P2 - Electricity Flashcards
Electric current
Is a flow of electric charge - the size is the rate of flow of electric charge
Equation which links charge flow, current and time
Charge flow (C) = current (A) x time (s) (Q = I t)
What does current through a component depend on
- resistance (R) and potential difference (V) across the component
- The greater the resistance of the component the smaller the current for a given pd across the potential difference
Equation which links current, potential difference and resistance
Potential difference (V) = current (A) x resistance (Ohms) (V = IR)
Required practical - Investigating the factors affecting the resistance of electrical circuits - method (6)
1) Attach a crocodile clip to the wire level with 0cm on the ruler
2) Attach a second to the wire e.g 10 cm away
3) close the switch the record the current and pd across it
4) Open the switch and move the second clip another 10cm
5) Close the switch and record the new length, current and pd
6) Repeat this for a number of different lengths of the test wire
Ohmic conductors (at a constant temp) e.g wire or a resistor
• directly proportional to the pd across the resistor - this means that the resistance remains constant as the current changes
Required practical: I-V characteristics
1) vary the variable resistor - to alter the current flowing through the circuit and pd across the component
2) take several pairs of readings from the ammeter and voltmeter to see how the pd across the component varies as the current changes - repeat to get an average
3) Swap over the wires connected to the cell, so the direction of the current is reversed
4) plot a graph,of current against voltage
Example of LDR’s being used in a circuit (3)
- Automatic night lights
- Outdoor lighting
- Burglar detectors
What is the relationship between the resistance of a thermistor and temperature
- A thermistor is a temperature dependant resistor
- In hot conditions, the resistance drops
- In cool conditions the resistance goes up
What is the relationship between the resistance of an LDR and light intensity
- In bright light, the resistance falls
* In darkness, the resistance is highest
Series circuits (4)
• there is the same current through each component
• the total potential difference of the power supply is shared between the components
• the total resistance of two components is the sum of the resistance of each components
R(total) = R1 + R2
Parallel circuits (3)
- The potential difference across each component is the same
- The total current through the whole circuit is the sum of the currents through separate components
- The total resistance of two resistors is less than the resistance of the smallest individual resistor
Why does adding resistors in series circuits increase the total resistance (3)
- This is because the two resistors have to share the total potential difference
- The pd across each resistor is lower so current through each resistor is also lower
- Total current is reduced when a resistor is added as pd is shared
Why does adding resistors in parallel circuits decrease the total resistance (5)
If you have two resistors in parallel their total resistance is less than the resistance of the smallest if the two resistors
• In parallel circuits both resistors have the same pd across the, as the source
• This means the ‘pushing force’ making the current flow is the same as the source pd for each resistor you add up
• But 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. Using V=IR an increase in current means a decrease in total resistance
Equation linking voltage, current and resistance
Voltage = current x resistance
Direct potential difference
Flows in only one direction
Alternating potential difference
Reverses its direction of flow
Live wire (2)
- Brown
* Carries alternating potential difference from the supply
Neutral wire (2)
- Blue
* Completes the circuit
Earth wire
- Green and yellow stripes
* Is a safety wire to stop the appliance becoming live
Why is a live wire dangerous even when a switch in the mains circuit is open (turned off) (3)
- There is still a danger of an electric shock
- A current isn’t flowing but there’s still a pd in the live wire
- If you make contact with it you body will provide a link between the supply and the earth, so a current would flow through you
What are the dangers of providing any connection between the live wire and earth
If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
How is the power transfer in any circuit device related to the potential difference across it and the current through it (4)
- When an electrical charge goes through a change in potential difference, then energy is transferred
- Energy is supplied to the charge at the power source to ‘raise it’ through a potential
- The charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit
- That means that a battery with a bigger pd will supply more energy to the circuit for every coulomb of charge which flows round it because the charge is raised at the start
Equation linking power, potential difference and current
Power (W) = potential difference (V) x current (A)
Equation linking power, current and resistance
Power (W) = (current)2 (A) x resistance (O)
How do kettles transfer energy from ac mains to the energy of heating
Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating elements inside the kettle
How do fans transfer energy from batteries to the kinetic energy of electric motors
Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor
Equation linking energy transferred, power and time
Energy transferred (J) = power (W) x time (s)
Equation linking energy transferred, charge flow x potential difference
Energy transferred (J) = charge flow (C) x potential difference (V)
How is the power of a circuit device related to the energy transferred over a given time (2)
- The power of an appliance is the energy that it transfers per second
- So the more energy it transfers in a given time, the higher its power is
What is the relationship between the power ratings for domestic electrical appliances and the changes in stored energy when they are in use (3)
- The power rating tells you the maximum amount of energy transferred between the stores per second when the appliance is in use
- The lower the power rating, the less electricity an appliance uses in a given time and so the cheaper it is to run
- The higher power doesn’t necessarily mean that it transfers more energy usefully because an appliance can be more powerful but less efficient
Why is the national grid system an efficient way to transfer energy (2)
- It uses high Pd and a low current
* Increasing the pd decreases the current which decreases the energy lost by heating the wires and the surroundings
What is the common characteristic of the resistance in components such as lamps, diodes, thermistors and LDR’s
Their resistance is not constant
Resistance in a filament lamp
The resistance increases as the temperature of the filament increases
Resistance in diodes (2)
- The current flows in only one direction
* The diode has a very high resistance in the reverse direction
Resistance in thermistors
Resistance decreases as the temperature increases
Resistance in LDR’s
The resistance decreases as light intensity increases
