SP10: Electricity and Circuits

Describe the structure of an atom

A nucleus containing protons and neutrons, surrounded by electrons in shells.

What are the two charged particles in an atom and what are their charges?

Protons (+1) and Electrons (-1)

How is current defined?

The rate of flow of charge (or electrons) around a circuit.

What are the units for charge?

Coulombs, C

What are the two ways that a component can be connected in a circuit?

Series (same loop) and Parallel (adjacent loop)

What does a cell do in a circuit?

The cell provides the energy and makes the charges move

What does a fuse do in a circuit?

It melts at a certain current and creates an opening or disconnect in the circuit.

What does a diode do?

Allows current to flow in only one direction

What does a resistor do?

It reduces the flow of current through the circuit

What does a thermistor do?

Resistance decreases as temperature increases

What does a variable resistor do?

It allows the current to be varied

What does a Light Dependant Resistor do?

They decrease the resistance as the light intensity increases.

What is potential difference?

The driving force that pushes the charge around - the Voltage

How does the potential difference across two components vary when connected in series and parallel?

Series: Total P.D. is shared between each component; Parallel: Total P.D. is the same across each component

If two resistors are connected in parallel, what can be said about their combined total resistance?

The total resistance is LESS than the smallest of the two individual resistances

If two resistors are connected in series, what can be said about their total resistance?

The total combined resistance is equal to the SUM of the two individual resistances.

Where must a voltmeter be placed in a circuit?

in parallel with the component that is being measured

Give an equation relating potential difference with energy transferred and charge.

Potential Difference \= Energy transferred / Charge (V\=E/Q)

How else can a volt be described as?

Joule per coulomb

What is electric current?

The rate of flow of charge

State the equation linking charge, current and time. Give the units for the quantities involved.

Charge (Coulombs, Q) \= Current (Amperes, I) x Time (Seconds, T)

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 (Voltage, V), and Resistance (R)

What equation links potential difference, current and resistance?

Potential Difference (V) \= Current (I) x Resistance (Ω)

What is an ammeter and where must it be connected in a circuit?

Ammeters measure current - connected in series with the component it is measuring.

What happens when current reaches in a junction in a parallel series?

Current is conserved; the total current remains the same and splits between branches

How does resistance affect current in a circuit?

As the total resistance of a circuit increases, the current flowing through the circuit decreases.

Describe an I-V graph for resistors and wires

Current is directionally proportional to the potential difference (if the temperature stays the same). Different resistors have different resistances so their I-V graphs have different LINEAR slopes.

What is an Ohmic conductor?

A resistor in which the current is directly proportional to the potential difference at a constant temperature

List four components for which resistance is not constant as current changes.

Filament Lamp, Diodes, Thermistors, Light Dependant Resistors (LDR).

What happens to the resistance of a filament lamp as the temperature increases? Why?

Resistance increases; metal ions have more kinetic energy, so vibrates more, colliding more frequently with the electrons as they flow through the metal, creating more resistance to current flow.

Describe an I-V graph measuring the resistance of a filament lamp

The increasing current increases the temoerature of the filament, which makes the resistance increase so its I-V graph are curved (non-linear)

What is different about the current flow in a diode?

The current only flows in one direction; Resistance is very high in the opposite direction, preventing current flow.

When would a thermistor be useful?

Thermostats to turn the heating on below a certain temperature; In a freezer to turn the cooler on when the temperature becomes too high

When would LDRs be useful?

Street lamps or night lights; When light levels drop at night, resistance increases and the light gains sufficient current to turn on.

How do diodes work?

A diode only allows current to flow in one direction. If current is flowing the right way, the resistance is large for small voltages (up until about 0.6V), but then at higher voltages becomes very small.

Describe an I-V graph for a diode

Current will only flow through a diode in one directions. The diode has a high resistance in the reverse direction. This means the I-V graph will have a curved shape (non-linear).

What factors affect the energy transferred when charge flows through a component?

Amount of charge and the potential difference across the component.

Give an equation linking energy, current and potential difference, giving all SI units.

Energy (Joules, J) \= Potential Difference (Voltage, V) x Current (Amperes, A) x Time (Seconds, T)

Describe potential difference in terms of charge

The work done per unit charge.

Give an equation relating potential difference to charge

Energy Transferred (Joules, J) \= Potential Difference (Voltage, V) x Charge (Coulombs, C)

When an electrical current flows through a resistor why does it heat up?

There are collisions between the electrons and ions in the resistor's lattice. This causes a transfer of kinetic energy into thermal energy, which is released into the surroundings.

How do low resistance wires reduce unwanted energy transfers?

A smaller resistance means there are less collisions, therefore less energy will be wasted through heating.

What is the heating effect?

When an electric current passes through a conductor (like a high resistance wire) the conductor becomes hot after some time and produces heat

What are some advantages of the heating effect?

It is useful for appliances such as toasters or electrical fires, where the heat is the desired product.

What are some disadvantages of the heating effect?

The loss of energy as heat energy can make an appliance inefficient; If an appliance overheats it can catch fire or overheat, which could ruin the device or injure the user

What is the energy transferred per second known as?

Power

Define power

The rate of energy transfer, or the rate at which work is done.

What are the units of power?

Watts, W

Give an equation for power including current and potential difference

Power (Watts, W) \= Current, (Amperes, A) x Potential Difference (Voltage, V).

Give an equation for power including current and resistance

Power (Watts, W) \= Current^2 (Amperes, A) x Resistance (Ohms, Ω).

Give an equation for power including energy and time

Power (Watts, W) \= Energy (Joules, J) / Time (Seconds, T)

Give an equation for power including potential difference and resistance

Power \= Potential Difference^2 (Voltage, V) x Resistance (Ohms, Ω)

What does A.C. mean?

Alternating current - This is current which constantly changing between a negative and positive maximum

What does D.C. mean?

Direct current - this is current which takes a constant value.

What supply does the mains electricity use: A.C. or D.C.?

Mains electricity uses A.C. supply

What is meant by the frequency of a supply?

This is the rate at which an AC current changes.

What is the frequency and voltage of the UK mains electricity supply?

Frequency: 50 Hz, Voltage: 230V

What type of current do batteries and cells supply?

D.C. (Direct current)

What is the power rating of an appliance?

It shows how much energy a device converts per second.

What wires does a typical domestic appliance have?

Live wire, Neutral wire, Earth wire

What are the colours of the live, neutral and earth wires?

Live wire is brown, neutral wire is blue, earth wire is green and yellow (striped).

What does the live wire do?

Carries alternating potential difference from the supply. Alternates between a high +ve and -ve voltage of about 230V

What does the neutral wire do?

Completes the circuit by connecting the appliance back to the mains supply. Carries the current out of the device. It is at around 0V

What does the earth wire do?

The earth wire is a safety wire connected to the metal casing of the appliance to stop it becoming live. Carries current away if something goes wrong e.g. live wire touches metal casing of appliance. It is at around 0V

What is the potential difference between the live and the earth wires?

230V

How does touching the live wire give you an electric shock

Human body is at 0V; If live wire is touched, a large potential difference is produced across your body and a current flows through you; This causes an electric shock which could injure or kill you.

How does a fuse wire stop the device from getting damaged?

When the current becomes too high, the fuse wire will melt and break, therefore stopping current from flowing to the appliance and preventing damage.

Why is it important to connect a fuse or switch to the live wire?

When the switch is turned off, or when the fuse breaks, it will break the circuit and stop current running through the wire. This provides a method to stop current flowing through the appliance.

Why is it dangerous to have a connection between the live wire and earth wire?

It can result in current surges, causing electric shocks.

How do circuit breaks work?

Instead of melting a fuse, large currents 'trips' a circuit breaker, which turn off quicker than the time taken for a fuse to melt. They can also be reset, which makes it much easier than having to replace a fuse.

What does double insulated mean?

An appliance that has a plastic casing covering the metal parts is considered to be double insulated. Double insulated appliances don't need an earth wire.

What are two-core cables?

Cables that only carry the live and neutral wires