Mains electricity and safety


Electrical charges

When two insulating materials are rubbed together, electrons are rubbed off one material and deposited on the other.

Electrons have a negative charge so the material that gains electrons becomes negatively charged and vice versa. This process is called charging by friction.

Two objects that have opposite electric charges will attract each other and two objects that have the same electric charges will repel each other. 

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Electric circuits

I = Q / t

I=current in amperes, A.
Q=charge in coulombs, C.
t=time in seconds, s.

[insert pic of symbols]

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The current through a component is measured with an anmeter. Anmeters are always placed in series with the component.

The potential difference across a component is measured with a voltmeter. Voltmeters are always placed in parallel with the component.

Resistance is the opposition to current flow. The unit of resistance is the ohm.

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Potential difference, work done and charge are related by this equation:

V = W / Q
V=PD in volts, V
W=work done in joules, J
Q=charge in coulombs, C

V = E / Q
E=energy transferred in joules, J

The resistance of a component is calculated using the equation:
R = V / I
R=resistance in ohms
V=PD in volts, V
I=current in amps, A

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Ohm's law

If a resistor is kept at a constant temperature, the current-potential difference graphs shows a straight line passing through the origin. This means the current is directly proportional to the potential difference (pd) across the resistor. This is known as Ohm's law. Any component that obeys Ohm's law is called an ohmic conductor.

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Current-potential difference graphs

The line on a current-potential different graph for a filament bulb is a curve. So the current is not directly proportional to the potential difference. 

The resistance of the filament increases as the current increases. This is because the resistance increases as the temperature increases.

The current through a diode flows in one direction only

The resistance of a light-dependent resistor decreases if the light intensity increases.

As the temperature goes up, the resistance of a thermistor goes down.

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Series circuits

- If there is a break in the circuit, charge stops flowing.

- The current is the same in each component.

- Adding the potential difference gives the total potential difference.

- Adding the resistances gives the total resistance.

I = V / R

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Parallel circuits

- If there is a break in one part of the circuit, charge can still flow in to other parts.

- The total current is the sum of the currents through the separate components.

- The bigger the resistance of a component the smaller the current is.

- Potential difference is the same across each component.

The current through a component can be calculated using 
I = V / R

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