Electricity Flashcards
What causes static electricity?
Friction causes the negative electrons to move from your hair to the balloon. Materials that gain electrons become negatively charged (the balloon), materials that lose electrons become positively charged (your hair)
How do materials become electrically charged?
When insulating materials are rubbed against each other (using friction) they become electrically charged e.g. rubbing a balloon against your hair.
What is current?
Current is the flow of charged particles, usually through a metal wire.
What is potential difference?
Potential difference is how much energy is transferred by each charged particle
What is resistance?
Resistance causes the battery’s chemical energy to be converted into thermal energy
Charge equation
Charge(coulomb)= current(amps) X time(s)
Energy transferred equation
Energy transferred(Joules)= potential difference(volts) X charge(coulomb)
Potential difference equation
Potential difference(volts)= current(amps) X resistance(ohms)
What is a filament lamp?
A filament lamp is an example of a non-ohmic conductor I.e. the current and potential difference are not directly proportional (unlike a resistor). The current passing through the filament causes the temperature to increase causing it to glow and further increase its resistance. As the temperature increases the resistance also increases I.e. it is more difficult for the electrons to get through the filament
What is a diode?
Current will only flow through a diode in one direction. The diode has very high resistance in the reverse direction.
What is an ohmic conductor?
The current through an ohmic conductor (at constant temperature) is directly proportional to potential difference so you get a straight line. Ohmic conductor- e.g. resistor
What is a light dependent resistor?
An LDR is a resistor that is dependent on the intensity of light.
In bright light the resistance falls, in darkness the resistance is highest.
They have lots of applications including automatic night lights, outdoor lighting and burglar detectors.
What is a thermistor?
A thermistor is a temperature dependent resistor. In hot countries, the resistance drops. I’m cool countries, the resistance goes up.
Thermistors make useful temperature detectors, e.g. car engine temperature sensors.
What are the properties in a series circuit?
Battery p.d is SHARED around the circuit
Total current is the SAME all around the circuit
Current is measured using ammeters that are placed in series in the circuit
In series circuits there is only one route for the electrons to travel around the circuit
What are the properties in a parallel circuit?
Battery p.d is the SAME for all components
Total current is SHARED between all branches in the circuit
The total resistance of the circuit is LESS THAN the smallest resistance
Potential difference is measured in voltmeters that are placed in parallel circuit
In parallel circuits there are two or more routes for the electron to travel around the circuit
The total resistance of the circuit is the SUM of each of the resistances
What is a Direct Current?
One that always flows around the circuit in the same direction. Cells and Batteries are sources of direct current because their voltages remain constant over time
Uses of a direct current:
Remote control, phone, cells and batteries are sources of DC because their voltages remain constant over time.
What is an alternating current?
One that is constantly changing direction. Mains electricity is a source of alternating current because its voltage alternates (changes) over time.
What is the voltage and frequency of the mains electricity in the UK?
In the UK the mains electricity supply has a voltage of about 230V and a frequency of 50Herts
What does oscilloscope mean?
To look at the direct and alternating current
What does power mean?
The amount of energy transferred every second
Power equation
Power(W)= energy transferred divided(J) divided by time(S)
Power equation
Power(W)= current(A) X potential difference(V)
Power equation
Power(W)=current(Asquared) X resistance(ohms)
What is the difference between UK and European plugs?
UK plugs have 3 pins but European plugs only have 2. This is because the European plugs don’t have an earth wire because they’ve got double insulation so you can’t get electrocuted so there is no need for an earth wire in European plugs.
How do large amounts of electricity not melt the cable?
An appliance that needs more power will have a thicker cable (to stop it from melting!!)
What colour and position is the earth wire?
The earth wire is positioned at the top of the plug. Safety wire, sends current to ground if a fault occurs. Green and yellow
What colour and position is the live wire?
Positioned to the right of the plug. Provides alternating potential difference. Provides 230V. Brown.
What colour and position is the neutral wire?
Positioned at the left of the plug. Completes the circuit and carries away the current. Has almost 0V. Blue
What is the national grid?
A giant system of cables and transformers that covers the UK and connects power stations to consumers.
What does a step-up transformer do?
Increases voltage, decreases current, reduces energy lost due to resistance in cables.
What is the voltage of the cables?
40,000V
What does the step-down transformer do?
Reduces voltage to 230V so it’s safe to use in our homes
Why is underground cables more safe than the overhead system?
Because all distribution wiring is placed underground and there are little chances of any hazard.
Does the underground or overhead system have more initial costs?
The underground system is more expensive due to the high cost of trenching, conduits cables, manholes and other special equipment.
Which system is harder to repair and more likely to suffer accidental damage?
Underground system because if a fault occurs in the underground system, it is difficult to locate and repair. Also suffer more frequent accidental damage from construction projects and other digging activities.
Why do electrical appliances waste energy?
Because the electrical components inside them have a small resistance- this causes a small heating effect.
Electrical devices with motors (e.g. hairdryers) also waste energy due to friction between the moving parts- this heat is transferred to the surroundings.
Efficiency equation
Efficiency= useful power output divided by total power input X100
Why does the national grid use a high PD and a low current?
To transmit the huge amount of power needed, you need either a high potential difference or a high current. The problem with a high current is that you lose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings. It’s much cheaper to boost the PD up really high and keep the current as low as possible. This makes the national grid an efficient way of transferring energy.
