# Physics Unit 2.3 - Currents in Electrical Circuits

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What happens when certain insulating materials are rubbed against each other?
They become electrically charged. Negatively charged electrons are rubbed off one material and onto the other.The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.
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What happens when two electrically charged objects are brought together?
They exert a force on each other.
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What is an electric current?
Electric current is a flow of electric charge. The size of the electric current is the rate of flow of electric charge.
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How do you calculate the potential difference (voltage) between two points in an electric circuit?
The work done (energy transferred) per coulomb of charge that passes between the points.
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How do LDRs work? (burgular alarms)
In the dark and at low light levels, the resistance of an LDR is high, and little current can flow through it. In bright light, the resistance of an LDR is low, and more current can flow through it.
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How does a thermister work? (fires)
At low temperatures, the resistance of a thermistor is high, and little current can flow through them. At high temperatures, the resistance of a thermistor is low, and more current can flow through them.
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How is a LDR useful?
For controlling how long the shutter should remain open on a digital camera. Changes in the resistance are measured and, if the light level is low, the shutter stays open for longer than if the light level is high.
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Why are potential-difference graphs used?
To show how the current through a component varies with the potential difference across it.
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How can the resistance of a componment be measured?
By measuring the current through, and potential difference across, the component.
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The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor.
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The current through a component depends on its resistance.
The greater the resistance the smaller the current for a given potential difference across the component.
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The potential difference provided by cells connected in series is the sum of the potential difference of each cell (depending on the direction in which they are connected).
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For componments connected in a series ...
The total resistance is the sum of the resistance of each component. There is the same current through each component the total potential difference of the supply is shared between the components.
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For components connected in parallel ...
The potential difference across each component is the same. The total current through the whole circuit is the sum of the currents through the separate components.
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The resistance of a filament bulb increases as the temperature of the filament increases.
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Describe the current in a diode.
It can only flow one way. The diode has a very high resistance in the reverse direction.
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What are ammeters?
Ammeters are placed in series with the component to measure the current through the component in amps/ amperes/A.
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What are voltmeters?
Voltmeters are always placed in parallel to a component to measure the pd across a component in volts.
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What is ohms law?
When the current is directly proportional to the voltage across a resistor. The component obeys ohms law.
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What is a series circuit?
Components are connected one after another, so if there’s a break anywhere in the circuit the charge stops flowing.
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What is a parallel circuit?
Each component is connected across the circuit-if break in 1 part of circuit, charge can still flow in other parts.
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## Other cards in this set

### Card 2

#### Front

What happens when two electrically charged objects are brought together?

#### Back

They exert a force on each other.

### Card 3

#### Front

What is an electric current?

### Card 4

#### Front

How do you calculate the potential difference (voltage) between two points in an electric circuit?

### Card 5

#### Front

How do LDRs work? (burgular alarms)