An introduction to electricity (electricity)

Question 1

What are the 14 circuit symbols?

Answer 1

• Switch (open)
• Switch (closed)
• Cell
• Battery
• Diode
• Resistor
• Variable resistor
• LED (light emitting diode)
• Bulb/lamp
• Fuse
• Voltmeter
• Ammeter
• Thermistor
• LDR (light dependent resistor)

Question 2

What is electric charge and current?

Answer 2

• Electric current is the flow of electrical charge –the greater the rate of flow, the higher the current.
• Current is measured in amperes (A), which is often abbreviated to amps, using an ammeter.
• As the current in a single, closed loop of a circuit has nowhere else to go (e.i. no branches to travel down), the current is the same in all points in the loop.
• Electric charge is measured in coulombs (C) and can be calculated with the equation: charge flow (Q is measured in coulombs, C) = current (I is measured in amps, A) x time (t is measured in seconds, s)

Question 3

What is resistance?

Answer 3

• The resistance of a component is the measure of how it measure of how it resists the flow of charge.
• Resistance is measured in ohms (Ω)

The higher the resistance:
* the more difficult it is for charge to flow.
* the lower the current.

Question 4

What is potential difference?

Answer 4

• Potential difference (or voltage) tells us the difference in electrical potential from one point in the circuit to another.
• Potential difference can be thought of as electrical push.
• Potential difference is measured in volts (V) using a voltmeter.
• Potential difference, current and resistance are linked by the equation: potential difference (V is measured in volts, V)= current (I is measured in amps, A) x resistance (R is measured in ohms, Ω)

The bigger the potential difference across a component:
* the greater the flow of the charge through the component.
* the bigger the current.

Question 5

What is potential difference–current graphs (V–I graphs)?

Answer 5

• Potential difference–current graphs (V-I graphs) are used to show the relationship between the potential difference (voltage) and current for any component.
• A straight line through the origin indicates that the voltage and current are directly proportional, i.e. the resistance is constant.
• A steep gradient indicates low resistance, as a large current will flow for a small potential difference.
• A shallow gradient indicates a high resistance, as a large potential difference is needed to produce small current.
• For some resistors the value of R is not constant but changes as the current changes, this results in a non linear graph.

Question 6

How is an ammeter and voltmeter connected?

Answer 6

• An ammeter is connected in series. The voltmeter is connected in parallel to the component.

Question 7

What is the role of resistors in a circuit?

Answer 7

• An ohmic conductor is a resistor in which the current is directlly proportional to the potential difference at a constant temperature.
• This means that the resistance remains constant as the current changes.
• It is indicated by a linear (straight line) graph.

Question 8

What is the role of filament lamps in a circuit?

Answer 8

• As the current through a filament lamp increases, its temperature increases.
• This causes the resistance to increasee as the current increases.
• It is incdicated by a curved graph.

Question 9

What is the role of diodes in a circuit?

Answer 9

• The current through a diode will only flow in one direction.
• The diode has a very high resistance in the reverse direction.
• This is indicated by horizontal line along the x axis, which shows no current flows.

Question 10

What is the role of thermistors in a circuit?

Answer 10

• The resistance of a thermistor decreases as the temperature increases.
• This makes them useful in circuits where temperature control or response is required.
• For example, a thermistor could be used in a circuit for a thermostat that turns a heater off at a particular temperature or an indicator light that turns on when a system is overheating.

Question 11

What is the role of light dependent resistors (LDR) in a circuit?

Answer 11

• The resistance of an LDR decreases as light intensity increases.
• This makes them useful where automatic light control or detection is needed.

Question 12

What are the features of a series circuit?

Answer 12

• 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 component. This is because the current has to travel through each component in turn.
• Adding resistors in series increases the total resistance (R) in ohms (Ω): total resistance = r1 + r2…

Question 13

What are the features of a parallel circuit?

Answer 13

• The potential difference across each component is the same.
• The total current drawn from the power supply is the sum if the currents through the separate components.
• The total resistance of the two resistors is less than the resistance of the smallest individual resistor. This is because, in parallel, there are more paths for the current to take – it can take one or the other – allowing the current to flow more easily.
• Adding resistors in parallel reduces the total resistance.

Question 14

What is power?

Answer 14

• The power of a device depends on the potential difference across it and the current flowing through it.
• A device with a higher potential difference or current will use more energy per second than one with a lower potential difference or current, e.i. it will be more powerful.
• power, P = potential difference, V multiplied by current, I
• power, P = (current)^2, I multiplied by resistance, R