2 Electrical resistance Flashcards
What is resistance?
It is the difficulty that current experiences in a circuit.
What do we describe as having very low resistance?
Connecting wires.
What is low resistance?
Low resistance is when there is nothing in opposition to the current flow. This means that it allows more current to flow through it.
What is high resistance?
It is when the flow of charge is no tso easy and a large amount of energy is used to move the charges through them. This energy is transferred, usually as heat. Components like these are said to have high resistance.
What do we describe as having high resistance?
Rubber.
What is the equation for Voltage?
V (Voltage volts) =
I (Current Amps) x R ( Resistance ohms)
What are the steps that we follow in the experiment for investigating how current varies with voltage for different components?
1) Set up the circuit (shown on next card).
2) Turn the variable resistor to its maximum value.
3) Close the switch and take the reading from the ammeter and voltmeter.
4) Alter the value of the variable resistor again and take a new pair of readings from the meters.
5) Repeat the whole process at least six times.
6) Place the results in a table, and draw a graph of current (I) against voltage (V).
What is the circuit that we use in the experiment regarding how current varies with voltage for different components?
What should the graph of current against voltage look like?
What does the slope reflect?
It shows the resistance of the wire. If it has a steeper slope, the smaller the resistance of the wire is.
What does the current against voltage graph look like for a wire or a resistor?
Describe this graph.
The graph is a straight line. It has a constant slope, so the resistance of this component does not change.
What does the current against voltage graph look like for a filament bulb?
Describe this graph.
This is not a straight line. The resistance of the bulb changes. At higher currents and voltages the slope of the graph shows us that the resistance of the filament bulb increases - that is, as the temperature of the filament increases the current decreases.
What does the current against voltage graph look like for a diode?
Describe this graph.
This strangely shaped graph shows that diodes have a high resistance when the current is in one direction and a low resistance when it is in the opposite direction.
What are fixed resistors?
They are included in circuits in order to control the sizes of currents and voltages.
Why are fixed resistors used?
Without the resistor, the voltage across the bulb may cause too large a current and the bulb may ‘blow’ or break.
What is a variable resistor?
They are called variable resistors as it is possible to alter their resistance.
What happens if the resistance in a variable resistor is decreased?
There will be a larger current and the bulb would shine more brightly.
What happens if the resistance in a variable resistor is increased?
The current will be smaller and the bulb will glow less brightly or not at all.
How do variable resistors usually behave when being used in a circuit with a bulb?
They act as a dimmer switch.
What else can variable resistors be used for?
They could be used in circuits containing electric motors, variable resistors can be used to control the speed of the motor.
What is a thermistor?
It is a conductor whose resistance changes a lot when its temperature changes.
Where would you place a thermistor?
On the left of the cell in the circuit.
What is a graph which represents resistance against temperature?
Explain the graph.
The thermistors resistance decreases when it senses a temperature increase.
Where and how are thermistors utilised in day to day life?
In temperature-sensitive circuits in devices such as fire alarms.
- Freezers.
- Computers.
What is a light dependent resistor (LDR)?
It has a resistance that changes when light is shone on it. In the dark its resistance is high but when light is shone on it its resistance decreases.
What is a graph that represents resistance against light intensity?
Explain the graph.
The LDR’s resistance decreases with an increase in light intensity.
Where and how are LDR’s used in every day life?
In light sensitive circuits in devices such as:
- Photography exposure equipment.
- Automatic lighting controls.
- Burglar alarms.
What are diodes?
They are very special resistors that allow charges to flow through them easily but only in one direction.
Which way does the diode face when it conducts?
The left.
Which way does the diode face when it doesn’t conduct?
The right.
When are diodes used?
In circuits where it is important that electrons flow in only one direction.
What kind of diodes glow when charges flow through them?
Light emitting diodes (LED’s)
