2. Electricity Flashcards
What is electric current?
The flow of electrical charge.
State the equation linking charge current and time.
Charge = Current x time Q=It
What is the unit of charge
Coulombs
What can be said about the value of current at any point in a single closed loop?
Current is the same at all points in a closed loop.
What two factors does the current in a circuit depend on?
Potential Difference (V), Resistance (R)
What equation should be used to calculate potential difference if current and resistance are known?
V = I R
What are the units for potential difference.
Volts
What are the units for current
Amps
What is the unit of resistance
Ohms
What is an Ohmic Conductor? State the condition required.
A conductor for which current and potential difference are directly proportional. Resistance remains constant as current changes. Temperature must be constant.
List four components for which resistance is not constant as current changes.
Lamps
Thermistor
Variable resistor
LDR
What happens to the resistance of a filament lamp as the temperature increases? Why?
Resistance increases. Ions in metal have more energy
What is different about current flow through a diode?
The current only flows in one direction. Resistance is very high in the other direction
State what happens to the resistance of a thermistor as temperature increases.
The thermistor’s resistance decreases.
Give an example of when a thermistor may be used.
In a thermostat to turn a heater on below a certain temperature
State what happens to the resistance of an LDR as light intensity decreases.
The LDR’s resistance increases.
Give an application for an LDR.
Street lights often use LDRs. When light levels become too low
What are the two ways that a component can be connected in a circuit?
Series (same loop)
How does the potential difference across two components vary when connected in series and parallel?
Series: Total P.D is shared between each component
If two resistors are connected in parallel
what can be said about their combined total resistance?
If two resistors are connected in series
what can be said about their total resistance?
Describe the current in a series circuit.
In a series circuit all components are in 1 complete loop
Describe the current in a parallel circuit.
In a parallel circuit the components are split across different loops in a circuit
How should you connect an ammeter in a circuit to measure current?
Ammeters should be connected in series with the component that they are measuring current through.
How should you connect a voltmeter in a circuit to measure potential difference?
Voltmeters should be connected in parallel to the component that they are measuring the potential difference of.
Why is it advantageous to connect lamps in parallel?
If one lamp blows current can take an alternative route and the circuit will not be broken
Is mains electricity an a.c. supply or a d.c. supply?
Mains electricity is an a.c. supply.
What do a.c. and d.c. stand for?
a.c.: Alternating Current
Define alternating current and direct current.
Alternating current: Current that continuously changes direction at a specific frequency. Direct current: One directional current flow.
What is the frequency and voltage of the UK mains electricity supply?
Frequency: 50 Hz
How many wires are usually in the cables connecting electrical appliances to the mains?
3 wires: Live wire, neutral wire earth wire
State the insulation colour used on the Earth wire.
Green and Yellow Stripes
State the insulation colour used on the live wire.
Brown
State the insulation colour used on the neutral wire.
Blue
Explain when the Earth wire does and doesn’t carry a current.
Under normal circumstances the earth does not carry a current, only when there is a fault does it carry current
What potential is the neutral wire at?
0 Volts
State the potential difference between the live and earth wires.
230 Volts
What is the purpose of the neutral wire?
To complete the circuit by connecting the appliance back to the mains supply.
State two equations for the power of a circuit.
P = I V, P = I squared R
State an equation linking energy transferred power and time.
Energy transferred = power / time
State an equation linking energy transferred
charge and potential difference.
What two factors affect the energy transferred by an appliance?
How long the appliance is used for, how powerful it is
Describe the energy transfers in a battery-powered torch.
Battery converts chemical energy into electrical energy. Bulb converts electrical energy into light and waste energy in the form of heating.
Describe the energy transfers in a battery-powered motor.
Battery converts chemical energy into electrical energy. Motor converts electrical energy into kinetic energy and waste energy in the form of heating.
What three things determine the power of a circuit device?
Potential difference, current, resistance
What is the purpose of the National Grid?
To link power stations to consumers so that they have access to electricity.
What are the two types of transformers used in the National Grid?
Step-Up Transformers, step down transformers
Where are step-up transformers found in the National Grid? What do they do?
Step-Up Transformers are used when connecting power stations to transmission cables. They increase potential difference.
Where are step-down transformers found in the National Grid? What do they do?
Step-Down Transformers are used when connecting transmission cables to domestic buildings. They decrease potential difference.
Why do transmission lines transfer electricity at high potentials?
A high potential results in a low current. The lower the current therefore less energy lost as heat.
Why does the potential need to be decreased between transmission lines and houses?
Lower potentials are safer for domestic use and reduce the likelihood of severe electrocution. Appliances are designed for 230V.
