Units 18, 19, and 20: Circuits, Motors, Generators, and Transformers

Many people have a hard time understanding these units, and I don't blame them.

There's a lot to learn, so lets get right into it.

Everyone knows the formula V=IR but don't understand what voltage and current actually are.

Voltage is the work done or energy transferred per unit charge: V = E/Q (unit for charge is coulomb, c)

Current is the number of charges transferred per unit time: I = Q/T

Note that the direction of the current is opposite to the direction of electron flow. In conventional current, current flows from the positive terminal to negative terminal.

The graph above is an ohmic resistor. (obeys ohms law)

Current is directly proportional to P.D.

However, a filament light bulb (and other components) do not obey ohms law: they are non-ohmic.

This is because at high currents, temperature increases, which increases the resistance. Therefore, current increases slower.

Power (W) is the rate at which energy is transferred.

P = E/T

It can also be defined as P = IV

which can be re-arranged to P = I^2*R

Therefore, energy transferred, E=IVt

All of these equations work because they are just re-arranged versions of each other, choose the best one for the question.

On to circuits. Here is a full list of circuit components you will have to know how to draw:

I'm sure everyone knows what a resistor is. Let's talk about some of the variations of the classic resistor:

Light-dependent resistor (LDR)

Light decreases resistance

Used in security lights and digital clocks

Thermistor

Resistance decreases when temperature increases

Used in thermometers

Resistance can be increased by using multiple resistors in a circuit. However, the result varies depending on how they are arranged. In series:

simply add up resistance

remember that the current is the same at all points in a series circuit.

the bigger the resistance, the bigger the PD.

In a parallel circuit, the effective resistance is less than the resistance of either resistor.

It can be calculated using:

where Rt is the total effective resistance.

The current in a parallel circuit is split between the parallel components.

total current = current 1 + current 2 + ...

A diode is a component that allows current to flow in one direction. the direction is shown by the direction of the arrow. A light emitting diode LED is a diode that emits a light when current flows through it. Used in screens. Power efficient.

There are also some safety concepts such as earth wires, but I will not be covering them here.

Now onto the part most of you are here for:

Motor Effect

When a current flows perpendicular to a magnetic field, a force is produced. The direction of the force is governed by the left hand rule.

The direction of the force can be reversed by reversing the direction of the current or reversing the direction of the magnetic field. A motor uses a commutator to make sure the current flows the same way so that the force acts in the same direction.

A use of the motor effect is in loudspeakers:

The alternating current passing through the coil is perpendicular to the magnetic field direction, causing the coil and cone to move up and down, causing vibrations in the air.

The turning effect (strength) of a motor can be increased by increasing number of turns in the coil, increasing current, increasing strength of magnetic field.

You can also apply the motor effect to electron deflection. Just remember that electrons travel in the opposite direction to conventional current.

Generator effect

a generator is the opposite of a motor.

Here is a summary of how this works:

The direction of induced emf can be found using the right hand rule. The direction of the induced voltage can reversed and its strength can be changed by changing the number of coils, direction/strength of magnetic field, speed of spinning, etc.

An AC generator uses slip rings.

To explain how AC current changes over time, say:

1. current produced will be max when the loop of wire cuts the magnetic field at 90˚

2. As the wire rotates, angle of wire cutting magnetic field decreases, current produced decreases

3. when movement of wire is parallel to magnetic field, no current is produced

4. current reverses as wire turns 180˚

5. process repeats to produce AC

In a DC generator, a commutator reverses the current therefore current production would remain in the same direction.

Full explanations of the left and right hand rules are in the 'formulae' section.

Lenz's law (modern dating)

When a magnet is moved towards an induced current, the induced emf/current will oppose this by repelling the current. But when the magnet moves away, it will oppose this change by attracting the magnet.

You can remember this law like a toxic relationship.

Transformers

oops, wrong transformer

this is what i'm talking about.

components: primary coil, secondary coil, soft iron core

step up transformer increases voltage, step down decreases voltage

number of coils and voltages can be expressed as Vp/Vs = Np/Ns

AC in the primary coil magnetises and induces a changing magnetic field in the soft iron core, which changing field lines cut the secondary coil, induces voltage and an AC current.

Step up Transformers are used to increase voltage from power plant to national grid, step down transformers are used to decrease voltage from national grid to consumers (for safety)

electricity is transmitted at a high voltage to reduce power loss, because the higher the current, the more power that is lost as heat.

That's it for now. It is important to note that I have omitted certain details to make this as concise and helpful as possible. If there is something I have not covered here, please leave a comment on the forums.