Ch 19 - (Electrical Circuits) Flashcards
What are power supplies?
cells, batteries, power supplies and generators all supply current to the circuit
What are resistors?
potential dividers, fixed and variable resistors, thermistors and light dependent resistors (LDR’s) control current
What are meters?
ammeters and voltmeters are used to measure the current and potential difference
- Ammeters in series
- Voltmeters in parallel
What are electromagnetic components?
Magnetizing coils, relays, and transformers use electromagnetic effects
- relay use small current in one circuit to switch on another
- transformers ‘step up’ and ‘step down’ current and potential difference.
What are fuses?
Protect expensive components from current surges and act as safety measures against fire
What are thermistors?
- non - ohmic conductor
- temperature dependent resistor
- resistance changes based on temp
- as temp increases, resistance of thermistor decreases
What are light - dependent resistors?
- non-ohmic conductor
- sensory resistor
- resistance changes depending on light intensity (illumination)
- as light intensity increases, resistance of LDR decreases
What is a diode?
allows a current in one direction
What happens if diode is placed in a.c. ?
Only allows a current half of the time (called as rectification)
How is the current in a series circuit? And why?
- same value at any point
- no. of electrons passing through per sec is same ate very part
What does the current flowing around series circuit depend on?
- Voltage of power source
- resistance of components in a circuit
What does increasing voltage do in a series circuit?
drives more current around the circuit
What does increasing number of components do in a series circuit?
increases the total resistance
What is the advantage in a parallel circuit?
- The components can be individually controlled, using their own switches
- If one component stops working the others will continue to function
How does Curr not flow in a parallel circuit?
- In a parallel circuit, the current splits up
- current in each branch will be smaller than the current from the power supply
What happens at every junction in parallel circuit and why?
-The current is conserved
- amount of current flowing into the junction is equal to the amount of current flowing out of it
Why does current split up in junctions?
charge is conserved
What happens when current does not split up equally at junctions in a series circuit?
The current in each branch will only be identical if the resistance of the components along each branch are identical
why does current behave a certain way in a series circuit?
- Electrons are physical matter – they cannot be created or destroyed
- total number of electrons (and hence current) going around a circuit must remain the same
- When the electrons reach a junction, however, some of them will go one way and the rest will go the other
How do you find the potential difference in series circuit?
several cells are connected together in series, their combined EMF is equal to the sum of their individual EMFs
How do you find potential difference in parallel circuits?
- potential difference across each component connected in parallel is the same
This is the opposite of the current, which is different in each branch
what are the advantages of parallel circuits?
- The components can be individually controlled, using their own switches
- If one component stops working the others will continue to function
How are resistors in series circuit?
The combined resistance of the components is equal to the sum of individual resistances
How are resistors in parallel circuits?
The combined resistance decreases and is less than the resistance of any of the individual components
How do you determine the resistance in a parallel circuit?
- 1/R = 1/R1 + 1 / R2
- To calculate the resistance:
First find the value of 1/R (by adding 1/R1 + 1/R2) - Next find the value of R by using the reciprocal button on your calculator (labelled either x-1 or 1/x, depending on your calculator)
What does the potential difference across each resistor depend on?
Resistance
What is a potentiometer?
single component that (in its simplest form) consists of a coil of wire with a sliding contact, midway along it
What effect does the sliding contact have?
separating the potentiometer into two parts – an upper part and a lower part – both of which have different resistances
What happens when 2 resistors are connected ins eries?
Kirchhoff’s Second Law, the potential difference across the power source is divided between them
What are potential dividers?
circuits which produce an output voltage as a fraction of its input voltage
What is the purpose of potential dividers?
produce an output voltage as a fraction of its input voltage
What are potential dividers used for?
- To provide a variable potential difference
- To enable a specific potential difference to be chosen
- To split the potential difference of a power source between two or more components
What are potential divider circuits based on?
ratio of voltage between components. This is equal to the ratio of the resistances of the resistors
What is the potential divider equation?
V out = R2 / R1 + R2 x V in
- R2 is the numerator and the resistance of the resistor over Vout
- R1 is the other resistance in series
- Vout is the output potential difference
- Vin is the input potential difference
What is the second way to write the potential divider equation?
V out = R1 / R1 + R2 x V in
- R1 is the numerator and the resistance of the resistor over Vout
- R2 is the other resistance in series
How is Vin and Vout measured?
input: input voltage Vin is applied to the top and bottom of the series resistors
Output: output voltage Vout is measured from the centre to the bottom of resistor R2
How does the potential difference V across each resistor depend on its resistance R?
- The resistor with the largest resistance will have a greater potential difference than the other one from V = IR
- If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share
What is p.d. across a component proportional to?
Resistance from V = IR
What are the common hazards?
- damaged insulation
- overheating of cables
- Damp conditions
- excess current from overloading of plugs, single and multiple sockets when using a mains supply
What is included in hazard of damaged insulation?
If someone touches an exposed piece of wire, they could be subjected to a lethal shock
What is included in hazard of overheating of cables?
Passing too much current through too small a wire (or leaving a long length of wire tightly coiled) can lead to the wire overheating. This could cause a fire or melt the insulations, exposing live wires
What is included in hazard of damp conditions ?
If moisture comes into contact with live wires, the moisture could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk
What is included in hazard of excess current from overloading of plugs, single and multiple sockets when using a mains supply ?
If plugs or sockets become overloaded due to plugging in too many components the heat created can cause fires
What is mains electricity?
electricity generated by power stations and transported around the country through the National Grid
alternating current (a.c.) supply
How is domestic supply energy in the UK?
- frequency of 50 Hz and a potential difference of about 230 V
- A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second
Does main electricity have a positive and negative side?
- does not have positive and negative sides to the power source
- The equivalent to positive and negative are called live and neutral and these form either end of the electrical circuit
What are safety features built into domestic appliances?
- Double insulation
- Earthing
- Fuses
- Circuit breakers
What is double insulation?
- Insulation around the wires themselves
- A non-metallic case that acts as a second layer of insulation
- do not require an earth wire
What is earthing and what does it pose?
- Many electrical appliances have metal cases
- live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted
What happens if the earth wire is an additional safety wire that can reduce the risk?
- The earth wire provides a low resistance path to the earth
- It causes a surge of current in the earth wire and hence also in the live wire
- The high current through the fuse causes it to melt and break
- This cuts off the supply of electricity to the appliance, making it safe
What is a fuse?
a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)
What do fuses consist of?
glass cylinder which contains a thin metal wire
What happens if current in wire becomes to large in a fuse?
- The wire heats up and melts
- This causes the wire to break, breaking the circuit and stopping the current
What does a trip switch do?
- When the current is too high the switch ‘trips’ (automatically flicks to the off position)
- This stops current flowing in that circuit
How do u choose which fuse to use?
- Fuses come in a variety of sizes (typically 3A, 5A and 13A) - in order to select the right fuse for the job, you need to know how much current an appliance needs
- If you know the power of the appliance (along with mains voltage), the current can be calculated using the equation:
Current = power / voltage
