# Physics 2.4

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• Created by: Laura
• Created on: 15-01-13 19:10

## What do oscilloscope traces show?

Oscilloscope traces show the waveform of electricty, or what is happening in a circuit. Effectively they are potential difference-time graphs.

For direct currents (d.c) the oscilloscope trace is just a constant horizontal line, so even over time the voltage of a direct current remains constant.

For alternating currents (a.c) the oscilloscope trace goes up and down in a wave, meaning that the potential difference goes from positive to negative.

From oscilloscope traces you can tell the potential difference or peak potential difference as this is where the line is on the y axis. For d.c this is obvious as the line is constant so where the line crosses the y axis is the potential difference. For a.c you trace across where the peak of the wave is along the y axis.

Direct Current                                Alternating Current

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## Period and frequency from an oscilloscope trace?

Period:
The period of an AC supply is the time taken for one complete oscillation. You can find this by looking at the time between one peak and the next, between one trough and the next, or between any two identical places on adjacent oscillations. Remember to check and take into account the scale of the oscilloscope.

Frequency:
The frequency of an AC supply is the number of oscillations per second, and it is measured in hertz, HZ. You can find the frequency by using Frequency =      1
Period (in seconds)
For example to work out the frequency of the UK mains supply you turn its period of 20ms into seconds by dividing it by 1000, so you end up with 0.02s, and divide 1 by this period.
1 / 0.02s = 50Hz

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## What is the frequency and p.d of the UK mains?

The frequency of the UK mains supply is 50Hz (hertz) (50 oscillations per second).

The potential difference (p.d) of the UK mains supply is 230V (volts).

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## What is found inside a plug?

• The earth wire which earths any current if the case becomes live due to a fault, as it has the lowest resistace.
• The live wire which takes the current to the appliance.
• The neutral wire which takes the current away from the appliance.
• The fuse which is between the live wire and live terminal, which will break if too much current flows.
• The cabel grip which secures the cabel to the plug, and should cover the collective wire, not the three seperate ones.
• The inner core of the wires is made from copper as it is an excellent conductor.
• The outer core of the wires is made from flexible plastic as it is a good insulator.
• The pins/terminals are made from brass as its a good conductor
• The casing is made from plastic or other good insulators
• Wires either have a two-core or three-core wire, the three-core carries the earth wire.
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## How does a fuse and circuit breaker work?

Fuse:
Fuses protect electrical appliances if a fault causes too much current to flow into the appliance. This is because in a fuse is a thin piece of wire which has a low melting point, if too much current is passed through the wire it will melt, therefore breaking the circuit and stoppoing the current. This protects both the wiring and the appliance from damage from the too high current. You are advised to have a fuse which will break at a slightly higher current than your appliance needs, for example if you have a 3A appliance you need a 5A fuse.

Circuit Breaker:
The circuit breaker does the same job as the fuse, but works in a different way. A spring-loaded push switch is held in the closed position by a spring-loaded soft iron bolt. An electromagnet is arranged so that it can pull the bolt away from the switch. If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the push switch into the open position, therefore breaking the circuit and stopping the current. Residual current circuit breakers (RCCB) protects some circuits by recognising a difference in current between the live wire and neutral wire which shouldn't occur as the current should be constant, it then breaks the circuit stopping the current. They work faster than fuses and this can help protect the user and appliance.

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## How dothe earth wire and fuse protect the circuit?

If a fault occurs in the circuit causing too much current to flow and the case to become live the earth wire and fuse will protect the circuit, any users and appliance. This is because when a fault occurs...

1. The case will become live and dangerous.
2. The current which makes the case live will "flow to earth" through the earth wire as this offers the least resistance.
3. The overload of the current will cause the fuse to melt (or circuit breaker to trip), which will break the circuit and stop any more current.
4. The appliance and user are then safe from the electrical current.

So the earth wire earths (when a current reaches earth it is safe as it disperses) any excess electrical current which causes the case to become live, as out of the three wires it offers the lowest restistance and therefore the current will flow through it as it "easiest".

And the fuse will break the circuit stopping any more current from flowing, as the increased current will melt the thin wire in the fuse, therefore breaking the circuit.

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## What happens to a resistor with charge?

When an electrical charge (current) flows through a resistor, which could be anything that adds resistance e.g a bulb or appliance, the resistor gets hot. Some of the electrical energy is used but most is lost as heat energy, for example only 5% of energy is turned into light in a filament bulb the rest is lost as heat energy.

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## What is the equation for power?

Power = Work Done               P = E                        E
Time Taken                      t                      P | t

Power is a mixture between current and potential-difference (think speed and strength).
Work done is the energy transfered.
Time taken is the time over which the appliance was on.

Power is measured in watts, W.
Work done is measured in joules, J.
Time taken is measured in seconds, s.

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## How do you use the equation P = I x V?

Power = Current x P.D      Current = Power       Potential Difference =  Power              P
P.D                                             Current           I | V

Power = P
Current = I
Potential Difference = V

Current is the flow of electrons around a circuit.
Power is a mixture between current and potential difference (speed and power).
Potential diference is the push of the electrons.

Current is measured in amps, A.
Power is measured in watts, W.
Potential Difference is measured in volts, V.

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## How do you use the equation E = V x Q?

Energy = P.D x Charge     Potential Difference = Energy    Charge = Energy            E
Charge                      P.D             V | Q

E is energy.
V is potential difference.
Q is charge.

Energy is the energy transfered in joules, J.
Potential difference is the push in volts, V.
Charge is the flow of electrons in coulombs, C.

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