Flashcards in P2:Electricity Deck (70):
Give 2 rules when drawing a circuit diagram:
1) make sure all the wires in your circuit are straight lines.2) make sure the circuit is closed.
Total charge through a circuit depends on....
Current and time.
Electric current is a flow of....
Current is measured in....
Amperes ( A)
Charge is measured in...
Electric charge will only flow around a complete circuit if something is providing a potential difference. Give an example of a PD:
A battery - it is a 'driving force' that pushed charge around the circuit.
What is resistance?
Anything that slows down the flow of charge.
What does the size of a current tell you?
How fast the charge is flowing - this is known as the rate of flow charge.
Give the equation for finding the rate of CHARGE FLOW:
Charge flow = CURRENT X TIME
The current flowing through a component depends on: (2)
- The potential difference across it.- The resistance of the component.
If there is a big resistance across a component, how does this affect the current that flows through?
The smaller the current.
Give the formulae for finding the potential difference:
CURRENT in amps X RESISTANCE in ohms
What are Ohmic Conductors?
Components with a fixed resistance. This means the resistance of the component doesn't change with current.
Ohmic Conductors only have a fixed resistance if .............. doesn't change.
Give 2 examples of Ohmic Conductors:
- Wires- Resistors
What is the link between the current and the potential difference in an Ohmic Conductor?
The current flowing through it is directly proportional to the potential difference across it. Meaning that of the potential difference doubles, so does the current.
Some components have a resistance that DOES change with the current. Explain how this works for a FILAMENT LAMP:
Filament lamps contain a wire which is designed to heat up and 'glow' as the current increases so as the current increases, the temperature of the filament increases too. Resistance increases with temperature so the resistance increases with current.
How does resistance work on DIODES?
The resistance depends on the direction of the current. It will let current flow in one direction but it has a very high resistance in the opposite direction which makes it hard for a current to flow that way.
The resistance of a circuit can depend on a number of things. Give 2 examples:
- If components are in series or parallel.- The length of wire used in the circuit.
Give the 7 steps to investigating the effect of wire length in a circuit:
1) Attach a crocodile clip to the wire level with 0cm on the ruler.2) Attach the second crocodile clip to the wire a short distance from the first clip.3) Write down the length of the wire between the clips.4) Close the switch, record the current through the wire and the potential difference across it..5) Calculate the resistance of the wire using the equation.6) Open the switch and move the second crocodile clip along the wire.7) Repeat steps for a range of wire lengths.
How to plot a graph of your results AFTER INVESTIGATING THE EFFECT OF WIRE LENGTH IN A CIRCUIT: (4)
1) Plot a graph of resistance against wire length.2) Draw a line of best fit through your points.3) Your graph should be a straight line through the origin.4) This means resistance is directly proportional to length - the longer the wire, the greater the resistance.
What is "I-V Characteristics"?
A graph showing how the current flowing through the component changes as the potential difference across it changes.
What are components with STRAIGHT LINE IV Characteristics called? Give an example.
Linear Components. (e.g. a fixed resistor)
What are components with CURVED IV Characteristics called? Give an example.
Non-linear components (e.g.filament lamp or diode)
Give the 6 steps to investigating IV Characteristics on different components:
1) Set up a test circuit (the one on page 183 in revision guide)2) The variable resistor is used to change the current in the circuit. This changes the potential difference across the component.3) Set the resistance of the variable resistor and measure the current through and potential difference across the component.4) Swap over the current connected to the cell to reverse the direction of the current. The ammeter should now display negative readings.5) Repeat step 3 to get results for negative values of current.6) Plot a graph with current on the y axis and the potential difference on the x axis.
LDR is short for...
Light Dependant Resistor.
LDR - In bright light, is the resistance high or low?
LDR - In darkness, is the resistance high or low?
LDR's have lots of uses. Give 2.
- Automatic night lights.- Burglar detectors.
What is a thermistor?
A temperature dependent resistor.
THERMISTORS - In hot conditions, does the resistance increase or decrease?
DROPS - decreases.
THERMISTORS - In cool conditions, does the resistance increase or decrease?
What do Thermistors do?
Turn the heating on when it's cool and off when it's warm. Can be used in car engines and for central heating.
What can sensing circuits be used for?
To automatically change the potential difference across components depending on changes in the environment.
What does a series circuit look like?
The components are all connected in a line between the ends of the power supply. Only voltmeters break this rule as they are always parallel.
What happens if you remove one component in a series circuit?
The circuit is broken so all the components stop working.
What is the potential difference like in series circuits?
The total potential difference of the supply is shared between all of the components - if you add the pd across each component you get the pd of the power supply. THE BIGGER THE RESISTANCE, THE BIGGER ITS SHARE OF THE TOTAL PD.
In a series circuit the same current flows through....
Give the equation to find the TOTAL RESISTANCE of a circuit:
Resistor TOTAL = resistance of 1 component + resistance of 2 component.
Give 5 points that explain WHY adding resistors in SERIES CIRCUITS increases the total resistance of the circuit:
- adding a resistor in series means the resistance have to share the total PD.-this means the PD across each resistor is lower so the current through each resistor is lower.- The current is the same everywhere.-so the total current in the circuit is reduced when a resistor is added.-this means the total resistance of the circuit has gone up.
What is a Parallel circuit like?
Each component is separately connected to the ends of the power supply. Only ammeters break this rule as they are always in series.
What happens if you take out one of the loops in a parallel circuit?
NOTHING. The things in the other loops won't be affected as they would in a series circuit.
What is the PD like in Parallel circuits?
All components get the full source PD so the potential difference is the same across all components.
In parallel circuits the total current in a circuit is equal to the....
Sum of all the currents through the separate components.
What happens AT JUNCTIONS in parallel circuits?
The current either splits or rejoins. The total current going into a junction must equal the total current leaving it.
Adding a resistor in PARALLEL reduces the total resistance. Give 4 points why:
1) if you had a resistor in parallel both a resistor still have the same potential difference across them as the power supply.2)This means the 'pushing force' for some making the current flow is still the same.3) but by adding another loop on the current has more than one direction to go in. 4) more current can flow around the circuit so the total current increases which means the total resistance of the circuit is lower.
INVESTIGATING CIRCUITSFIRST SET UP THE BASIC CIRCUITGive 5 steps to doing this.
1) Find at least 4 identical resistors.2) Build the circuit shown on the right PAGE 187 revision guide.3) Write down the PD of the battery. This is the pd of the circuit.4) Read the current in the circuit from the ammeter.5) Calculate the resistance.
INVESTIGATING CIRCUITSSTEP 2 ADDING RESISTORS IN SERIES Give the 3 steps
1) Add another resistor, in series with the first.2) Measure the current again and calculate resistance again. The pd is the same as the pd of the battery.3) Repeat steps 1 and 2 until you've added all of your resistors.
INVESTIGATING CIRCUITSSTEP 3 INVESTIGATING ADDING RESISTORS IN PARALLELGive the 5 steps.
1) Build the basic circuit again. you already know it's resistance.2) Use the same equipment so it's a fair test.3) Add another resistor, in parallel with the first.4) Measure the total current through the circuit and calculate the overall resistance of the circuit. The pd is still the same as before.5) repeat steps 3 and 4 until you've added all of your resistors.
INVESTIGATING CIRCUITSFINAL STEP DRAW GRAPHS TO COMPARE RESULTSWhat should results be?
SeriesAdding resistors in series increases the total resistance of the circuit.ParallelThe more resistors you add, the smaller the overall resistance becomes.Page 187 revision guide look at graphs.
What is the national grid?
A giant system of cables and transformers that covers the U.K. It transfers electrical power from power stations to consumers across the U.K.
Electricity production has to meet demand. Explain this.
Throughout the day, the amount of electricity used changes and the power stations have to produce enough electricity.
How does the national grid cope with high demand?
Power stations often run at well below their maximum power output so that they can increase their power if needed.
Why does the National Grid use a high PD and a low current?
To transmit a huge amount of power you either need a high potential difference or a high current. A high current means lots of energy is lost to thermal energy as wires are heated up so high PD is used to less energy is lost.
What are step up transformers?
Things used to increase the potential difference from power stations to electric cables.
What are Step DOWN transformers?
Things used to bring the potential difference back down to safe levels before the electricity gets to homes.
The energy transferred by a component depends on the potential difference across it and the charge flowing through it. Give the formulae for ENERGY TRANSFERRED:
CHARGE FLOW X POTENTIAL DIFFERENCE.
Power also depends on current and potential difference. Give the equation for the power of an appliance:
Potential difference x Current
Explain what the Live Wire does and what colour is it?
BROWNThe live wire provides the alternating potential difference from the mains supply - 230V.
Explain the Neutral Wire and what colour is it?
BlueCompletes the circuit. Usually the current flowed in through the live Wire and out through the neutral Wire. 0V.
Explain what the Earth Wire does and what colour is it?
Green and YellowA safety Wire, it stops the appliance becoming live: it is connected to the metal casing of an appliance. If a fault caused the live Wire to touch the casing, the current flows away through the earth Wire. 0V.
Explain how the live Wire (brown) can give you an electric shock.
There is a potential difference between the live Wire and your body. Touching it can cause a current to flow through your body because it has a pd of 230V. Our body is at 0V.
What is an ALTERNATING POTENTIAL DIFFERENCE?
A potential difference that is constantly changing direction. It produces an alternating current.
In an alternating current, the current is also constantly changing direction. Give an example of an Alternating current supply:
The UK Mains supply (electricity in your homes) is an ac supply at around 230V.
What is the frequency of the AC MAINS SUPPLY?
50Hz ( hertz)
What is direct current (dc)?
A current that is always flowing in the same direction. It is created by a direct potential difference.
When a charge moves around the circuit, work is done against the resistance of the circuit. Then what happens?
Whenever work is done, energy is transferred. When the work is done by a charge, the energy is transferred electrically. Electrical appliances transfer energy to components in the circuit when a current flows.
The total energy transferred by an appliance depends on : (2)
1) how long it's on for2) it's power (the energy that it transfers per second)
Give the equation that finds Energy Transferred:
Energy Transferred = POWER X TIME