Electricity - Paper 1 - Section 2

What is current?

The net rate of flow of charge.

* Electrons carry the charge.
* Current flows from the positive to the negative terminal.
* Current is the same in series; splits in parallel.
* 1 Ampere = when 1 Coulomb (C) moves past a point in 1 second.

What is voltage?

Voltage is what drives the current round the circuit.

It is a measure of how much energy is lost by electrons carrying that energy between two points.

It is the energy transferred per unit charge passed.

What is resistance?

Anything that opposes the flow of current.

* Adding more component to a circuit gives a higher resistance.
* If you increase resistance, more voltage will be needed to keep the same current flowing.

What happens as you vary the variable resistor?

It alters the current flowing through the circuit.

How do light-emitting diodes work?

* When current (electrons) flow through them in the forward direction, they emit light.
* They are used in digital clocks, traffic lights, and remote controls.
* Unlike a bulb, they don’t have a filament that can burn out. They are more energy efficient as they don’t create heat.

How do light dependant resistors (LDRs) work?

LDR’s resistance increases as light intensity decreases.

* With
* more light (daytime), the LDR’s resistance decreases, having the path of least resistance, so the current flows to the LDR instead of the bulb.
* With less light (night time), the LDR’s resistance increases, having a path of greater resistance, so the current flows to the bulb instead, lighting it.

How do thermistors work?

Thermistors are temperature-dependant resistors. They make useful temperature detectors e.g. car engine temperature sensors, thermostats, and fire alarms.

* In hot conditions, the resistance drops.
* In cool conditions, the resistance increases.

This is because increasing temperature frees more electrons to carry the current, so resistance falls.

Discuss series circuits

* If you remove/disconnect a component, the circuit is broken and they all stop working.
* They are only in a few practical devices, e.g. fairy lights.
* Current is the same everywhere
* Voltage is shared between the components, and the voltage for each component depends on its resistance.
* The total resistance is the sum of the resistance of each component in the circuit.

Discuss parallel circuits.

* Each component is separately connected to the positive and negative terminals of the supply (except ammeters).
* Removing/disconnecting one component will hardly affect the others at all.
* Current the shared between branches. The total current = the total of all the currents through the separate components.
* The current through a branch depends on the resistance of the branch - the higher the resistance, the harder it is for charge to flow, so the current is lower in that branch.
* Voltage is the same across all the branches.
* The total resistance of the circuit decreases if you add a second resistor in parallel. The resistance is the sum of the reciprocal of each individual resistor. The overall resistance is less than the resistance of any individual branch.

What does the live wire do?

It alternates between a high positive and negative voltage of about 230V.

What is the voltage of the neutral wire?

Always 0V

Which two wires does the electricity normally flow in?

The live and neutral wire.

* The earth wire and fuse (or circuit breaker) are just for safety and work together.

What does ‘earthing’ mean?

The case must be attached to an earth wire. An earthed conductor can never become live because the current flows to the path of least resistance, which is into the ground.

Why do devices with double insulation not need an earth wire?

Because the plastic around them is an insulator, so you cannot get a shock by touching it.

Discuss fuses

* If a fault develops in which the live wire touches the metal case, then, because the case is earthed, a big current flows through the live wire, the case, and into the earth wire into the ground.
* This surge draws a large current through the live wire, which melts fuse, which cuts of the live supply.
* The isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of a fire caused by the heating effect of a large current.

Discuss RCCBs

* Usually the neutral and live wire carrying the same amount of current.
* However, if someone touches the live wire, a current will flow through the to the earth, meaning the live wire carries more current than the neutral wire.
* The RCCB detects this difference in current and opens a switch, cutting off the power.

What is a problem sometimes caused by the heating effect of a resistor when current passes through them?

The heating effect can cause components in the circuit to melt - which means the circuit will stop working, or not work properly.

Explain two ways in which the heating effect of a resistor when a current passes through them is used.

* Fuses use this to protect circuits - they melt and and break the circuit if the current gets too high.
* Toasters contain a coil of wire with a very high resistance. When a current passes through the coil, its temperature increases so much that it glows and gives off infrared radiation (heat), which cooks the bread.

What does an appliance with a high power rating do?

It transfers a lot of energy in a short amount of time.

What should the current rating of fuses be?

As near as possible but just higher than the normal operating current.