Electricity (P4 & P5)

Need to learn all the component circuit symbols...

• Switch (open)
• Switch (closed)
• Cell
• Battery
• Diode
• Resistor
• Variable Resistor
• LED (Light emitting diode)
• Bulb
• Fuse
• Voltmeter
• Ammeter
• Thermistor
• LDR (Light dependent resistor)

Electric charge and current.

Electric current is the flow of electrical charge - the greater the rate of flow, the higher the current. 

An ammeter is connected in series. A voltmeter is connected in parallel to the component.

Charge flow (C) = current (A) x time (S) 

As the current in a single, closed loop of a circuit has nowhere else to go, the current is the same at all points in the loop.

Resistance and Potential difference.

The resistance of a component is the measure of how it resists the flow of charge. The higher the resistance:

• The more difficult it is for charge to flow.
• The lower the current.

Potential difference tells us the difference in electrical potential from one point in a circuit to another - an electrical push. The bigger the potential difference across a component:

• The greater the flow of charge through the component.
• The bigger the current.

Potential difference (V) = current (A) x resistance (Ohms)

Increasing the resistance reduces the current & Increasing the voltage increases the current.

Resistors and other components.

Potential difference-current graphs:

• A straight line through the origin indicates that the voltage and current are directly proportional, i.e. the resisitance is constant.
• A steep gradient indicates low resistance, as a large current will flow for a small potential difference.
• A shallow gradient indicates a high resistance, as a large potential difference is needed to produce a small current.

Potential difference-current graphs show the relationship between voltage and current. It, therefore, can be used to determine the resistance. 

Resistors:

An ohmic conductor is a resistor in which the current is directly proportional to the potential difference at a constant temperature.

This means that the resistance remains constant as the current changes.

It is indicated by a linear graph. 

Filament Lamps:

As the current through a filament lamp increases, its temperature increases.

This causes the resistance to increase as the current increases.

It is indicated by a curved graph.

Diodes:

The current through a diode will only flow in one direction.

The diode has a very resistance in the reverse direction.

This is indicated by a horizontal line along the x-axis, which shows that no current flows. 

Thermistors:

The resistance of a thermistor decreases as the temperature increases.

This makes them useful in circuits where temperature control or response is required.

For example, a thermistor could be used in a circuit for a thermostat that turns a heater off at a particular temperature or an indicator light that turns on when a system is overheating.

Light Dependent Resistors.

The resistance of an LDR decreases as light intensity increase.

This makes them useful where automatic light control or detection is needed. E.g. in dusk till dawn garden lights/stret lights and in cameras/phones to dtermine if a flash is needed.

Series circuits.

There is the same current through each component.

The total potential difference of the power supply is shared between the components.

The total resistance of two components is the sum of the resistance of each component. This is because the current has to travel through each component in turn.

Adding resistors in series increases the total resistance in ohms.

Parallel circuits.

The potential difference across each component is the same.

The total current drawn from the power supply is the sum of the currents through the seperate components.

The total resistance of the two resistors is less than the resistance of the smallest individual resistor. This is because, in parallel, there are more paths for the current to take - it can take one or the other, allowing it to flow more easily.

Adding resistors parallel reduces the total resistance.

Power in circuits.

The power of a device depends on the potential difference across it and the current flowing through it.

A device with a higher potential differnece or current will use more energy per second that one with a lower potenital difference or current - i.e. it will be more powerful.

Power (v) = potential difference (A) x current (Ohms)

Power = (current)^2 x resistance

A direct supply:

Has a direct potential difference, i.e one that is always positive or always negative - this makes the current direction constant.

Is the type of current that is supplied by cells and batteries.

An alternating current supply:

Has an alternating potential difference, i.e. one that alternates from positive to negative - this makes the current direction alternative.

Is the type of current used in mains electricty.

Mains electricity.

• In the UK - 230v and changes direction 50 times a second, i.e. it has a frequency of 50Hz.
• Three core cables involved.
• Live wire (brown) - 230v potential.
• Neutral wire (blue) - at or close to 0v earth potential.
• Earth wire (green and yellow stripes) - 0v potential. (A safety wire, which stops the exterior of an appliance becoming live).

During operation:

• The potential difference causes current to flow thorugh the live and the neutral wires.
• The live wire carries the alternating potential from the supply.
• The neutral wire completes the circuit.
• Current will only flow in the earth wire if there is a fault connecting it to a non-zero potential.

Dangers of mains electricty.

Touching the live wire can create a large potential difference across the body and result in a large current flowing through the body. The live wire can be dangerous even if a switch in the circuit is open.

For example, a television might be switched off (so no current flows), but still plugged in and switched on at the wall:

• The live wire between the wall and the switch on the television is still at an alternating potential.
• All it needs is a path for the electricity to flow through.
• This path could be provided by a damaged cable exsposing the live wire.
• If someone then touches the live wire, creating a potential difference from the live to the earth and causing current to flow, they will get an electric shock.

Power and efficiency.

Power is the rate at which energy is transferred or work is done:

Power (W) = Energy transferred (J) / Time (S) or Power = Work done / Time

An energy transfer of 1J per second is equal to 1W of power.

If two kettles are used to bring the same amount of water to the boil and one takes less time, it is because it has a higher power.

Power and efficiency.

In an energy transfer, efficiency is the ratio of useful energy out to total energy in:

• An efficiency of 0.5 or 50% means that half the energy is useful, but half is wasted.
• An efficiency of 0.75 or 75% means that three-quarters of the energy is useful, but a quarter is wasted.

Efficiency = Useful energy transfer / Total energy transfer 

Efficiency = Useful power output / Total power input