Current electricity

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  • Created by: Chiara
  • Created on: 07-04-13 15:05

Electrical charges

When two electrically insulating materials are rubbed together, electrons are rubbed off one material and deposited on the other. The material that has gained electrons be omens negatively charged. The one that has lost electrons becomes positively charged. This is called charging by friction.

Two objects that have opposite charges attract each other, but two that have the same repel each other. The bigger the distance between the objects, the weaker the force between them.

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Electric circuits

This is how you find out the current:
I = Q I = current (A)
T Q = Charge (C)
T = time (s)

In a circuit there can be many components, that all have different jobs. These include:

  • a cell - a heater
  • a switch - a voltmeter
  • an indicator
  • a diode
  • a light emitting diode
  • an ammeter
  • a fixed resistor
  • a variable resistor
  • a fuse
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The current through a component is measured with an ammeter. They are placed in a series with a component. Potential difference across a component is measured with a voltmeter. They are place in parallel with the component.
This is the equation to work out potential difference:
V = W V = potential difference (v) Q = charge (c)
Q W = work done (j) or it can be energy transferred (e) as they are equal.
This is the equation to work out resistance:
R = V R = resistance (ohms) I = current (a)
I V = potential difference (v)
Ohms law
If a resistor is kept at a constant temperature, the current-potential difference graph shows a straight line, passing through the origin. So the current is directly proportional to the potential difference across the resistor. This is ohms law. Any component that obeys ohms laws is called an ohmic conductor.

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More current-potential difference graphs

The line on a current-potential difference graph for a filament bulb is a curve. So the current is not directly proportional to the potential difference. The resistance of a filament bulb increases as the current increases. This is because the resistance increases as the temperature increases. Reversing the potential difference makes no difference to the shape of the curve.

The current through a diode flows in one direction only. In the reverse direction the diode has a very high resistance so the current is zero.

As the light falling on it gets bigger the resistance of a light-dependent resistor decreases.

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

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Series circuit

In a series circuit the components are connected one after another. So if there is a break anywhere in the circuit, the charge stops flowing. There is only one way that the. Charge can flow, so the current going through each component is the same.

The current depends on the potential difference of the supply, and the total resistance:
I = V

The potential difference of the supply is shared between all the components. So the pd of each component add up to give the total pd of the supply. The bigger the resistance of a component, the bigger its share of the supply pd.

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Parallel circuits

In a parallel circuit each component is connected across the supply, so if there is a break in one part, the charge can still flow in the other parts. The pd across each component is the same.

There are junctions in the circuit so different amount of charge can flow through different components. The current through each component depends on its resistance. The bigger the resistance of a component, the smaller the current through it.

The total current thorough the whole circuit is equal to the sum of the currents through the separate components.

In everyday life, parallel circuits are much more useful than series circuits. That is because a break in one part of the circuit does not not stop charge flowing in the rest of the circuit.

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