Electricity - Paper 1 Flashcards
To revise the key points from the Electricity topic (paper 1)
Draw the circuit symbol for a lamp
an X in a circle
Draw the circuit symbol for a diode
An arrow head in a circle
Draw the circuit symbol for a resistor
A rectangular box
Draw the circuit symbol for a variable resistor
A rectangular box with a diagonal arrow
Draw the circuit symbol for an LED
Same as a diode with 2 arrows points away
Draw the circuit symbol for a fuse
A rectangular box with a horizontal one running through it
Draw the circuit symbol for a voltmeter
V in a circle
Draw the circuit symbol for an ammeter
A in a circle
Draw the circuit symbol for a thermistor
a rectangular box with a diagonal line through it
Draw the circuit symbol for an LDR
A rectangular box in a circular with 2 arrows pointing towards it
Define Current
Flow of charge (electrons) measured in amps (A)
State the equation that links charge, current and time.
Charge (C)= Current (A) x time (s)
Q = It
What are the units of charge
Coulombs (C)
What is the relationship between current and resistance?
They have an inverse relationship. As resistance increases, current decreases and vice versa
State the equation that links potential difference, current and resistance
Potential difference (V) = Current (A) x resistance (ohms)
V = IR
State the units of potential difference
Volts (V)
State the units of resistance
Ohms
State the relationship between the length of wire and its resistance.
As the length of the wire increases the resistance increases. They are directly proportional. If the lenght of wire doubles then the resistance will double.
As more resistors are added in series what happens to the total resistance.
Total resistance increases (Rt = R1 + R2 + …..) as the current has to flow through every resistor.
As more resistors are added in parallel what happens to the total resistance.
As more resistors are added in parallel then the total resistance decreases. This is because the electrons have more paths they can take back to the battery.
State ohms law
Current is directly proportional to potential difference at a constant temperature. This means resistance remains constant.
Do the following components obey ohms law:
resistor?
Lamp?
Diode?
Resistor - yes
Lamp - No
Diode - No
Sketch the I/V graph for a resistor
A straight line through the origin (current and potential difference are directly proportional - an ohmic conductor)
Sketch the I/V graph for a bulb
It starts off as a straight line through the origin but the the current levels offs so the graph curves. Non ohmic conductor as current is not proportional to potential difference
How does the resistance of a lamp change as it heats up?
As temperature increases resistance increase. Atoms/ions have more energy so vibrate more taking up more space making it harder for electrons to travel down the wire. This causes the I/V graph for a bulb to level off.
Sketch the I/V graph for a diode
Current can only flow in one directions. So zero current in the negative direction but in the positive direction current increases just after the origin. Non ohmic conductor
State the relationship between temperature and resistance for a thermistor
As temperature increases resistance decreases.
State some uses of thermistors
In thermostats to turn circuits on/off if they get too hot to cold e.g. in an iron, oven, central heating, hair straighteners etc
State the relationship between temperature and light intensity for a LDR
As light increases increases, resistance decreases
State some uses of LDRs
In circuits to switch lights on when it gets dark e.g. street lights
state the rules for current, potential difference and resistance in a series circuit
- Current - same flowing through each component
- Potential difference - the p.d. of the power supply is shared between the other components
- Resistance - the total resistance increases as more components are added. the total resistance is the sum of all the individual components resistance
state the rules for current, potential difference and resistance in a parallel circuit
- current - splits up but the total current stays the same
- Potential difference - across each brach is the same as from the battery
- Resistance - decreases as more branches are added. the electrons have more choice of paths to take.
What happens when electrical current flows through a resistor?
Electrical energy is transformed into heat energy
What is a short circuit?
An electrical circuit that allows current to travel along a paths with no or very low resistance Resistance low = current very high. This will cause the battery to overheat
Why do lamps blow when they have just been turned on?
The lamp is cold so it has low resistance, this means a high current flows - sometimes enough to melt the wire in the bulb
In a potential divider, what happens to the potential difference from the battery when two components have equal resistance?
They share the potential difference from the battery equally.
In a potential divider, a current passes through a resistor then through a thermistor. How does the current and voltage across each component change as temperature increases?
The thermistor heats up. The resistance of the thermistor decreases, and then takes a smaller share of input voltage from the battery (voltage decreases). As the input from the battery is constant the voltage increases across the fixed resistor.
As the resistance deceases the current decreases.
In a potential divider, a current passes through a resistor then through an LDR. How does the current and the voltage across each component change as light intensity increases?
If light intensity increases the resistance of the LDR decreases. The LDR will take a smaller share of the input voltage (voltage decreases). As the input voltage from the battery is fixed the voltage across the fixed resistor must increase.
If resistance decreases then the current will increase.
