Topic 10 - Electricity and their circuits Flashcards
Define current
The flow of electrical charge
what 2 things must be done in order for current to flow through a component?
- There must be a P.D across the component
2. the circuit must be complete (closed)
What is Potential difference (V)?
The driving force that pushes the charge round
What is resistance (ohms)?
Anything that can slow a current down
is the current of a component affected by Potential difference and resistance?
Yes
The higher the P.D, the …… the flow of current. The greater the resistance, the …….. the current
- higher
2. smaller
What is the formula that relates charge (C)
, current and time?
charge = current * time
memorise 16 circuit symbols
you can do this by finding the symbols in the cgp guide
The ……………. is the energy transferred per unit charge
Potential difference
What is the formula to work out energy transferred (relates to P.D and charge moved?
Energy transferred = charge moved * P.D
What is the formula to work out P.D? (relates to current and resistance)
Potential difference = current *resistance
Why does a resistor heat up when current flows through it?
The electrons collide with the ions in the lattice that make up the resistor as they flow through it. This gives the ions energy, which causes them to vibrate and heat up
Why does current decrease as a resistor heats up? (for a given P.D)
The more the ions vibrate, the harder it is for electrons to get through the resistor
What happens if a resistor gets too hot and what is the only exception to this rule?
Current will not be able to flow through the component ( exception: resistance of a thermistor decreases with an increase in temp)
What is an ammeter and where can it be placed?
A device that measures current and it must be placed in series with a component. (anywhere in the circuit)
What is a voltmeter and where must it be placed?
A device that measures the P.D across a component. It must be placed in parallel with the component
What happens if you swap the direction of a diode in a circuit?
Current cannot flow through the diode anymore
What happens to resistance and current if you heat a thermistor whilst keeping the supply P.D constant in a circuit? (clue: THIS IS THE ONLY EXCEPTION TO THE RULE ‘RESISTANCE INCREASES WITH TEMP’!
As the temp increases, the current through the thermistor increases as the resistance decreases
What happens to current and resistance if you keep the p.d of a supply constant but adjust the light level near an LDR? (in a circuit)
as the light level gets brighter, the current through an LDR increases and the resistance decreases.
Why is current reading always the same in a series circuit?
Current is never used up in a series circuit so it remains the same all the way around
What direction does current flow in?
positive to negative
For parallel circuits, the current in the branches…..to the total current leaving the cell
adds up to
What is the difference between parallel and series circuits?
Parallel circuits have branches whilst series do not
In parallel circuits, is the pd of each component the same or different?
the same
What is a battery?
2 or more cells joined together`
What does a current - PD graph look like for resistors and wires
It would be a straight line going through (0,0) because current is directly proportional to Potential difference
What does an I-V graph for a filament lamp look like?
It would be curved (like a slanted ‘s’) because the increasing current increases the temperature of the filament , which makes the resistance increase
What does an I-V graph for a diode look like and why does current only flow through a diode in one direction?
straight horizontal line then a curve.
the current only flows through one direction as the diode has very high resistance in the opposite direction
What is an LDR?
A resistor that is dependent on light
What happen to LDR’s in bright light and in darkness?
In bright light, the resistance falls and in darkness, the resistance is highest
What are applications of LDR’s?
- Automatic night lights
- Outdoor lighting
- Burglar detectors
What is a thermistor?
A temperature dependent resistor
What happens to thermistors in hot and cold conditions?
In hot conditions, the resistance drops and in cold conditions, the resistance goes up
Where can themistors be used?
- In temperature detectors:
- Car engine temperature sensors
- Electronic thermostats
What are 3 differences between series and parallel circuits?
- in series, the current is the same everywhere but in parallel, the current is shared
- In series, the V is shared between components but in parallel, the voltage is the same across all components
- In series, the total resistance increases as you add resistors but in parallel, the total resistance decreases if you add a 2nd resistor
In series, Why does total resistance increase as you add resistors?
- In series, total resistance of 2 components is the sum of their resistances
- This means by adding a resistor, the 2 resistors have to share the total V
- The pd across each resistor is lower as its now shared so the current through each resistor is also lower.
- In series, 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 increases
In parallel, Why does total resistance decrease as you add resistors?
- In parallel, the pd of resistors is the same as the pd leaving the cell
- By adding another loop, the current has more than one direction to go in
- This increases the total current that can flow around the circuit. An increase in current means a decrease in the total resistance of the circuit
Describe a practical to investigate the properties in series and parallel circuits
- Set up circuit (look at book to find circuit)
- Vary the output potential difference from the power supply. Record the readings from the ammeter and voltmeter for each change
- Replace the resistor with a filament lamp and repeat step 2
- Connect a second filament lamp in parallel to the first (to change circuit from series to parallel) Connect ammeters and a second voltmeter
- Vary the output pd of the supply and record the current through each ammeter and the pd across each component
What results should you find from the practical for the series circuits? (circuits)
For the series circuit, as the pd increases, the current through the resistor increases/filament bulb is brighter
What results should you find in the practical for parallel circuits?
- For the parallel circuit, you should find that as pd increases, the current through each bulb also increases.
- The pd across each bulb is the same as the pd of the power supply (cell/battery).
- The total current in the parallel circuit is the sum of the current through the two branches
Give an example of how an electrical appliance may be designed to transfer energy to components in the circuit when a current flows?
Kettles transfer energy electrically from the mains a.c. supply to the thermal energy store of the heating element inside the kettle
What are the advantages of the heating effect of an electrical current? (use an example for the first advantage)
- It can be used for when you want to heat something e.g. toasters contain a coil of wire with a really high resistance. When a current passes through the coil, its temperature increases so much that it glows and gives off infrared radiation. This radiation transfers energy to the bread and cooks it
- Fuses use the heating effect to protect circuits
The higher the current in an electrical appliance, the more energy is transferred to the ………… energy store, and ……. usually increases the resistance of the components
- Thermal
2. Heating
What is power? (in terms of energy transferred)
The energy that an appliance transfers per second
What is the formula for power (in terms of energy transferred?)
Power = energy transferred/ time
What is the formula for power? (in terms of pd and current?
Power = current * pd
If you dont know the pd, what equation could you use to find power?
P = I(squared)R
Is mains supply a.c or d.c?
a.c
Is battery supply a.c or d.c.?
d.c
What is the main difference between alternating current and direct current?
Alternating current is always changing direction as they are produced by alternating voltages. (the positive and negative ends of the pd keep alternating) whilst Direct current is always flowing is the same direction as it created by direct voltage (either positive or negative, not both)
What is the frequency of the a.c. mains supply and what is the voltage of it?
The frequency 50 Hz and the voltage is around 230V
What is the function of the neutral wire (blue) and what is its voltage?
The neutral wire completes the circuit - electricity normally flows in through the live wire and out through the neutral wire. The neutral wire is always at 0V.
What is the function of the live wire (brown) and what is its voltage?
The live wire carries the voltage. It alternates between high positive and negative voltage of about 230V
What is the function of the earth wire (green and yellow) and what is its voltage?
The earth wire is for safety and protecting the wiring. It carries the current away if something goes wrong and stops the appliance casing becoming live. It’s also at 0V,
Why do plug sockets have switches connected in the live wire of the circuit?
This is so the circuit can be broken - the appliance becomes isolated and the risk of an electric shock is reduced
Why can touching the live wire give you an electric shock?
Your body is at 0V so if you touch the live wire, a large pd is produced across your body and a current flows through you. This causes a large electric shock that could injure/kill you
Why could you get an electric shock even if a plug socket is switched off
there is still pd in the live wire so if you made contact with the live wire, your body would provide a link between the supply and the earth, so a current would flow through you
Why could a connection between the live wire and the neutral wire be dangerous?
If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
Explain how earthing and fuses prevent electrical overloads?
- If the live wire touched metal case, then because the case is earthed, too great a current flows in through the live wire, through the case, and out down the earth wire
- This surge in current melts the fuse when the amount of current is greater than the fuse rating. Fuses are connected to the live wire, so breaking the fuse cuts of the live supply. This makes it impossible to get an electric shock
How high should fuses be rated?
As near as possible (but just higher) than the normal operating system (H)
Why does fuse rating needed for cables usually increase with cable thickness?
The larger the current, the thicker the cable you need to carry it (to stop the cable getting too hot and melting)
Evaluate circuit breakers?
1) Instead of melting a fuse, a large current may instead turn off a circuit breaker
2) circuit breakers turn off quicker than the time taken for a fuse to melt
3) They can also be reset, instead of having to replace them (fuse)
4) But they are more expensive than fuses
What is double insulation in terms of appliances?
This is if an appliance has a plastic casing and no metal parts showing so that the risk of an electric shock is reduced
What are two-core cables?
Cables that only carry the live and neutral wire and not the earth wire (as perhaps the cable is double insulated)
Why may a microwave with a power rating of 500W take longer to cook food than one with a power rating of 750W
This is because the 500W transfers less energy per second to the thermal energy store of the food, so it takes longer to cook
