distances with the help of transformers

what is root mean squared

the value of an alternating current that would produce the same amount of power as the value of the direct current

how does voltage change in ac

sinusoidally. In the shape of a sin wave

resistors in AC

• Vr and I are in phase

• Vc is 90 degrees behind V and Vr

• The phase angle is how far ahead/behind the supply voltage is compared to Vr (and I)

reactance - what happens if frequency increases

as f increases the current changes direction more frequently so the capacitor accumulates less charge (and voltage) and opposes the flow of charge less (Xc decreases)

reactance - what happens if capacitance increases

The capacitor can store more charge per volt and the current can flow for longer before the VC is large enough to oppose the current (Xc decreases)

Impedence

- overall opposition to the flow of current
- the vector sum of the reactance and the resistance of the resistor

effect of capacitor in AC bulb

a bulb will stay bright. the changing direction of the current is constantly charging and discharging the capacitor so it doesn't get a large enough charge to reduce the current

formula for calculating current through the component

I = Vc/Xc

when is the voltage on a capacitor peak

Vr = O (Vc = IXc)

when is the voltage on a resistor peak

Vc = 0 (Vr = IR)

how do you calculate Vpeak and why

Vsupply is always changing and the vector sum of Vr and Vc is too. So to calculate Vpeak use the 2 max values of Vc and Vr and pythag

LR circuit explanation of voltage

a voltage is induced across an inductor whenever the current changes (FL). In AC circuits the current is constantly changing this means there will be a back emf opposing the current

current - ahead or behind? (inductor)

Vl is 90 degrees ahead of I and Vr

what does an increased frequency of supply voltage result in

the greater the frequency of the supply voltage than the greater the change in I/change in t is. As the back emf opposes current XL increases as F increases

what happens when the inductance of the inductor is increased

the greater the inductance of the inductor, the greater the back emf ie opposition to current so L increase XL increases

resonance

Xc = Xl and the only impedence is due to the resistance of the resistor
F^o is the frequency at resonance
Z = R and the largest possible current flows

what is a transformer

A device that will increase or decrease the voltage of an AC circuit

Equation for transformer and effect of I on V

Np/Ns = Vp/Vs. If I increaases voltage decreases