5. Electricity Flashcards
What is electric current? State its units
The rate of flow of charge. It is measured in amperes (amps)
What is potential difference?
The work done moving a unit charge between 2 points in a circuit
V = W / Q
What is resistance?
How difficult it is for current to flow through an appliance.
A component has a resistance of 1Ω if 1A flows through it when a p.d of 1V is applied across it.
R = V / I
What is meant by an ohmic conductor?
A conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant.
How can you measure the current in a circuit?
You can measure the current in a circuit with an ammeter connected in series with the component.
How do you measure potential difference across a component?
Using a voltmeter, connected in parallel across the component being measured.
What does the gradient of a current-potential difference graph represent?
Rate of change of current with respect to voltage. This is not the same as 1/R
Does this graph represent an ohmic conductor?
. . . . . .Current . . . . . . . . . | . . . . . . . ̷ . . . . . . . . . | . . . . ̷ . . . . . . . . . | . . ̷ -------------- ̷ ------------- Voltage . . . . . . .̷ . . | . . . . ̷ . . . . .| .. ̷ . . . . . . . |
Yes, as the line has a constant gradient and passes through the origin.
This shows that voltage is directly proportional to current.
Which graph represents an appliance with higher resistance?
A . . . . . .Current . . . . . . . . . | . . . . . . . ̷ . . . . . . . . . | . . . . ̷ . . . . . . . . . | . . ̷ -------------- ̷ ------------- Voltage . . . . . . .̷ . . | . . . . ̷ . . . . .| .. ̷ . . . . . . . |
B . . . . . .Current . . . . . . . . . | . . . . . . . . . | . . . . . . . . ̷ . . . . . . . . . | . . . . ̷ -------------- ̷ ------------- Voltage . . . . .̷ . . . . | ̷ . . . . . . . . | . . . . . . . . . |
B
Higher voltage is required in B for the same change in current in comparison to A
What common appliance could this curve represent?
. . . . . .Current . . . . . . . . . | . . . . . . . . . | . . . . \_\_---- . . . . . . . . . | . . ̷ -------------- ̷ -------------Voltage . . . . . . .̷ . . | \_\_----. . . . | . . . . . . . . .|
A filament lamp. As the current increases the the resistance also increases. A big increase in the voltage produces only a small increase in current
Why does the current increasing on a filament lamp cause an increase in the resistance?
As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions (resistance has increased).
What is a diode?
A diode is a appliance that only allows current to flow in one direction.
. . . . . .Current . . . . . . . . | . . . . ./ . . . . . . . . | . . . ./ . . . . . . . . | . . ̷ \_\_\_\_\_\_\_| ̷\_\_\_\_\_\_\_Voltage . . . . . . . . | . . . . . . . . | . . . . . . . .|
Unless stated in the question, should you assume that voltmeters to have zero resistance or infinite resistance?
You should assume they have infinite resistance. Current takes the path of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow through the appliance.
Why should you assume that an ammeter has zero resistance unless stated otherwise?
This assumption means that there would be 0 potential difference across the ammeter and no energy is lost across it; it does not affect the circuit
What is an Light Dependent Resistor (LDR)?
A semiconductor that is sensitive to light.
As the light intensity increases, its resistance decreases.